Journal of Nanoparticle Research

, Volume 12, Issue 1, pp 327–335

Microwave technique applied to the hydrothermal synthesis and sintering of calcia stabilized zirconia nanoparticles

Authors

    • Department of Materials and Environmental EngineeringUniversity of Modena and Reggio Emilia
  • Anna Corradi
    • Department of Materials and Environmental EngineeringUniversity of Modena and Reggio Emilia
  • Cristina Leonelli
    • Department of Materials and Environmental EngineeringUniversity of Modena and Reggio Emilia
  • Roberto Rosa
    • Department of Materials and Environmental EngineeringUniversity of Modena and Reggio Emilia
  • Roman Pielaszek
    • Laboratory of Nanomaterials, Institute of High Pressure PhysicsPolish Academy of Science
  • Witold Lojkowski
    • Laboratory of Nanomaterials, Institute of High Pressure PhysicsPolish Academy of Science
Research Paper

DOI: 10.1007/s11051-009-9619-9

Cite this article as:
Rizzuti, A., Corradi, A., Leonelli, C. et al. J Nanopart Res (2010) 12: 327. doi:10.1007/s11051-009-9619-9

Abstract

This study is focused on the synthesis of zirconia nanopowders stabilized by 6%mol calcia prepared under hydrothermal conditions using microwave technology. Sodium hydroxide-based hydrolysis of zirconyl chloride solution containing calcium nitrate followed by microwave irradiation at the temperature of 220 °C for 30 min was sufficient to obtain white powders of crystalline calcia stabilized zirconia. By means of X-ray diffraction and transmission electron microscopy, it was shown that tetragonal zirconia nanocrystallites with a size of ca 7 nm and diameter/standard deviation ratio of 0.10 were formed. The effects of the [Ca2+] and [NaOH] as well as temperature and time of microwave irradiation on the density and specific surface area were evaluated. Sintering test of the tetragonal nanopowders at 1,300 °C in air was performed in a monomode microwave applicator. The sample was sintered to the density of 95% and the grain size, analyzed by field emission scanning electron microscopy, was in the range from 90 to 170 nm.

Keywords

ZirconiaNanostructuresMicrowaveSinteringX-ray diffractionElectron microscopy

Introduction

Calcia as well as di- or tri-valent oxides stabilised zirconia (CaSZ) have received much attention recently because of their high oxygen-ion conductivity, which combined with their high thermochemical stability makes them the best candidate for electrochemical devices, such as oxygen sensors (Zhou and Ahmad 2006; Williams and McGeehin 1984; Fray 1996), solid oxide fuel cells (Fergus 2006; Ringuede et al. 2006) and oxygen pumps (Caproni and Muccillo 2006; Takehira et al. 2002) which operate at high temperatures. The main preparation methods of these compounds are solid state reactions (Durrani et al. 2006) and sol-gel route (Muccillo et al. 2001), which require high temperatures and complex interdiffusion processes. Fine and uniform ceramic powders of high purity can be readily prepared through the hydrothermal method in a single step (Pyda 2006; Dell’Agli and Mascolo 2000). Thus, hydrothermal synthesis appears to be a simple method for the preparation of zirconia nanopowders of high purity and high degree of compositional homogeneity, which ensure an appropriate sinterability (Park et al. 2007; Haberko and Pyda 1984). The co-precipitation of calcium and zirconium hydroxides followed by treatment in pressurised water was an efficient route for the inclusion of calcium into the crystalline structure of zirconia, compared to the solid state methods (Cushing et al. 2004). This inclusion stabilizes the tetragonal phase whose stability at high temperatures is crucial for sintering of ZrO2 powders. Microwave technology combined with hydrothermal processing is, as broadly reported (Leonelli and Lojkowski 2007), an advantageous tool for the efficient obtainment of inorganic oxides as well as doped zirconia nanoparticles due to the uniform and fast heating and good control of the process time as well as high purity conditions of the process (Opalinska et al. 2006; Lojkowski et al. 2004). In this study, we tested the optimal conditions of microwave heating for hydrothermal synthesis of calcia stabilised zirconia.

The use of dense and monodispersed nanometric particles with phase uniformity is a crucial factor for obtaining nanocrystalline ceramics, which recently have found several industrial applications such as cosmetic sunscreens (O’Brien and Yin 2005) and pigments (Rangappa et al. 2007), mechanical seals (Greenberg and Mech 2003; Ederyd et al. 2001), catalysts (Zheng et al. 2005; Zyryanov et al. 2004), inks for high-opacity jettable inks products (Kumar et al. 2006; Buretea et al. 2004) and medical devices (Schneiders et al. 2008). During the last decade, microwave sintering of a considerable number of functional ceramic materials was developed (Kaysser et al. 2003). The advantages of microwave sintering are the rapid and volumetric heating that leads to a fine microstructure and good mechanical properties and reduced porosity. Moreover, it has been proved that under microwave irradiation, the densification process is enhanced by surface atom mobility due to localized electric field intensification. The smaller the grain size of the starting powder the higher the electrical field density (Agrawal et al. 2003). This peculiarity of microwave heating can be exploited during nanocrystalline powder sintering, as proposed in this paper, where high densification degree can be reached in short times maintaining the grain size in the nano range.

In this study, microwave irradiation is used to optimize the hydrothermal method for the preparation of CaO-doped zirconia nanoparticles via hydroxides co-precipitation route. The influence of [Ca2+], [NaOH], temperature and time of synthesis on the phase composition and morphology of the powders was investigated. The aim of the optimisation process was to obtain a rapid conversion of the hydroxides to the oxides as well as highly dense nanoparticles with a pure tetragonal phase. Specific surface area versus density diagrams has been used as a simple tool for the evaluation of the quality of nanopowders in relation to their preparation methods. Finally, microwave technology has also been used for sintering of the CaSZ nanoparticles with a particular attention to the crystalline phase effect. With this in mind, a mixture of monoclinic and tetragonal phases nanoparticles were used as starting powders for sintering.

Experimental

First, reference 6%mol CaSZ nanoparticles (sample A1, Tables 1, 2) were obtained by adding calcium nitrate powder (Ca(NO3)2 · 4H2O, Fluka) to a 0.5 M ZrOCl2 · 8H2O aqueous solution. The solution was neutralized with NaOH 1 M to a pH of 10. A total of 12 mL of the mixture was transferred to a Teflon® lined digestion vessel and hydrothermally treated using a Milestone ETHOS TC microwave digestion system operating at 2.45 GHz and 210 °C for 90 min with computer control of time, temperature, pressure and power. After the reaction, the powder was agglomerated and was centrifuged, washed and dried in a conventional oven at 100 °C for several hours.
Table 1

Reactions undertaken to investigate the effect of [Ca2+] and [NaOH] on the microwave assisted syntheses of the CaSZ nanoparticles conducted at T = 200 °C for 90 min

Sample

[ZrOCl2] (mol/l)

[NaOH] (mol/l)

%mol Ca

Monoclinic %b

Tetragonal %b

BET analysis

Density (g cm−3)

S (m2 g−1)

ϕ (nm)

A1a

0.5

1.0

6

17 ± 5

83 ± 5

134 ± 2

8.0 ± 0.5

4.9 ± 0.1

B

0.5

1.0

3

15 ± 5

85 ± 5

138 ± 2

7.7 ± 0.5

5.3 ± 0.1

C

0.5

1.0

8

18 ± 5

82 ± 5

138 ± 2

7.8 ± 0.5

5.2 ± 0.1

D

0.5

0.5

6

21 ± 5

79 ± 5

122 ± 2

8.8 ± 0.5

5.2 ± 0.1

E

0.5

0.25

6

21 ± 5

79 ± 5

142 ± 2

7.6 ± 0.5

4.9 ± 0.1

aReference synthesis

bOn the basis of powder X-ray diffraction analysis

Table 2

Reactions undertaken to investigate the effect of the temperature and time on the syntheses of the CaSZ nanoparticles ([ZrOCl2] = 0.5 M, [NaOH] = 1 M, %mol Ca = 6)

Sample

T (°C)

t (min)

Monoclinic %b

Tetragonal %b

BET analysis

Density (g cm−3)

S (m2 g−1)

ϕ (nm)

A1a

200

90

17 ± 5

83 ± 5

134 ± 2

8.0 ± 0.5

4.9 ± 0.1

A2

200

60

14 ± 5

86 ± 5

133 ± 2

8.1 ± 0.5

5.2 ± 0.1

A3

200

30

16 ± 5

84 ± 5

174 ± 2

6.1 ± 0.5

4.8 ± 0.1

G1

210

60

18 ± 5

82 ± 5

136 ± 2

7.9 ± 0.5

5.3 ± 0.1

G2

210

30

17 ± 5

83 ± 5

140 ± 2

7.6 ± 0.5

5.2 ± 0.1

G3

210

15

12 ± 5

88 ± 5

173 ± 2

6.2 ± 0.5

5.0 ± 0.1

H1

220

30

4 ± 5

96 ± 5

148 ± 2

7.2 ± 0.5

4.9 ± 0.1

H2

220

20

10 ± 5

90 ± 5

148 ± 2

7.2 ± 0.5

5.3 ± 0.1

H3

220

10

8 ± 5

92 ± 5

150 ± 2

7.1 ± 0.5

5.0 ± 0.1

aReference synthesis

bOn the basis of powder X-ray diffraction analysis

On the basis of the reference reaction conditions (sample A1), new syntheses were undertaken to investigate the effect of [Ca2+] and [NaOH] as well as the temperature and time on the crystalline phase and size of the CaSZ nanoparticles. The experimental conditions are shown in Tables 1 and 2. Samples B and C have been prepared by changing the content of calcia from 6, the reference value, to 8 and 3, respectively. Samples A1, D and E have been obtained using a decreasing mineraliser concentration of 1.0 M, 0.25 M and 0.5 M, respectively. Samples labeled A, G and H were obtained using a reaction temperature of 200, 210 and 220 °C, respectively, and the associated numbers 1, 2 and 3 indicate the decreasing reaction time (for samples A 90, 60 and 30 min, respectively; for sample G 60, 30 and 15 min, respectively; for samples H 30, 20, 10 min, respectively).

Discs of CaSZ powders mixed with 5% polyvinyl alcohol binder solution were formed using uniaxial press. Microwave sintering was performed under single mode microwave irradiation at 2.45 GHz on the sample in the form of a pressed pellet at a temperature of 1,300 °C in air for 5 min.

Characterization

The dried powders and the sintered samples were analysed by a computer assisted X-ray powder diffractometer (Model X’Pert PRO Philips, Eindhoven, Netherlands) using CuKα radiation. The X-ray diffraction (XRD) patterns were collected in a 2θ range of 5 to 80° at room temperature, with a step size of 0.02° and a time per step of 6 s. Raman spectra were obtained with a confocal Raman microscope (LabRAM system, Jobin-Yvon) consisting of a Raman spectrometer coupled to a microscope (model BX 40, Olympus) using a 632.81 nm beam wavelength of an He–Ne laser and a Peltier-cooled detector. The grain size distributions (GSD) were determined using a newly developed method of XRD peak fine structure analysis of polydisperse powders (XRD-GSD) (Wejrzanowski et al. 2006; Pielaszek 2003). This method permits to fit the peaks using an analytical function with fitting parameters: average particle diameter and dispersion of particle sizes are fitting parameters. The ratio of the volume fraction of the monoclinic and tetragonal phases was determined by measuring the peak area belonging to the respective phases.

In order to cross check the results of XRD analysis, the nanoparticle size was determined using TEM (Model JEM 2010, Jeol, Akishima Tokyo, Japan) equipped with X-ray energy dispersive spectroscopy (EDS). Specimens were prepared by dispersing the powders in distilled water using an ultrasonic stirrer and then placing a drop of suspension on a copper grid covered with a transparent polymer film, followed by drying and carbon coating. The size distributions were obtained by TEM image analysis using the ImageJ software (freeware software: http://rsb.info.nih.gov/ij/). The specific surface area analysis was conducted by means of the multipoint B.E.T method (Gemini 2360, Micromeritics Instruments, Norcross, GA, U.S.A) using nitrogen as an adsorbate. Based on B.E.T. data, the particle size was calculated, assuming that the particles are spherical, using the equation:
$$ \phi = 6/S\rho , $$
where ϕ (m) is the average diameter of a spherical particle, S (m2 g−1) is specific surface area of powder and ρ (g m−3) is the density value of crystalline zirconia (5.6 × 106 g m−3). The density of the powders was measured by helium picnometry (Model AccuPyc 1330, Micrometrics Instruments, Norcross, GA, U.S.A.).

The sintered sample morphology was examined by a Field Emission Scanning Electron Microscope (FE-SEM, LEO1530). The final density was determined by quantitative microscopy.

Results

The phase composition of the reference sample (sample A1, Tables 1, 2) was 83% tetragonal phase, 17% monoclinic phase and 0% cubic as revealed by XRD analysis (Fig. 1). The size of tetragonal zirconia particles was 7.9 ± 1.0 nm and that of the monoclinic phase was 8.1 ± 3.0 nm with a particle size dispersion of 0.12 and 0.36, respectively, where the particle dispersion index is the ratio of the diameter to the standard deviation.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9619-9/MediaObjects/11051_2009_9619_Fig1_HTML.gif
Fig. 1

XRD spectra of the CaSZ nanoparticle samples (a) A1 (synthesized at 200 °C in 90 min) and (b) H1 (synthesized at 220 °C in 30 min) (T: tetragonal phase; M: monoclinic phase)

Transmission electron microscopy (TEM) images and selected area electron diffraction, shown in Fig. 2 and in its inset, respectively, confirmed the formation of crystalline nanoparticles of CaSZ of 6.9 ± 1.1 nm. The detection of both monoclinic and tetragonal polymorphs in sample A1 was confirmed (Castro et al. 2008) using the Raman spectrum recorded under the microscope (Fig. 3). Monoclinic zirconia is characterized by the doublet at 183 and 195 cm−1, whereas the two distinct bands at 151 and 274 cm−1 as well as at 467 and 480 cm−1 are assigned to the tetragonal phase. A broad peak at 590–610 cm−1 corresponding to the cubic phase is not detected.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9619-9/MediaObjects/11051_2009_9619_Fig2_HTML.jpg
Fig. 2

TEM image of sample A1 (reference synthesis) Ca-stabilized zirconia nanoparticles (synthesized at 200 °C in 90 min). Inset: electron diffraction related to the TEM image acquired

https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9619-9/MediaObjects/11051_2009_9619_Fig3_HTML.gif
Fig. 3

RAMAN spectrum of sample A1 (synthesized at 200 °C in 90 min). t-ZrO2, m-ZrO2, c-ZrO2 indicate the tetragonal, monoclinic and cubic zirconia, respectively

Effect of [Ca2+] and [NaOH]

The result of increasing CaO content while keeping temperature, time and NaOH concentration constant was as follows: zirconia nanopowders obtained from mixtures with CaO content up to 8%mol were solid solutions with both monoclinic and tetragonal phase structures as also reported in literature (Dell’Agli and Mascolo 2000). In samples A1, B and C (see Table 1), the tetragonal zirconia was detected as the main phase in a constant amount. The results of decreasing the mineraliser concentration were as follows: the increase of [NaOH] from 0.25 to 0.5 M did not substantially influence the content of tetragonal zirconia, which was 79% as determined from XRD (Table 1). When the mineralizer concentration was 1.0 M, the amount of the tetragonal phase increased up to 83%. The particle size was relatively independent of [NaOH] with accuracy of 1 nm. For low NaOH content, the density of the particles decreased (Table 1).

Effect of temperature and time

At 200 and 210 °C, the effect of an increase of the reaction time was a corresponding increase of the nanoparticle size from 6.1 to 8.1 nm and from 6.2 to 7.9 nm respectively, as reported in Table 2. The crystal growth was accompanied by a decrease of the tetragonal phase content, as depicted in Fig. 4. The increase in the temperature of the microwave hydrothermal process up to 220 °C, maintaining the same reaction time of 30 min, favoured the tetragonal phase (compare sample A3, G2 and H1 in Table 2).
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9619-9/MediaObjects/11051_2009_9619_Fig4_HTML.gif
Fig. 4

Average size and tetragonal phase content of CaSZ nanoparticles as a function of time and temperature as measured using BET method and XRD analysis, respectively. Lines are drawn as a guide to the eye

In Fig. 5, the specific surface area versus density diagram for zirconia nanoparticle samples prepared under the reaction conditions described in Table 2 is shown. The increase in reaction time led to zirconia nanoparticles with lower specific surface area as well as higher densities, indicating the good quality of the nanopowders. The increase of the temperature up to 220 °C stabilized the zirconia nanopowder structure.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9619-9/MediaObjects/11051_2009_9619_Fig5_HTML.gif
Fig. 5

Specific surface area versus density diagram for zirconia nanoparticle samples prepared as reported in Table 2. Lines are drawn as a guide to the eye

Microwave sintering of CaSZ nanoparticles

Samples A1 and H1 have been chosen for the microwave sintering process. In Table 3, the average particle size determination by means of TEM for nanoparticles A1 and H1 and FE-SEM for the relative sintered samples, SA1 and SH1, respectively, are reported. The samples were sintered to a density of 95%. Both SH1 and SA1 had a tetragonal phase even if SA1 still contained some parts in a monoclinic phase. The sintered sample SA1 obtained from almost complete densification of CaSZ tetragonal nanoparticles with a low content of monoclinic phase presented grains with size in the range of 120–240 nm. The sintering of pure tetragonal CaSZ nanoparticles led to the sample SH1 with grains in the range of 90 and 170 nm, as depicted in Fig. 6.
Table 3

Size characterization of the CaSZ nanoparticles and the relative sintered samples

Sample

TEM

Size (nm)

σ (nm)

A1

6.9

1.1

H1

6.3

0.6

Sample

Sintering T (°C)

FE-SEM

Grain size (nm)

σ (nm)

SA1

1,300

180

60

SH1

1,300

130

40

https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9619-9/MediaObjects/11051_2009_9619_Fig6_HTML.gif
Fig. 6

SEM image of the sample SH1 obtained by microwave sintering at 1,300 °C of H1 (synthesized at 220 °C in 30 min)

Discussion

The current results confirmed the advantages of the high pressure microwave technique for the production of stable as well as metastable nanocrystalline powders with a narrow particle size distribution. Moreover, the microwave technology could be successfully applied to the sintering process, assuring the nanometric grain size of the densified sample.

The mechanisms of crystallization of the tetragonal and monoclinic phases and their mutual transformation under hydrothermal conditions have been explained (Yashima et al. 1996) in terms of dissolution/precipitation, structural rearrangement of the tetragonal phase or simultaneous nucleation of the two phases of zirconia. On this basis, unstabilized crystals transform from tetragonal to monoclinic phase soon after the crystallite size of 5 nm is reached (Leonelli and Lojkowski 2007). Moreover, for doped zirconia crystals, as reported in a previous study (Opalinska et al. 2006), it was observed that at a pressure above 8 MPa, the smallest monoclinic crystals transform to the tetragonal one, while the big ones remain stable according a threshold grain size of 13 nm (Simeone et al. 2003). Therefore, in sample A1 prepared at 200 °C, due to the lower working pressure, monoclinic nanoparticles of 8.1 ± 3.0 nm, smaller than their unstability threshold size, were found. At 220 °C, the nucleation of the tetragonal zirconia nanoparticles was favoured with respect to the crystal growth as evidenced by the constant nanoparticles size of ca 7 nm (Fig. 4, Table 3) at all reaction times investigated. This effect is typically due to the presence of calcium ions that stabilise the tetragonal phase, which shifted its limit size of stability over 5 nm.

Due to the difficulty in peak assignment performed in XRD patterns in which, due to the peak broadness, the tetragonal and the cubic zirconia phases are hardly distinguished, the Raman analysis of the zirconia samples was fundamental because it resulted in the non-detection of the broad peak at 590–610 cm−1 ascribable to the cubic phase (Castro et al. 2008).

The best reaction conditions for the synthesis of tetragonal zirconia were found for sample H1. In fact, it was constituted by 96% tetragonal CaSZ nanoparticles with a size of 7.9 ± 0.8 nm, as shown by XRD analysis (Fig. 1) and confirmed by the size determination using the BET method. The TEM and SAED images of the sample H1 are depicted in Fig. 7. The SAED image of the sample H1 revealed the crystalline nature of the tetragonal-rich Ca-stabilized zirconia nanoparticles and their size as 6.3 ± 0.6 nm extrapolated by TEM image analysis. By means of both XRD and TEM analyses, a particle size dispersion of 0.10 was obtained. Each nanoparticle corresponds to a single crystalline domain, thus explaining the high densification and complete hydroxide to oxide conversion in only 30 min.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-009-9619-9/MediaObjects/11051_2009_9619_Fig7_HTML.jpg
Fig. 7

TEM images of the Ca-stabilized zirconia nanoparticle samples H1 (synthesized at 220 °C in 30 min) at different magnifications. Inset in b: electron diffraction related to the selected area defined by the white circle of the TEM image acquired

The increase of the temperature up to 220 °C stabilizes the zirconia nanopowders structure: H1, H2 and H3 samples had the same specific surface areas and densities comparable to those of ion divalent-doped zirconia reported in literature (Castro et al. 2008).

The sintering results are in the direction of that found for microwave assisted sintering of ZrO2-8%mol Y-doped samples, but starting from nanopowders one order of magnitude larger (Mazaheri et al. 2008): microwave radiation increases the diffusion driving force, declining the required time for densification, and is, therefore, capable of providing denser specimens within the shorter sintering times compared to conventional sintering. In our experiments, sintered temperature was chosen and optimised after several attempts and was measured with a sapphire optical fibre in contact with sample surface. The transversal section of the pellet presented homogenous nanostructure, indicating the absence of ‘hot spots’ (Rizzuti et al. 2008). Reduced grain size for short time treatments have been recently recorded for zirconia derived from zircon sintered and decomposed samples (Ebadzadeh and Valefi 2008). Starting from a sample of tetragonal-rich zirconia nanoparticles, the sintering process produced a sintered (SH1) with narrow distribution of nanometric size lower than sample SA1.

Conclusions

Tetragonal-rich CaSZ nanoparticles of an average grain size of 7 nm were rapidly synthesized by microwave hydrothermal hydrolysis of a 0.5 M zirconyl chloride solution added with 6%mol of calcium at 220 °C for 30 min. Application of microwaves for heating of the solution permitted optimisation of the process time in the batch process and obtained a narrow particle dispersion index of 0.10. Positioning the samples in the diagram with coordinates specific surface area versus density was an useful tool to determine the reaction conditions for a good quality of the zirconia nanopowders. Microwave technology was also applied to sinter the tetragonal-rich CaSZ nanoparticles (sample H1). After 5 min of microwave heating at 1,300 °C, the sintered sample displayed a uniform nanostructure with grains in the range of 90 to 170 nm, while sintered sample derived from CaSZ nanoparticles with a low content of monoclinic phase (sample A1) had higher grain size and dispersion of 180 ± 60 nm.

Acknowledgments

Authors are grateful to COST Action D-32 CHEM-Chemistry in High Energy Environment for supporting student mobility within this activity. Part of the work was funded by Polish Ministry of Science and High Education under the DONANO project. Authors thank Prof. P. Baraldi for the acquisition of RAMAN spectra.

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© Springer Science+Business Media B.V. 2009