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Effect of mechanochemical activation on the synthesis of NaNbO3 and processing of environmentally friendly piezoceramics
Journal of Alloys and Compounds 395 (2005) 166–173
Effect of mechanochemical activation on the synthesis of NaNbO3 and processing of environmentally friendly piezoceramics Teresa Hungr´ıa, Lorena Pardo, Alberto Moure, Alicia Castro∗ Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 8 September 2004; accepted 12 October 2004 Available online 5 January 2005
Abstract High-energy milling has been applied to obtain NaNbO3 ceramic precursor powders and dense ceramics by a one-step thermal treatment. Stoichiometric mixtures of Nb2 O5 and different sodium reactants (Na2 CO3 , Na2 O and NaOH) were mechanically activated. The obtained results are discussed regarding the starting reactants and the milling media. The evolution of the powder mixtures and the thermal behavior of the activated samples were investigated by X-ray powder diffraction and thermal analysis. The most crystalline NaNbO3 powders were obtained when a 2NaOH/Nb2 O5 mixture was homogenized using a wet-chemistry technique followed by mechanochemical activation in a planetary mill and treated at 900 ◦ C for 2 h. Dense ceramics were processed from the different precursors by conventional-sintering and hot-pressing and their microstructure and piezoelectric properties characterized. © 2004 Elsevier B.V. All rights reserved. Keywords: Ferroelectrics; High-energy ball milling; X-ray diffraction; Dielectric response
1. Introduction Niobates are a technologically important class of inorganic materials because of their broad range of physical properties and applications. Alkaline niobate ceramics are considered a good alternative for the substitution of commercial piezoceramics by lead-free materials, based on highly toxic lead compounds. Sodium niobate (NaNbO3 ) is antiferroelectric at room temperature [1], but the application of an electric field or the substitution of Li [2] or K [3] for Na, induces a ferroelectric phase that provides piezoelectric activity that is interesting for high frequency devices. It is known that NaNbO3 presents a large number of structural phases, variants of lower symmetry of the ideal perovskite structure, having six phase transitions [4] from the non-polar high temperature phase to the antiferroelectric room temperature phase and the low temperature ferroelectric one. Sodium niobate is of particular interest because in this antiferroelectric phase are based some solid solutions [5–7] with good ferroelectric ∗
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and piezoelectric properties. Recent studies [6,7] are focused on the analysis, involving dielectric and thermal expansion measurements, of the phase transitions in the polar phase of ceramics of Na1−x Lix NbO3 compositions. Knowledge of such transitions is a key factor to assess the thermal stability of their piezoelectric properties. On the other hand, the electrical and optical properties of dielectric ceramics strongly depend on the chemical and microstructural homogeneity, which is mainly influenced by the powder synthesis method and sintering process. Alkali metal niobate powders are usually prepared via a solid-state reaction between Nb2 O5 and alkali metal carbonates. This classical method involves high temperatures and long reaction times, produces the volatilization of the alkali metal, leads to poor compositional homogeneity and provides products with large particles [2]. Particularly for piezoceramics the control of the porosity and pore size is a key issue, since the dielectric strength is dependent on them [8] and a high dielectric strength is required during the poling process. Alternative routes need to be developed to facilitate the production of desired materials in a controllable way. Powder synthesis using evaporation [9], sol–gel [10], hydrothermal
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[11] and polymeric precursor processes [12] have been described in the literature to facilitate the reactions. However, most of these chemistry-based routes require high purity inorganic or organometallic reactants, more expensive than the widely available oxides and carbonates. Mechanochemical activation is a way to modify the conditions in which chemical reactions usually take place. During the mechanical treatment the particle size of the crystals is reduced, the homogeneity of the mixture is increased and, in most of the cases, the solid becomes more reactive [13]. Mechanical activation is usually a result of disorder of the crystal and generation of defects that cause the decrease of activation barrier for the reaction [14]. Solid-state mechanochemistry is a powerful technique for synthesizing many kinds of materials from metallic [15] to amorphous and nanocrystalline materials [16], as well as to prepare new oxides or to improve the properties of known functional materials [17–26]. This paper describes a comparative study of routes based on high energy milling to improve the reaction conditions to synthesize NaNbO3 and to allow the processing of dense piezoceramics in a single thermal treatment in which synthesis, grain growth and sintering takes place. Different sodium reagents and milling systems were used and results compared.
2. Experimental procedure Mechanical activation techniques have been applied to the preparation of NaNbO3 ceramic powders. Stoichiometric mixtures of analytical-grade Nb2 O5 and sodium reactants (Na2 CO3 , Na2 O or NaOH) were mixed by hand in an agate mortar or also in water media with the NaOH reactants. Then, the initial mixtures were mechanically activated using vibrating or planetary mills (Fristch, pulverisette models 0 and 6, respectively). In both cases, the mixture was placed in an agate vessel, using one ball of 5 cm diameter for the vibrating mill, whereas for the planetary mill five balls of 2 cm diameter each were used. The milling bowl of the planetary system was rotated at 200 rpm. For the sake of comparison, NaNbO3 powder was also prepared by solid-state reaction from Nb2 O5 and Na2 CO3 , by successive thermal treatments at increasing temperatures from 400 to 800 ◦ C, during 12 h each, followed by cooling by quenching in air. The effect of both milling and thermal treatments in the particle morphology and surface area were investigated by standard scanning electron microscopy (SEM) and BET surface area analysis. The milled powders were characterized by differential thermal analysis (DTA), thermogravimetric analysis (TG) and X-ray powder diffraction (XRD) at room and increasing temperatures. The DTA and TG measurements were taken with a Seiko 320 instrument, with ␣-Al2 O3 as the inert reference material,
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between room temperature and 800 ◦ C at 10 ◦ C min−1 , in the heating and cooling process, in argon. The evolved gases were analyzed with a Pfeiffer ThermoStar GSD 301T (quadrupole mass spectrometer), with argon as gas carrier to determine the molar mass. XRD patterns at room temperature were measured in a Bruker AXS D8 advance diffractometer, between 5 and 80◦ (2Θ), with 2Θ increments of 0.05◦ and counting time of 4 s per step. For high-temperature measurements, a Philips PW1310 diffractometer, coupled with an Anton Paar HTK 10 attachment to stabilize the temperature, was used. The patterns were obtained by depositing a small quantity of powder onto a platinum sheet placed on a tantalum strip, which was the heating element. The recordings were taken from 5 to 70◦ (2θ) with a scan rate of 0.02◦ s−1 . The temperature was monitored by a Pt–Pt 13% Rh thermocouple welded in the center of the platinum sheet. The heating rate was 10 ◦ C min−1 and the temperature was stabilized during ˚ was used in all X-ray 1 h. The Cu K␣ doublet (λ = 1.5418 A) experiments. Powders were shaped by uniaxial pressing at 210 kg cm−2 as thin disks of ∼10 mm diameter and 2 mm thickness. Disks were isostatically pressed at 2000 kg cm−2 and then conventional-sintering was carried out on a Pt foil at 1200 ◦ C for 2 h. Also, disks were hot-uniaxially pressed in alumina dies and surrounded by alumina powder, at temperatures of 1000 and 1100 ◦ C and ∼200 kg/cm−2 . Ceramic surfaces were polished and analyzed by optical microscopy (Leitz Laborlux 12 ME S/ST), in order to examine the ceramic porosity. Quantitative characterization of the porosity, i.e., determination of the pore area distributions on statistical samples of more than 750 pores, was carried out on the optical micrographs by a computerized image analysis and measurement system (IMCO10-KAT386 system, Kontron Elektronic GMBH, 1990) by a procedure explained in detail elsewhere [27]. The percentage of porosity was characterized by the fraction of the examined area which is occupied by pores. The average A and standard deviation, σ, of the distributions of pore area were obtained by linear fitting of the experimental data represented in probability plots [28]. Ceramic disks were lapped to a ratio thickness/diameter of 1/10, typically 8 mm diameter and 0.8 mm thickness, and electrodes were painted on the major faces, so as to prepare the capacitor samples for the electrical measurements. The samples were poled in a silicon oil bath at 50 kV cm−1 , for 20 min at 180 ◦ C. The piezoelectric d33 coefficient was measured in a Berlincourt-meter by the direct piezoelectric effect at 100 Hz. Dielectric, elastic and piezoelectric constants, real and imaginary parts, thus accounting for their respective losses [29] and electromechanical coupling factors, corresponding to the radial extensional vibration modes of the thin disk shaped ceramic resonators, were calculated. An automatic iterative method, described elsewhere [30], of calculation of these parameters from complex impedance measurements at the appropriated frequencies was used.
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Fig. 1. SEM images of the Na2 CO3 /Nb2 O5 mixture after different milling times: (a) 0 days, (b) 10 days, (c) 20 days and (d) 30 days.
3. Results and discussion 3.1. Na2 CO3 reagent A study concerning the synthesis of NaNbO3 was previously carried out by the authors, using a mechanical activation route, from a stoichiometric Na2 CO3 /Nb2 O5 mixture and a vibrating-type mill [31]. During the activation process, even after long time of milling, only a significant broadening in the diffraction peaks of the Nb2 O5 phase took place, while the Na2 CO3 became amorphous. NaNbO3 was obtained as single phase, with quite small particle size, after thermal treatment at 600 ◦ C, from powder activated during 30 days, but not for shorter milling time. Nb2 O5 /Na2 CO3 mixture was investigated by SEM, after both milling and annealing treatments. As the milling time increased, particle size decreased up to 100 nm and agglomerates of fine grains with irregular shapes can be observed together with bigger particles corresponding to the remaining Nb2 O5 observed by XRD (Fig. 1). Table 1 shows an important increase of the BET surface area value of the raw materials as a result of the activation process.
Table 1 BET surface area measurements of the Na2 CO3 /Nb2 O5 mixture after different treatments (m2 /g) Annealing temperature (◦ C) 25 600 800
Milling times (days) 0 10 20
30
0.46 – 1.31
7.27 5.74 –
2.90 – –
4.64 3.39a –
a A small quantity of Na Nb O was obtained together with the NaNbO 2 4 11 3 phase.
The properties of the powder NaNbO3 sample strongly depend on the synthesis method. As can be observed in Fig. 2, NaNbO3 prepared by classical solid-state method is constituted by large crystals, while NaNbO3 obtained by mechanical activation assisted route is composed of agglomerates with irregular shapes and formed by the coalescence of grains hundreds of nanometers in size. Moreover, the surface area of the NaNbO3 synthesized by the milling route is significantly higher (Table 1). When Na2 CO3 was used, long milling times were needed; therefore, other reactants need to be tested mainly in order to
Fig. 2. Scanning electron micrographs of: (a) NaNbO3 powder obtained by solid-state classic method at 750 ◦ C and (b) NaNbO3 phase prepared by annealing at 600 ◦ C after 30 days of milling (Na2 CO3 /Nb2 O5 mixture).
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Fig. 3. XRD patterns of the (a) Na2 O/Nb2 O5 and (b) 2NaOH/Nb2 O5 mixtures after different milling times in a vibrating mill. For the sake of comparison the Nb2 O5 XRD pattern is also plotted.
decrease the mechanical activation time, while effects on the ceramic sintering and final density and piezoelectric performance were also examined. 3.2. Na2 O reagent Fig. 3a shows the XRD patterns of the Na2 O/Nb2 O5 mixture activated in the vibrating-type mill. After 1 day of milling, the only detectable phase is Nb2 O5 . After 4 days of milling, some broad diffraction peaks appeared at 2Θ between 10 and 13◦ . Longer milling times produced an increase of the intensity of these peaks together with a broadening of the Nb2 O5 diffraction lines, showing that a reduction of the crystallite size of this phase took place. The diffraction peaks appearing at low 2Θ angles were attributed to the compound Na7 (H3 O)Nb6 O19 (H2 O)14 (JCPDS-ICDD file no. 77-0059) mechanosynthesized during the activation step. This was probably due to the high moisture-sensitivity of the Na2 O, increased by the mechanical activation process. TG tracing acquired from a heating/cooling cycle of the mixture Na2 O/Nb2 O5 activated for 7 days displayed several weight-loss steps: the most important (about 10%) between room temperature and 200 ◦ C, and three less important at 200–415, 415–455 and 455–575 ◦ C. As it can be observed in Fig. 4a, the DTA curve shows the existence of one endothermic peak at 120 ◦ C and three exothermic peaks at 325, 430 and 460 ◦ C. Each one of these peaks is related with one of the weight-loss steps detected by TG. The mass spectrometer measurement (Fig. 4a) confirms that the endothermic peak is related with the elimination of H2 O molecules of the Na7 (H3 O)Nb6 O19 (H2 O)14 phase, while the exothermic peaks are related with the loss of the H2 O from the H3 O+ ion and the CO2 , incorporated during the milling from the ambient. These last eliminations are exothermic processes because they are associated to the simultaneous formation of NaNbO3 . This indicates that part of the Na2 O present in the initial mixture has reacted with the CO2 to form carbonates with very small particle size, and then non-detectable by XRD (Fig. 3a).
Fig. 4. DTA tracing and QMS measurements for masses corresponding to H2 O and CO2 , for the (a) Na2 O/Nb2 O5 and (b) 2NaOH/Nb2 O5 mixtures milled for 7 days in a vibrating mill.
XRD at increasing temperatures (Fig. 5a) displayed an important decrease in the intensity of the diffraction peaks of the Na7 (H3 O)Nb6 O19 (H2 O)14 phase between room temperature and 250 ◦ C. At temperatures above 410 ◦ C the formation of NaNbO3 was triggered, although a small quantity of Nb2 O5 was still observed. Moreover, traces of Na2 Nb4 O11 started to be visible at 600 ◦ C and remained stable at the final room temperature. In order to isolate NaNbO3 as single phase, in a single thermal treatment, the precursor milled for 7 days was heated in a
Fig. 5. XRD patterns at increasing temperatures of the (a) Na2 O/Nb2 O5 and (b) 2NaOH/Nb2 O5 mixtures activated during 7 days in a vibrating mill (X, Na2 Nb4 O11 ; N, Nb2 O5 ; Pt, platinum; RT, room temperature and FRT, final room temperature).
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furnace at different temperatures. At temperatures higher than 600 ◦ C, Nb2 O5 completely reacted, but the most important diffraction peaks of the Na2 Nb4 O11 phase became observable, together with those of NaNbO3 and remain stable even after thermal treatment at 900 ◦ C for 12 h. The appearance of this secondary phase suggests a defect of sodium in the activated mixture. This defect could be solved by adding an excess of ≈5% of Na2 O to the activated precursor before the annealing. Longer milling times did not improve the reactivity of the milled mixture, and it could be also observed the formation of the Na2 Nb4 O11 phase. This defect in the stoichometry, observed in all the cases, can be attributed to the high reactivity and very difficult handling of the sodium oxide. 3.3. NaOH reagent When NaOH and Nb2 O5 were used as starting materials, similar results to those above mentioned for the sodium oxide were obtained. Fig. 3b shows the XRD patterns of 2NaOH/Nb2 O5 mixture at increasing activation times in a vibrating mill. NaOH was not detected by XRD during the milling process. The only difference with respect to the Na2 O/Nb2 O5 mixture is the higher crystallinity of the mechanosynthesized Na7 (H3 O)Nb6 O19 (H2 O)14 phase. In this case, the presence of the OH groups in the activated mixture facilitated the mechanosynthesis of the Na–Nb hydrated phase. In the TG curve only two weight-losses were observed: about 10 wt.% between room temperature and 150 ◦ C and 4.5 wt.% between 150 and 660 ◦ C. The DTA recording (Fig. 4b) exhibits one endothermic peak at 105 ◦ C due to the elimination of H2 O and several very weak exothermic bands around 390, 605, 640 and 670 ◦ C, these three last bands are overlapped. Close to the temperatures of each of the exothermic bands, the mass spectrometer measurements show the loss of CO2 . The 2NaOH/Nb2 O5 milled precursor needs a higher treatment temperature to form the pure NaNbO3 phase as it is confirmed in the XRD at increasing temperatures (Fig. 5b). The diffraction peaks of the Na7 (H3 O)Nb6 O19 (H2 O)14 phase became less intense as the temperature increases. At 410 ◦ C a peak at 2Θ ≈ 22.5◦ was detected, which corresponds to the onset of the NaNbO3 formation and a process of crystal growth took place at higher temperatures. The XRD patterns at the final room temperature confirm the existence of a small quantity of Nb2 O5 and Na2 Nb4 O11 together with the NaNbO3 phase. 3.4. Mixed technique: wet-chemistry plus mechanochemical activation Wet-chemistry technique aims to a better mixing of the constituent elements, before the mechanical treatment and thus, a better reactivity during the activation step. NaOH was dissolved in deionised water and the stoichiometric quantity
Fig. 6. XRD recordings at room temperature, after cumulative thermal treatments up to 600 ◦ C, measurement of: (a) precursor DOH obtained from aqueous solution of the stoichiometric mixture 2NaOH/Nb2 O5 , (b) DOH activated in a vibrating mill for 7 days and (c) DOH milled in a planetary system for 2 days (NN, NaNbO3 ; N, Nb2 O5 ; X, Na2 Nb4 O11 and Pt, platinum).
of Nb2 O5 was added into the aqueous solution. This suspension was stirred for 4 h and then dried at 150 ◦ C overnight to obtain the precursor, hereinafter called DOH, for the mechanical activation process. The use of this wet-chemistry step improve the homogeneity of the mixture but not too much the reactivity, as it can be noticed in the XRD recording at room temperature after the cumulative treatments for 1 h each at 250, 410, 500 and 600 ◦ C (Fig. 6a). The evolution of the precursor DOH with the activation time in a vibrating mill was very similar to that mentioned in the case of the non-dissolved 2NaOH/Nb2 O5 mixture, but in the latter case the sample activated for 7 days has a lower crystallinity. As it can be observed in Fig. 6b, the activation step enhanced the reactivity of the DOH precursor, only a small quantity of Nb2 O5 is detected by XRD after the cumulative treatments up to 600 ◦ C. When the DOH precursor activated for 7 days was annealed at 900 ◦ C for 2 h, the NaNbO3 phase obtained is more crystalline than in the case of non-dissolved 2NaOH/Nb2 O5 milled mixture, as it is shown in Fig. 7a and b. To make a comparative study of the mechanical activation method, the DOH precursor was also activated in a planetary mill. The results obtained were similar to those mentioned in the case of the vibrating mill, although the time scale involved was shorter (Fig. 6c). This difference is attributed to the higher energy supplied by the planetary system. The DOH precursor activated using a planetary system is able of faster reaction and the NaNbO3 phase could be obtained in a single annealing step at 700 ◦ C from the powder milled for 2 days. Fig. 7c shows the increase of the crystallinity induced by the planetary system after the thermal treatment at 900 ◦ C for 2 h.
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Fig. 7. XRD patterns of NaNbO3 obtained from different activated samples annealed at 900 ◦ C for 2 h: (a) 2NaOH/Nb2 O5 milled for 7 days in a vibrating mill, (b) DOH precursor activated in a vibrating mill for 7 days and (c) DOH precursor milled in a planetary system for 2 days.
3.5. Ceramic characterization XRD of the ceramics show that all of them are single phase and isostructural with NaNbO3 (JCPDS-ICDD file no. 33-1270).
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The ceramics naturally sintered from Na2 CO3 based precursors show densities, which are lower than the green density [31]. An expansion of the green pellets during air sintering was observed. The origin of this was the formation of porosity while the weight-loss step corresponding to the decarboxylation of the precursor takes place, extending up to temperatures higher than 600 ◦ C. A way to avoid this generation of porosity, while keeping the advantage of ceramic processing by a one-step thermal treatment, is the hot-uniaxial pressing (HP). Fig. 8a shows the polished surface of hot-pressed ceramics at 1000 ◦ C for 2 h. Reasonably dense ceramics (porosity = 6.4%) were obtained. The polished surfaces of the ceramics naturally sintered at 1200 ◦ C for 2 h from Na2 O and DOH activated in a vibrating mill based precursors, Fig. 8b and c, respectively, show also reasonably low remnant porosity (4.5 and 3.0%, respectively). When the quantitative analysis of the porosity of these ceramics is carried out and the probability plots of the determined pore area distributions compared (Fig. 9), it is clearly observed that the air-sintered ceramics have pore sizes comparable to the previous hot-pressed ceramics. This is the result of the higher reactivity of the precursors based in this two reactants. Further improvement of the porosity is achieved when DOH precursors are hot-pressed, as the micrograph in Fig. 8d and the probability plot in Fig. 9 reveal.
Fig. 8. Optical micrographs of the polished surfaces of ceramics obtained from: (a) Nb2 O5 /NaCO3 mixture after 30 days of milling precursor and hot pressing at 1000 ◦ C for 2 h; (b) Nb2 O5 /Na2 O mixture activated in a vibrating mill for 7 days precursor and sintering at 1200 ◦ C for 2 h and (c) DOH precursors, also activated in a vibrating mill for 7 days, after sintering at 1200 ◦ C for 2 h and (d) the same precursor after hot pressing at 1100 ◦ C for 2 h.
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Fig. 9. Probability plots of the pore area distributions of ceramics obtained by hot pressing at 1000 ◦ C for 2 h and 1100 ◦ C for 2 h and sintering at 1200 ◦ C for 2 h from some of the ceramic precursor powders.
As a proof of the ceramic performance improvement gained by the use of the DOH based precursors, Fig. 10 shows the planar resonance spectra at room temperature for the hotpressed ceramics at 1000 ◦ C for 2 h from Na2 CO3 based precursors and for the hot-pressed ceramics at 1100 ◦ C for 2 h from DOH precursors. The insets in Fig. 10 of the resonance
spectra show in each case the complex values, real and imagE , and compliances, sE inary part, of the elastic stiffness, c11 11 E , dielectric, εT , and piezoelectric, d , coefficients, and s12 31 33 together with the planar electromechanical coupling factor, kp , and the frequency number, Np , and Poisson ratio. It is noticeable that all losses are low. The piezoelectric coefficient at room temperature of these ceramics was d33 = 40 and 32 × 10−12 C N−1 , respectively, and piezoelectric activity was detected up to 300 ◦ C. As insets in the graphs of the planar resonance spectra in Fig. 10, and the spectra themselves (the R and G peaks of the DOH ceramic are noticeably narrower) show, the mechanical characteristics of the hot-pressed DOH ceramics are better than the ones of the hot-pressed ceramics from Na2 CO3 . Namely, the frequency number, Np , and the elastic stiffness, E , increases from 3571 to 3778 kHz mm and from 12.76 to c11 15.70 × 1010 N m−2 , in accordance with the reduction of the percentage of porosity (to 2.1% in the DOH ceramic) and average pore area (to 0.4 m2 in the DOH ceramic) shown in Figs. 8 and 9, respectively. This improvement is obtained together with similar electromechanic characteristics for both ceramics and a slight increase in the relative dielectric permittivity, εT33 , is also gained, from 144.4 to 158.2. Thus, the hot-pressed DOH ceramics have better mechanical and dielectric properties and have comparable piezoelectric performance than hot-pressed ceramics from Na2 CO3 , while they are obtained by a much more efficient processing route.
4. Conclusions
Fig. 10. Fundamental planar mode spectra of ceramics obtained by hot pressing (a) at 1000 ◦ C for 2 h from Na2 CO3 based precursors and (b) at 1100 ◦ C for 2 h from DOH based precursors () conductance, G, experimental values; () resistance, R, experimental values; lines: reproduced spectra after the calculated coefficients shown, G: dashed line and R: dotted line.
The synthesis of NaNbO3 from mechanochemically activated precursors and processing of environmentally friendly piezoceramics in a one-step thermal treatment, in which synthesis, grain growth and sintering takes place, has been comparatively studied for different Na reagents. The use of Na2 CO3 leads to single phase NaNbO3 after treatment at 600 ◦ C but implies long mechanical activation times and leads to severe weight-loss due to decarboxilation, detrimental for ceramic preparation. For Na2 O and NaOH with all milling systems used, similar steps during both milling and annealing processes could be observed. In all cases Na7 (H3 O)Nb6 O19 (H2 O)14 transient phase was mechanosynthesised, which indicates a higher degree of chemical activation achieved in these mixtures. In both cases a remarkable reduction of the activation time to 7 days is also obtained. When Na2 O was used as starting material, Na2 Nb4 O11 was obtained as secondary phase, due to a loss in the stoichiometry but it could be solved by adding a small excess of Na2 O. However, the use of NaOH always leads to obtaining the pure phase NaNbO3 . The reactivity of the 2NaOH/Nb2 O5 mixture can be improved by using a solution technique before the mechanical process. Furthermore, the use of the planetary mill supplies more energy to the reagents and produces the mechanical activation in even shorter milling times (2 days).
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The results of the quantitative ceramic porosity characterization clearly show that remarkable enhancement of the sinterability of the ceramic can be obtained by proper choice of the starting reagents. The ceramic piezoelectric performance improvement gained by the use of the DOH based precursors was also proved.
Acknowledgments Dr. C. Pithan is gratefully acknowledged for the SEM and BET surface measurements. One of the authors, T. Hungr´ıa, is indebted to the MECD of Spain for the postgraduate FPU grant awarded (AP2000–3477). Thanks are also given to Ms. M. Ant´on for the powder samples preparation. This work was supported by EC Project “LEAF” G5RD-CT2001-00431 and MAT2001-4818E (MCyT Spain).
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Effect of mechanochemical activation on the synthesis of NaNbO3 and processing of environmentally friendly piezoceramics Teresa Hungr´ıa, Lorena Pardo, Alberto Moure, Alicia Castro∗ Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain Received 8 September 2004; accepted 12 October 2004 Available online 5 January 2005
Abstract High-energy milling has been applied to obtain NaNbO3 ceramic precursor powders and dense ceramics by a one-step thermal treatment. Stoichiometric mixtures of Nb2 O5 and different sodium reactants (Na2 CO3 , Na2 O and NaOH) were mechanically activated. The obtained results are discussed regarding the starting reactants and the milling media. The evolution of the powder mixtures and the thermal behavior of the activated samples were investigated by X-ray powder diffraction and thermal analysis. The most crystalline NaNbO3 powders were obtained when a 2NaOH/Nb2 O5 mixture was homogenized using a wet-chemistry technique followed by mechanochemical activation in a planetary mill and treated at 900 ◦ C for 2 h. Dense ceramics were processed from the different precursors by conventional-sintering and hot-pressing and their microstructure and piezoelectric properties characterized. © 2004 Elsevier B.V. All rights reserved. Keywords: Ferroelectrics; High-energy ball milling; X-ray diffraction; Dielectric response
1. Introduction Niobates are a technologically important class of inorganic materials because of their broad range of physical properties and applications. Alkaline niobate ceramics are considered a good alternative for the substitution of commercial piezoceramics by lead-free materials, based on highly toxic lead compounds. Sodium niobate (NaNbO3 ) is antiferroelectric at room temperature [1], but the application of an electric field or the substitution of Li [2] or K [3] for Na, induces a ferroelectric phase that provides piezoelectric activity that is interesting for high frequency devices. It is known that NaNbO3 presents a large number of structural phases, variants of lower symmetry of the ideal perovskite structure, having six phase transitions [4] from the non-polar high temperature phase to the antiferroelectric room temperature phase and the low temperature ferroelectric one. Sodium niobate is of particular interest because in this antiferroelectric phase are based some solid solutions [5–7] with good ferroelectric ∗
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and piezoelectric properties. Recent studies [6,7] are focused on the analysis, involving dielectric and thermal expansion measurements, of the phase transitions in the polar phase of ceramics of Na1−x Lix NbO3 compositions. Knowledge of such transitions is a key factor to assess the thermal stability of their piezoelectric properties. On the other hand, the electrical and optical properties of dielectric ceramics strongly depend on the chemical and microstructural homogeneity, which is mainly influenced by the powder synthesis method and sintering process. Alkali metal niobate powders are usually prepared via a solid-state reaction between Nb2 O5 and alkali metal carbonates. This classical method involves high temperatures and long reaction times, produces the volatilization of the alkali metal, leads to poor compositional homogeneity and provides products with large particles [2]. Particularly for piezoceramics the control of the porosity and pore size is a key issue, since the dielectric strength is dependent on them [8] and a high dielectric strength is required during the poling process. Alternative routes need to be developed to facilitate the production of desired materials in a controllable way. Powder synthesis using evaporation [9], sol–gel [10], hydrothermal
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[11] and polymeric precursor processes [12] have been described in the literature to facilitate the reactions. However, most of these chemistry-based routes require high purity inorganic or organometallic reactants, more expensive than the widely available oxides and carbonates. Mechanochemical activation is a way to modify the conditions in which chemical reactions usually take place. During the mechanical treatment the particle size of the crystals is reduced, the homogeneity of the mixture is increased and, in most of the cases, the solid becomes more reactive [13]. Mechanical activation is usually a result of disorder of the crystal and generation of defects that cause the decrease of activation barrier for the reaction [14]. Solid-state mechanochemistry is a powerful technique for synthesizing many kinds of materials from metallic [15] to amorphous and nanocrystalline materials [16], as well as to prepare new oxides or to improve the properties of known functional materials [17–26]. This paper describes a comparative study of routes based on high energy milling to improve the reaction conditions to synthesize NaNbO3 and to allow the processing of dense piezoceramics in a single thermal treatment in which synthesis, grain growth and sintering takes place. Different sodium reagents and milling systems were used and results compared.
2. Experimental procedure Mechanical activation techniques have been applied to the preparation of NaNbO3 ceramic powders. Stoichiometric mixtures of analytical-grade Nb2 O5 and sodium reactants (Na2 CO3 , Na2 O or NaOH) were mixed by hand in an agate mortar or also in water media with the NaOH reactants. Then, the initial mixtures were mechanically activated using vibrating or planetary mills (Fristch, pulverisette models 0 and 6, respectively). In both cases, the mixture was placed in an agate vessel, using one ball of 5 cm diameter for the vibrating mill, whereas for the planetary mill five balls of 2 cm diameter each were used. The milling bowl of the planetary system was rotated at 200 rpm. For the sake of comparison, NaNbO3 powder was also prepared by solid-state reaction from Nb2 O5 and Na2 CO3 , by successive thermal treatments at increasing temperatures from 400 to 800 ◦ C, during 12 h each, followed by cooling by quenching in air. The effect of both milling and thermal treatments in the particle morphology and surface area were investigated by standard scanning electron microscopy (SEM) and BET surface area analysis. The milled powders were characterized by differential thermal analysis (DTA), thermogravimetric analysis (TG) and X-ray powder diffraction (XRD) at room and increasing temperatures. The DTA and TG measurements were taken with a Seiko 320 instrument, with ␣-Al2 O3 as the inert reference material,
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between room temperature and 800 ◦ C at 10 ◦ C min−1 , in the heating and cooling process, in argon. The evolved gases were analyzed with a Pfeiffer ThermoStar GSD 301T (quadrupole mass spectrometer), with argon as gas carrier to determine the molar mass. XRD patterns at room temperature were measured in a Bruker AXS D8 advance diffractometer, between 5 and 80◦ (2Θ), with 2Θ increments of 0.05◦ and counting time of 4 s per step. For high-temperature measurements, a Philips PW1310 diffractometer, coupled with an Anton Paar HTK 10 attachment to stabilize the temperature, was used. The patterns were obtained by depositing a small quantity of powder onto a platinum sheet placed on a tantalum strip, which was the heating element. The recordings were taken from 5 to 70◦ (2θ) with a scan rate of 0.02◦ s−1 . The temperature was monitored by a Pt–Pt 13% Rh thermocouple welded in the center of the platinum sheet. The heating rate was 10 ◦ C min−1 and the temperature was stabilized during ˚ was used in all X-ray 1 h. The Cu K␣ doublet (λ = 1.5418 A) experiments. Powders were shaped by uniaxial pressing at 210 kg cm−2 as thin disks of ∼10 mm diameter and 2 mm thickness. Disks were isostatically pressed at 2000 kg cm−2 and then conventional-sintering was carried out on a Pt foil at 1200 ◦ C for 2 h. Also, disks were hot-uniaxially pressed in alumina dies and surrounded by alumina powder, at temperatures of 1000 and 1100 ◦ C and ∼200 kg/cm−2 . Ceramic surfaces were polished and analyzed by optical microscopy (Leitz Laborlux 12 ME S/ST), in order to examine the ceramic porosity. Quantitative characterization of the porosity, i.e., determination of the pore area distributions on statistical samples of more than 750 pores, was carried out on the optical micrographs by a computerized image analysis and measurement system (IMCO10-KAT386 system, Kontron Elektronic GMBH, 1990) by a procedure explained in detail elsewhere [27]. The percentage of porosity was characterized by the fraction of the examined area which is occupied by pores. The average A and standard deviation, σ, of the distributions of pore area were obtained by linear fitting of the experimental data represented in probability plots [28]. Ceramic disks were lapped to a ratio thickness/diameter of 1/10, typically 8 mm diameter and 0.8 mm thickness, and electrodes were painted on the major faces, so as to prepare the capacitor samples for the electrical measurements. The samples were poled in a silicon oil bath at 50 kV cm−1 , for 20 min at 180 ◦ C. The piezoelectric d33 coefficient was measured in a Berlincourt-meter by the direct piezoelectric effect at 100 Hz. Dielectric, elastic and piezoelectric constants, real and imaginary parts, thus accounting for their respective losses [29] and electromechanical coupling factors, corresponding to the radial extensional vibration modes of the thin disk shaped ceramic resonators, were calculated. An automatic iterative method, described elsewhere [30], of calculation of these parameters from complex impedance measurements at the appropriated frequencies was used.
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Fig. 1. SEM images of the Na2 CO3 /Nb2 O5 mixture after different milling times: (a) 0 days, (b) 10 days, (c) 20 days and (d) 30 days.
3. Results and discussion 3.1. Na2 CO3 reagent A study concerning the synthesis of NaNbO3 was previously carried out by the authors, using a mechanical activation route, from a stoichiometric Na2 CO3 /Nb2 O5 mixture and a vibrating-type mill [31]. During the activation process, even after long time of milling, only a significant broadening in the diffraction peaks of the Nb2 O5 phase took place, while the Na2 CO3 became amorphous. NaNbO3 was obtained as single phase, with quite small particle size, after thermal treatment at 600 ◦ C, from powder activated during 30 days, but not for shorter milling time. Nb2 O5 /Na2 CO3 mixture was investigated by SEM, after both milling and annealing treatments. As the milling time increased, particle size decreased up to 100 nm and agglomerates of fine grains with irregular shapes can be observed together with bigger particles corresponding to the remaining Nb2 O5 observed by XRD (Fig. 1). Table 1 shows an important increase of the BET surface area value of the raw materials as a result of the activation process.
Table 1 BET surface area measurements of the Na2 CO3 /Nb2 O5 mixture after different treatments (m2 /g) Annealing temperature (◦ C) 25 600 800
Milling times (days) 0 10 20
30
0.46 – 1.31
7.27 5.74 –
2.90 – –
4.64 3.39a –
a A small quantity of Na Nb O was obtained together with the NaNbO 2 4 11 3 phase.
The properties of the powder NaNbO3 sample strongly depend on the synthesis method. As can be observed in Fig. 2, NaNbO3 prepared by classical solid-state method is constituted by large crystals, while NaNbO3 obtained by mechanical activation assisted route is composed of agglomerates with irregular shapes and formed by the coalescence of grains hundreds of nanometers in size. Moreover, the surface area of the NaNbO3 synthesized by the milling route is significantly higher (Table 1). When Na2 CO3 was used, long milling times were needed; therefore, other reactants need to be tested mainly in order to
Fig. 2. Scanning electron micrographs of: (a) NaNbO3 powder obtained by solid-state classic method at 750 ◦ C and (b) NaNbO3 phase prepared by annealing at 600 ◦ C after 30 days of milling (Na2 CO3 /Nb2 O5 mixture).
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Fig. 3. XRD patterns of the (a) Na2 O/Nb2 O5 and (b) 2NaOH/Nb2 O5 mixtures after different milling times in a vibrating mill. For the sake of comparison the Nb2 O5 XRD pattern is also plotted.
decrease the mechanical activation time, while effects on the ceramic sintering and final density and piezoelectric performance were also examined. 3.2. Na2 O reagent Fig. 3a shows the XRD patterns of the Na2 O/Nb2 O5 mixture activated in the vibrating-type mill. After 1 day of milling, the only detectable phase is Nb2 O5 . After 4 days of milling, some broad diffraction peaks appeared at 2Θ between 10 and 13◦ . Longer milling times produced an increase of the intensity of these peaks together with a broadening of the Nb2 O5 diffraction lines, showing that a reduction of the crystallite size of this phase took place. The diffraction peaks appearing at low 2Θ angles were attributed to the compound Na7 (H3 O)Nb6 O19 (H2 O)14 (JCPDS-ICDD file no. 77-0059) mechanosynthesized during the activation step. This was probably due to the high moisture-sensitivity of the Na2 O, increased by the mechanical activation process. TG tracing acquired from a heating/cooling cycle of the mixture Na2 O/Nb2 O5 activated for 7 days displayed several weight-loss steps: the most important (about 10%) between room temperature and 200 ◦ C, and three less important at 200–415, 415–455 and 455–575 ◦ C. As it can be observed in Fig. 4a, the DTA curve shows the existence of one endothermic peak at 120 ◦ C and three exothermic peaks at 325, 430 and 460 ◦ C. Each one of these peaks is related with one of the weight-loss steps detected by TG. The mass spectrometer measurement (Fig. 4a) confirms that the endothermic peak is related with the elimination of H2 O molecules of the Na7 (H3 O)Nb6 O19 (H2 O)14 phase, while the exothermic peaks are related with the loss of the H2 O from the H3 O+ ion and the CO2 , incorporated during the milling from the ambient. These last eliminations are exothermic processes because they are associated to the simultaneous formation of NaNbO3 . This indicates that part of the Na2 O present in the initial mixture has reacted with the CO2 to form carbonates with very small particle size, and then non-detectable by XRD (Fig. 3a).
Fig. 4. DTA tracing and QMS measurements for masses corresponding to H2 O and CO2 , for the (a) Na2 O/Nb2 O5 and (b) 2NaOH/Nb2 O5 mixtures milled for 7 days in a vibrating mill.
XRD at increasing temperatures (Fig. 5a) displayed an important decrease in the intensity of the diffraction peaks of the Na7 (H3 O)Nb6 O19 (H2 O)14 phase between room temperature and 250 ◦ C. At temperatures above 410 ◦ C the formation of NaNbO3 was triggered, although a small quantity of Nb2 O5 was still observed. Moreover, traces of Na2 Nb4 O11 started to be visible at 600 ◦ C and remained stable at the final room temperature. In order to isolate NaNbO3 as single phase, in a single thermal treatment, the precursor milled for 7 days was heated in a
Fig. 5. XRD patterns at increasing temperatures of the (a) Na2 O/Nb2 O5 and (b) 2NaOH/Nb2 O5 mixtures activated during 7 days in a vibrating mill (X, Na2 Nb4 O11 ; N, Nb2 O5 ; Pt, platinum; RT, room temperature and FRT, final room temperature).
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furnace at different temperatures. At temperatures higher than 600 ◦ C, Nb2 O5 completely reacted, but the most important diffraction peaks of the Na2 Nb4 O11 phase became observable, together with those of NaNbO3 and remain stable even after thermal treatment at 900 ◦ C for 12 h. The appearance of this secondary phase suggests a defect of sodium in the activated mixture. This defect could be solved by adding an excess of ≈5% of Na2 O to the activated precursor before the annealing. Longer milling times did not improve the reactivity of the milled mixture, and it could be also observed the formation of the Na2 Nb4 O11 phase. This defect in the stoichometry, observed in all the cases, can be attributed to the high reactivity and very difficult handling of the sodium oxide. 3.3. NaOH reagent When NaOH and Nb2 O5 were used as starting materials, similar results to those above mentioned for the sodium oxide were obtained. Fig. 3b shows the XRD patterns of 2NaOH/Nb2 O5 mixture at increasing activation times in a vibrating mill. NaOH was not detected by XRD during the milling process. The only difference with respect to the Na2 O/Nb2 O5 mixture is the higher crystallinity of the mechanosynthesized Na7 (H3 O)Nb6 O19 (H2 O)14 phase. In this case, the presence of the OH groups in the activated mixture facilitated the mechanosynthesis of the Na–Nb hydrated phase. In the TG curve only two weight-losses were observed: about 10 wt.% between room temperature and 150 ◦ C and 4.5 wt.% between 150 and 660 ◦ C. The DTA recording (Fig. 4b) exhibits one endothermic peak at 105 ◦ C due to the elimination of H2 O and several very weak exothermic bands around 390, 605, 640 and 670 ◦ C, these three last bands are overlapped. Close to the temperatures of each of the exothermic bands, the mass spectrometer measurements show the loss of CO2 . The 2NaOH/Nb2 O5 milled precursor needs a higher treatment temperature to form the pure NaNbO3 phase as it is confirmed in the XRD at increasing temperatures (Fig. 5b). The diffraction peaks of the Na7 (H3 O)Nb6 O19 (H2 O)14 phase became less intense as the temperature increases. At 410 ◦ C a peak at 2Θ ≈ 22.5◦ was detected, which corresponds to the onset of the NaNbO3 formation and a process of crystal growth took place at higher temperatures. The XRD patterns at the final room temperature confirm the existence of a small quantity of Nb2 O5 and Na2 Nb4 O11 together with the NaNbO3 phase. 3.4. Mixed technique: wet-chemistry plus mechanochemical activation Wet-chemistry technique aims to a better mixing of the constituent elements, before the mechanical treatment and thus, a better reactivity during the activation step. NaOH was dissolved in deionised water and the stoichiometric quantity
Fig. 6. XRD recordings at room temperature, after cumulative thermal treatments up to 600 ◦ C, measurement of: (a) precursor DOH obtained from aqueous solution of the stoichiometric mixture 2NaOH/Nb2 O5 , (b) DOH activated in a vibrating mill for 7 days and (c) DOH milled in a planetary system for 2 days (NN, NaNbO3 ; N, Nb2 O5 ; X, Na2 Nb4 O11 and Pt, platinum).
of Nb2 O5 was added into the aqueous solution. This suspension was stirred for 4 h and then dried at 150 ◦ C overnight to obtain the precursor, hereinafter called DOH, for the mechanical activation process. The use of this wet-chemistry step improve the homogeneity of the mixture but not too much the reactivity, as it can be noticed in the XRD recording at room temperature after the cumulative treatments for 1 h each at 250, 410, 500 and 600 ◦ C (Fig. 6a). The evolution of the precursor DOH with the activation time in a vibrating mill was very similar to that mentioned in the case of the non-dissolved 2NaOH/Nb2 O5 mixture, but in the latter case the sample activated for 7 days has a lower crystallinity. As it can be observed in Fig. 6b, the activation step enhanced the reactivity of the DOH precursor, only a small quantity of Nb2 O5 is detected by XRD after the cumulative treatments up to 600 ◦ C. When the DOH precursor activated for 7 days was annealed at 900 ◦ C for 2 h, the NaNbO3 phase obtained is more crystalline than in the case of non-dissolved 2NaOH/Nb2 O5 milled mixture, as it is shown in Fig. 7a and b. To make a comparative study of the mechanical activation method, the DOH precursor was also activated in a planetary mill. The results obtained were similar to those mentioned in the case of the vibrating mill, although the time scale involved was shorter (Fig. 6c). This difference is attributed to the higher energy supplied by the planetary system. The DOH precursor activated using a planetary system is able of faster reaction and the NaNbO3 phase could be obtained in a single annealing step at 700 ◦ C from the powder milled for 2 days. Fig. 7c shows the increase of the crystallinity induced by the planetary system after the thermal treatment at 900 ◦ C for 2 h.
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Fig. 7. XRD patterns of NaNbO3 obtained from different activated samples annealed at 900 ◦ C for 2 h: (a) 2NaOH/Nb2 O5 milled for 7 days in a vibrating mill, (b) DOH precursor activated in a vibrating mill for 7 days and (c) DOH precursor milled in a planetary system for 2 days.
3.5. Ceramic characterization XRD of the ceramics show that all of them are single phase and isostructural with NaNbO3 (JCPDS-ICDD file no. 33-1270).
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The ceramics naturally sintered from Na2 CO3 based precursors show densities, which are lower than the green density [31]. An expansion of the green pellets during air sintering was observed. The origin of this was the formation of porosity while the weight-loss step corresponding to the decarboxylation of the precursor takes place, extending up to temperatures higher than 600 ◦ C. A way to avoid this generation of porosity, while keeping the advantage of ceramic processing by a one-step thermal treatment, is the hot-uniaxial pressing (HP). Fig. 8a shows the polished surface of hot-pressed ceramics at 1000 ◦ C for 2 h. Reasonably dense ceramics (porosity = 6.4%) were obtained. The polished surfaces of the ceramics naturally sintered at 1200 ◦ C for 2 h from Na2 O and DOH activated in a vibrating mill based precursors, Fig. 8b and c, respectively, show also reasonably low remnant porosity (4.5 and 3.0%, respectively). When the quantitative analysis of the porosity of these ceramics is carried out and the probability plots of the determined pore area distributions compared (Fig. 9), it is clearly observed that the air-sintered ceramics have pore sizes comparable to the previous hot-pressed ceramics. This is the result of the higher reactivity of the precursors based in this two reactants. Further improvement of the porosity is achieved when DOH precursors are hot-pressed, as the micrograph in Fig. 8d and the probability plot in Fig. 9 reveal.
Fig. 8. Optical micrographs of the polished surfaces of ceramics obtained from: (a) Nb2 O5 /NaCO3 mixture after 30 days of milling precursor and hot pressing at 1000 ◦ C for 2 h; (b) Nb2 O5 /Na2 O mixture activated in a vibrating mill for 7 days precursor and sintering at 1200 ◦ C for 2 h and (c) DOH precursors, also activated in a vibrating mill for 7 days, after sintering at 1200 ◦ C for 2 h and (d) the same precursor after hot pressing at 1100 ◦ C for 2 h.
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Fig. 9. Probability plots of the pore area distributions of ceramics obtained by hot pressing at 1000 ◦ C for 2 h and 1100 ◦ C for 2 h and sintering at 1200 ◦ C for 2 h from some of the ceramic precursor powders.
As a proof of the ceramic performance improvement gained by the use of the DOH based precursors, Fig. 10 shows the planar resonance spectra at room temperature for the hotpressed ceramics at 1000 ◦ C for 2 h from Na2 CO3 based precursors and for the hot-pressed ceramics at 1100 ◦ C for 2 h from DOH precursors. The insets in Fig. 10 of the resonance
spectra show in each case the complex values, real and imagE , and compliances, sE inary part, of the elastic stiffness, c11 11 E , dielectric, εT , and piezoelectric, d , coefficients, and s12 31 33 together with the planar electromechanical coupling factor, kp , and the frequency number, Np , and Poisson ratio. It is noticeable that all losses are low. The piezoelectric coefficient at room temperature of these ceramics was d33 = 40 and 32 × 10−12 C N−1 , respectively, and piezoelectric activity was detected up to 300 ◦ C. As insets in the graphs of the planar resonance spectra in Fig. 10, and the spectra themselves (the R and G peaks of the DOH ceramic are noticeably narrower) show, the mechanical characteristics of the hot-pressed DOH ceramics are better than the ones of the hot-pressed ceramics from Na2 CO3 . Namely, the frequency number, Np , and the elastic stiffness, E , increases from 3571 to 3778 kHz mm and from 12.76 to c11 15.70 × 1010 N m−2 , in accordance with the reduction of the percentage of porosity (to 2.1% in the DOH ceramic) and average pore area (to 0.4 m2 in the DOH ceramic) shown in Figs. 8 and 9, respectively. This improvement is obtained together with similar electromechanic characteristics for both ceramics and a slight increase in the relative dielectric permittivity, εT33 , is also gained, from 144.4 to 158.2. Thus, the hot-pressed DOH ceramics have better mechanical and dielectric properties and have comparable piezoelectric performance than hot-pressed ceramics from Na2 CO3 , while they are obtained by a much more efficient processing route.
4. Conclusions
Fig. 10. Fundamental planar mode spectra of ceramics obtained by hot pressing (a) at 1000 ◦ C for 2 h from Na2 CO3 based precursors and (b) at 1100 ◦ C for 2 h from DOH based precursors () conductance, G, experimental values; () resistance, R, experimental values; lines: reproduced spectra after the calculated coefficients shown, G: dashed line and R: dotted line.
The synthesis of NaNbO3 from mechanochemically activated precursors and processing of environmentally friendly piezoceramics in a one-step thermal treatment, in which synthesis, grain growth and sintering takes place, has been comparatively studied for different Na reagents. The use of Na2 CO3 leads to single phase NaNbO3 after treatment at 600 ◦ C but implies long mechanical activation times and leads to severe weight-loss due to decarboxilation, detrimental for ceramic preparation. For Na2 O and NaOH with all milling systems used, similar steps during both milling and annealing processes could be observed. In all cases Na7 (H3 O)Nb6 O19 (H2 O)14 transient phase was mechanosynthesised, which indicates a higher degree of chemical activation achieved in these mixtures. In both cases a remarkable reduction of the activation time to 7 days is also obtained. When Na2 O was used as starting material, Na2 Nb4 O11 was obtained as secondary phase, due to a loss in the stoichiometry but it could be solved by adding a small excess of Na2 O. However, the use of NaOH always leads to obtaining the pure phase NaNbO3 . The reactivity of the 2NaOH/Nb2 O5 mixture can be improved by using a solution technique before the mechanical process. Furthermore, the use of the planetary mill supplies more energy to the reagents and produces the mechanical activation in even shorter milling times (2 days).
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The results of the quantitative ceramic porosity characterization clearly show that remarkable enhancement of the sinterability of the ceramic can be obtained by proper choice of the starting reagents. The ceramic piezoelectric performance improvement gained by the use of the DOH based precursors was also proved.
Acknowledgments Dr. C. Pithan is gratefully acknowledged for the SEM and BET surface measurements. One of the authors, T. Hungr´ıa, is indebted to the MECD of Spain for the postgraduate FPU grant awarded (AP2000–3477). Thanks are also given to Ms. M. Ant´on for the powder samples preparation. This work was supported by EC Project “LEAF” G5RD-CT2001-00431 and MAT2001-4818E (MCyT Spain).
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