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Preparation of nanocrystalline lead zirconate powder by homogeneous precipitation using hydrogen peroxide and urea

Preparation of nanocrystalline lead zirconate powder by homogeneous precipitation using hydrogen peroxide and urea

Materials Letters 57 (2003) 2472 – 2479 www.elsevier.com/locate/matlet Preparation of nanocrystalline lead zirconate powder by homogeneous precipitat...

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Materials Letters 57 (2003) 2472 – 2479 www.elsevier.com/locate/matlet

Preparation of nanocrystalline lead zirconate powder by homogeneous precipitation using hydrogen peroxide and urea Taegyung Ko *, Deok-Ki Hwang Department of Ceramic Engineering, Institute of Advanced Materials, Inha Unversity, Inchon 402-751, South Korea Received 8 September 2002; accepted 10 September 2002

Abstract Nano-sized lead zirconate (PZ) powder has been prepared with a modified urea-based homogeneous precipitation process. In this route, hydrogen peroxide was used prior to the addition of urea to allow the formation of peroxo complexes in the mixed solution of lead nitrate and zirconyl chloride. The use of hydrogen peroxide effectively suppressed to the formation of crystalline Pb-carbonate phases during the precipitation. The obtained precipitate contained only a small amount of poorly crystallized cerussite. The as-dried precipitate converted to the crystalline lead zirconate phase, when calcined at 600 jC. The crystalline grains appeared in a spherical shape and uniform size of about 20 nm. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Lead zirconate; Urea; Hydrogen peroxide; Homogeneous precipitation; Nanomaterials; Piezoelectric

1. Introduction Lead zirconate (PZ) is an antiferroelectric material that can show phase switching from the antiferroelectric to ferroelectric phase by applying electric field or raising temperature [1]. In recent years, some interest on the preparation of nano-sized PZ powder appeared to be revived with regard to potential applications of its phase transformation for electromechanical actuators [2] or the use of its ferroelectric phase for pyroelectric sensors [3]. Conventionally, PZ powder was prepared by the mixed oxide process associated with calcination at higher temperatures than 1200 jC [4,5]. In this process, supplementary PbO addition was required to suppress PbO volatilization. * Corresponding author. Tel.: +82-32-860-7526; fax: +82-32874-3382. E-mail address: [email protected] (T. Ko).

As a result, wet chemical routes were considered as alternatives to avoid such a loss of PbO by producing PZ powder in nano size. Size reduction to a nanometer level can make precursor grains highly reactive, which enables the PZ phase form below the vaporization temperature of PbO. Among the chemical routes, the homogeneous precipitation commonly uses urea as a precipitant. Urea thermally decomposes to produce carbon dioxide and ammonia in aqueous solution, resulting in continuous pH increase. Cation species precipitate successively depending on the pH of solution containing multi-components of dissimilar solubility in aqueous solution. In these systems, precipitates likely have a core-shell structure [6]. With the urea route, Oren et al. [7] reported that pure PZ powder was successfully prepared following calcination at 700 jC for 6 h. In their result, the X-ray diffraction (XRD) pattern of the as-dried precipitate showed that crys-

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01296-X

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talline Pb-containing carbonates exist substantially. This indicates that strong chelation of Pb-species with carbonate ions occurred in Pb-precipitation. Presumably, the Pb-carbonates might result in raising both temperature and time required for the formation of crystalline PZ phase during calcination. On the other hand, Camargo et al. [8] reported that a reactive amorphous Pb – Zr precipitate was obtained directly from ammonia-based coprecipitation by adding H2O2 in Pb- and Zr- containing aqueous solution. They showed that a zirconyl-peroxo complex formed by adding hydrogen peroxide, resulting in that the Pb –Zr precipitate was free of undesirable anions such as Cl or NO3 ions contained in Pb and Zr sources. Therefore, if H2O2 is employed for the urea process, it may act as a prohibitor for the chelation of CO3 2 anion ligands with Pb-aquo species and suppress the formation of Pb-containing carbonates. In this study, a urea process with the use of H2O2 is presented for the preparation of nanocrystalline PZ powders at a lower calcination temperature.

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2. Experimental Starting reagents were Pb(NO3)2 (99+%, Aldrich) and ZrOCl2?8H2O (98%, Aldrich). The procedure of preparing PZ powders is shown in Fig. 1. An equimolar aqueous solution of Pb(NO3)2 and ZrOCl2 was prepared with a concentration of 0.032 mol dm 3, followed by the addition of H2O2 (30%, Ducksan). The solution was heated up to 95 jC with stirring. After that, urea (99%, Aldrich) was rapidly added to the solution. The molar ratio of solute/urea was 1:32.5. The reaction solution was refluxed with vigorous stirring at 95 jC during the precipitation. The temperature and the pH of the solution were being recorded every 5 s with a pH meter (125 PD, ISTEC) interfaced to PC. The precipitation was considered to be completed, when the variation of pH against time became flat. The precipitate was filtered and washed four times with distilled/deionized water. The washed precipitate was dried at 102 jC overnight and calcined in air with a heating rate of 3 jC min 1.

Fig. 1. Flow chart of the preparation of PbZrO3 powder by homogeneous precipitation.

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Thermal analysis for the as-dried precipitate was carried out using a TG – DTA analyzer (TA Instruments, SDT 2960) in air between 30 and 700 jC with a heating rate of 10 jC min 1. XRD patterns of the dried and calcined powders at room temperature were obtained with a powder X-ray diffractometer (Rigaku, D/MAX-2500) equipped with a Cu-rotating anode. The morphology of the powders was characterized with a magnification of 100,000 , using a field emission scanning electron microscopy (Hitachi, S4300). For IR measurements, pressed pellets were prepared from blending powers with KBr (Aldrich, FT-IR grade). IR spectra were measured at room temperature using a FTIR spectrometer (Bio-Rad, FTS 165) in transmission mode.

3. Results and discussion Fig. 2 presents the change of pH and temperature during the precipitation. It was observed that break in pH curve was associated with decrease in temperature. For comparison, two pH curves are presented in Fig. 1 to show the effect of H2O2 on the pH variations. One is for the solution containing H2O2 (A). The other is

for the solution free of H2O2 (B). For the solution A, precipitation was associated with sharp break in both pH and temperature curves, compared to that of the solution B. In the both solutions, Zr-precipitation began with the evolution of gas, which ceased when Pb-precipitation started. For a solution similar to the solution B, Oren et al. [7] reported the presence of both cerussite (PbCO3) and hydrocerussite (Pb3(CO3)2(OH)2) in precipitate. We confirmed that these minerals were also present in the precipitate of the solution B by XRD. The Zr- and Pb-precipitation occurred at the lower pH values in the solution A, compared to those for the solution B. This indicates that H2O2 causes Zr and Pb cations to be coordinated in part with peroxide ions in aqueous solution. Besides, the solution A effervesced more vigorously, compared to the solution B. H2O2 appeared to increase the oxidation potential of the solution, which facilitated decomposition of urea. A steep increase of pH was correspondingly observed in the pH curve of the solution A. The XRD pattern of the as-dried precipitate exhibited two weak broad humps centered around 28j and 51j of 2H, along with weak peaks belonging to cerussite (PbCO3). The similar humps were also

Fig. 2. The change of pH and temperature in Pb- and Zr-precipitation with and without hydrogen peroxide.

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reported in a dried precipitate prepared for amorphous zirconia, indicating the presence of short-range structures of Zr – O [9]. The peaks of cerussite indicated that this phase was poorly crystallized. Fig. 3 shows the phase development of the as-dried powder during calcination for 30 min at temperatures ranging from 200 to 700 jC. When the dried precipitate was subjected to thermal treatment at 200 jC, a Pb2OCO3 phase, a tetragonal zirconia (t-ZrO2) and PZ appeared. A phase of Pb2OCO3 is identified as an intermediate product of the decomposition of cerussite [10]. This phase can further decompose into a PbO phase (litharge) as the temperature is raised [10]. The PbO phase was observed in the powder calcined at 450 jC. The t-ZrO2 phase transformed to a monoclinic ZrO2 (m-ZrO2) phase in the powder calcined at 300 jC. It was noted that a PZ phase could form in the calcination even at 200 jC. A greater amount of PZ phase was obtained in the powder calcined at 550 jC. The XRD patterns of the powders calcined at 600 and 700 jC agree well with that of orthorhombic PZ [11]. The formation temperature of the PZ phase is 100 jC

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lower than the one reported for the powder preparation of the PZ phase [7] from a urea-based precipitation. Furthermore, our result could be comparable to that of a microemulsion process [12], in which a crystalline PZ phase was obtained from calcination at 600 jC for 1 h. Fig. 4 presents the TG – DTA curves of the dried powders from the precipitation with H2O2 (the solution A) and without H2O2 (the solution B) for comparison. In the both cases, a broad endothermic peak appeared at f 90 jC, corresponding to the decomposition of physically absorbed H2O. The dried powder obtained from the solution A showed a steady weight loss in TG curve without being associated with a recognizable peak in DTA curve as temperature increased. In contrast, stepwise weight loss appeared in the TG curve of the precipitate of the solution B. A shoulder at 170 jC in the TG curve is assigned to the decomposition of NH4 + ion [10,13]. Peaks observed in the temperature range from 200 to 400 jC are attributed to the decomposition reactions of cerussite and hydrocerussite [7].

Fig. 3. XRD patterns of the PbZrO3 precursor powders calcined at various temperatures for 30 min.

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Fig. 4. TG – DTA curves of the PbZrO3 precursor powder prepared with and without hydrogen peroxide.

Fig. 5. IR spectra of the as-dried PbZrO3 powders calcined at various temperatures.

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Fig. 6. SEM images of (a) the as-dried and the calcined powders at (b) 200 jC, (c) 300 jC, (d) 450 jC, (e) 550 jC, (f) 600 jC, (g) 700 jC for 30 min and (h) 600 jC for 40 min.

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In both precipitates, an exothermic peak above 500 jC was recognized, which was not associated with weight loss in TG. This could correspond to the formation of the PZ phase from the reaction between litharge and m-ZrO2. In the precipitate from the solution A, the peak temperature was 525 jC, while that of the precipitate from the solution B was 592 jC. This result also confirms that the addition of H2O2 can lead to lowering the formation temperature of the PZ phase. Above f 450 jC, no further weight loss was observed. Fig. 5 exhibits IR spectra for the precursor powder and the calcined powders obtained at various temperatures ranging from 200 and 700 jC for 30 min. A broad absorption between 3409 and 3229 cm 1 in the IR spectra of the as-dried powder can be attributed to the vibrations of absorbed water and NH4 + ion [13,14]. Its intensity decreased as the calcination temperature was increased. A peak at 1487 cm 1 observed in the as-dried powder is assigned to an absorption due to NH4 + ion. This peak disappeared in the powder calcined at 200 jC. A shoulder at 915 cm 1 belongs to a band of the PZ phase [15]. The XRD showed that the PZ phase began to appear in the powder calcined at 200 jC. This indicates that some degree of bonding of Pb –O – Zr could form during the precipitation.

IR bands at 839, 1053, 1083, 1335 and 1540 cm 1 correspond to the vibrations of coordinated carbonate ions, which are coordinated with metal ions in a molecular complex [14]. These peaks disappeared when the calcination temperature reached to 600 jC. In contrast, two strong absorptions at 839, 1053 and 1407 cm 1 are assigned to the bands of structural carbonate ions [14]. The IR peaks of the structural carbonate ions existed in the powder calcined at up to 600 jC and disappeared at a higher calcination temperature such as 700 jC. A peak at 1638 cm 1 is attributed to a vibration of structural water [14], which was not observed in the powder calcined at 700 jC. From the IR study, it is considered that the elimination of hydroxyl and carbonate ions may require calcination at the higher temperatures than 600 jC. SEM micrographs are shown in Fig. 6. Grains of the as-dried and calcined powders were more or less spherical and uniformly sized. Fig. 7 presents the variation of average grain size as a function of temperature for the powders calcined for a constant dwelling time of 30 min. The average grain size was calculated using the line intercept method. The average grain size of the as-dried powder is about 16 nm. Grain shrinkage was observed in the powders calcined at below 450 jC, while the grain size increased above

Fig. 7. Variation of grain size depending on calcination temperature in the powders calcined for 30 min.

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450 jC. The grains occurred in agglomerated cluster. The average grain size of the powder calcined at 600 jC was about 20 nm, which consisted of a few primary particles of f 6 nm. For calcination at 700 jC, severe neck formation occurred among grains and resulted in apparent decrease of the grain size. In particular, the powder calcined at 600 jC for 40 min appeared in a massive texture, indicating that it can be highly sinterable.

4. Conclusions In the preparation of PZ powder using the ureabased homogeneous precipitation, the addition of H2O2 resulted in a higher rate of hydrolysis of urea and precipitation at lower pH values for Pb2 + and Zr4 + species. It is believed that in the presence of H2O2, a zirconyl amorphous peroxo complex and cerussite in a poorly crystalline state precipitated for Zr2 + and Pb2 + species, respectively. XRD analysis shows that the as-dried precipitate converted to a crystalline lead zirconate at 600 jC only for 30 min, indicating that the temperature and the time required for the formation of the PZ phase were significantly reduced. From XRD and IR analysis, it is evident that some amount of the PZ phase began to form at 200 jC, indicating that the Zr-peroxo complex can allow oxo-bonding between Pb2 + and Zr4 + ions during the precipitation. This PZ phase formed at the earlier stage likely acts as a nucleus for the formation of the PZ phase at higher temperatures. The powder

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calcined at 600 jC consisted of nano-sized grains and was highly reactive, presenting that the process developed in this study was simple and easily applicable for preparing nanocrystalline PZ powder.

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