J Sol-Gel Sci Technol (2009) 49:228–232 DOI 10.1007/s10971-008-1848-3
ORIGINAL PAPER
Preparation of calcium plumbate via different route Katerˇina Rubesˇova´ Æ Dagmar Sy´korova´
Received: 7 July 2008 / Accepted: 22 October 2008 / Published online: 14 November 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Calcium plumbate Ca2PbO4 was prepared by sol–gel methods (Pechini complex route with two varieties and water soluble polymer method) and by solid state reaction. The sol–gel prepared samples contained calcium plumbate as the only one phase as early as after 2 h annealing at 800 °C. Phase composition was detected by XRD measurement and by Raman spectroscopy. The next annealing at 800 °C for 24 h induced weak Pb losses displayed by present CaO (according to phase equilibrium) and confirmed by XRF measurement. The Pb losses were smaller for the sol–gel prepared samples probably due to earlier formation of Ca2PbO4. Microstructure and grain size were also established. Sol–gel prepared samples had regularly distributed grains with a small distribution interval with median value in order of 1 lm. Differences in microstructure of solid state and sol–gel samples are presented on SEM micrographs. Keywords Calcium plumbate Sol–gel X-ray diffraction SEM Raman spectroscopy
1 Introduction Calcium plumbate is used as a substitute for red lead in primers, especially for galvanized steel. However, the application is suppressed due to toxicity of lead substances. Besides, Ca2PbO4 is also playing crucial role during K. Rubesˇova´ (&) D. Sy´korova´ Department of Inorganic Chemistry, Institute of Chemical Technology, Technicka´ 5, Prague 6 166-28, Czech Republic e-mail:
[email protected] D. Sy´korova´ e-mail:
[email protected]
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(Bi, Pb)-2223 superconducting cuprates preparation from mixture of oxides and carbonates of Bi, Pb, Sr, Ca and Cu. The exact role of calcium plumbate has been discussed many times. One of the assumed mechanism of the (Bi, Pb)-2223 evolution [1] is that the first arising Bi-2212 phase is enriched by calcium plumbate to decrease its melting point. Such partially melted mixture facilitates dissolving of present Ca/Sr cuprates to nucleate the (Bi, b)2223 phase. The favorable partial melting can be also enhanced by other lead containing phases—such as 3321 phase (Pb3Sr3Ca2CuOx) [2]. Our aim is to use a sequential reaction of precursors (such as Bi-2212, Ca2PbO4, (Ca,Sr)2CuO3, CuO) from the middle stage of the (Bi, Pb)-2223 phase preparation. By means of properties of the precursors we could eventually affect the final superconductor quality. Therefore we would like to normalize the procedure of the optimal precursors’ preparation. Calcium plumbate crystallizes with orthorhombic symmetry in Sr2PbO4 structure type [3, 4]. Ca2PbO4 is stable in air up to 980 °C [5, 6] where it melts incongruently and decomposes into a liquid and CaO. Ca2PbO4 is prepared by solid state reaction from CaCO3 (CaO) and PbO as the only solid state product of the reaction [7]. It is not known to us that sol–gel preparation of calcium plumbate would have been published before. Sol–gel routes are suitable for mixed oxides’ preparation due to high homogeneity of a reaction mixture [8] followed by smaller grain size of a product in comparison with samples prepared by solid state reaction [9]. Losses of Pb during Ca2PbO4 preparation mentioned several times would be also suppressed by a smaller grain size of a sol–gel intermediate. We decided to prepare calcium plumbate via different routes: by solid state reaction and by two sol–gel routes. We chose so-called polymerized complex sol–gel
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route based on the Pechini polyester reaction [10] and sol– gel method using a water soluble polymer [11]. 2 Experimental procedure Samples of Ca2PbO4 were prepared by three different methods—solid state reaction (sample SS), water-soluble polymer method (sample PP) and polymerized complex route (samples PCI and PCII—see below). Temperature scheme of gels decomposition (250-500-800 °C) was the same for all the sol–gel samples and is based on our previous DTA/TG study of related systems (not published). This temperature treatment is sufficient to pyrolyze an organic resin in the gels. The final stage of samples’ preparation was identical for all the methods—annealing at temperature of 800 °C that was chosen on the basis of phase diagram of pseudobinary system PbO(PbO2) –CaO [5]. Temperature of the final annealing was also a compromise between the highest possible temperature enhancing the solid-state reaction and minimizing of eventual Pb losses at higher temperatures. Starting compounds in all the methods were weighted along the stoichiometry of Ca2PbO4. The methods are listed below: (1)
(2)
(3)
Solid-state reaction—sample SS: starting compounds—CaCO3 and PbO—were homogenized in an agate mortar for 15 minutes. Mixture was then annealed 24 h at 800 °C in alumina crucible in air. Water-soluble polymer method—sample PP: Ca(CH3COO)2H2O and Pb(CH3COO)23H2O were dissolved in an acetic acid diluted 1:1 in volume ratio. Then solution of polyethyleneimine (average Mw *2,000, 50 wt. % in H2O) was added. The ratio of PEI to present metals was 180 g of 50% PEI solution to 1 mol of Men? (based on previously published results [11]). The mixture was heated at 80 °C up to formation of white gelo-paste, which was then decomposed gradually at 250 °C (2 h), at 500 °C (2 h) and at 800 °C (2 h) with intermediate grinding. The retrieved intermediate was annealed for 24 h at 800 °C. Polymerized complex route—sample PCI: Pb(CH3 COO)23H2O was dissolved directly in ethyleneglycol (EG) at 80 °C. Then anhydrous citric acid (CA) and CaCO3 were added. Molar ratio was following: n(Men?)/n(CA)/n(EG) = 1/4/16 and is based on works preparing mixed oxides [12] and confirmed in our previous experiments [13]. No water or acidification was needed in this procedure, clear solution without any visible precipitate originated after a few hours. Temperature was then increased up to 120 °C to enhance polyesterification until clear brownish-yellow gel was formed. The gel was decomposed gradually at
(4)
250 °C (2 h), 500 °C (2 h) and 800 °C (2 h) with intermediate grinding. The retrieved intermediate was annealed for 24 h at 800 °C. Polymerized complex route—sample PCII: Pb(NO3)2 and Ca(NO3)24H2O were dissolved in concentrated solution of citric acid (CA) at 80 °C. After full dissolution ethyleneglycol (EG) was added. Ratio of CA, EG and present metals was the same as for the previous sample. Temperature was then increased up to 120 °C to enhance polyesterification until clear brownish-yellow gel was formed. The gel was decomposed gradually at 250 °C (2 h), 500 °C (2 h) and 800 °C (2 h) with intermediate grinding. The retrieved intermediate was annealed for 24 h at 800 °C.
X-ray powder diffraction data were collected at room temperature with an powder diffractometer (X’Pert PRO) with parafocusing Bragg–Brentano geometry using CuKa radiation. Phase composition evaluation was performed using the X’pert HighScore Plus software. Determination of cell parameters was also refined by this software using indexing by Treor method [14]. A sequential WD-XRF spectrometer (ARL 9400 XP) was used to perform XRF analysis. The obtained data were evaluated by standardless software Uniquant 4. Raman analyses were performed on dispersion spectrometer Labram HR (Jobin Yvon) equipped by Olympus microscope. Scanning electron microscopy was carried out by Hitachi S-4700 equipment. Prepared powders were stuck on a graphite tape. Grain size distribution was measured by laser technique on Analizeta 22 Fritsch equipment.
3 Results and discussion XRD diffraction measurement was carried out for the sol–gel samples after a gel decomposition step at 500 °C. Samples
Fig. 1 XRD patterns for samples PP, PCI and PCII after 2 h annealing at 800 °C, 2 h (before the final annealing) with pattern of Ca2PbO4
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contained oxides and carbonates of Ca and Pb, no detectable amount of Ca2PbO4 was registered. Figure 1 presents diffractograms of the samples prepared by sol–gel methods (PP, PCI, PCII) after decomposition of gels at 800 °C (before final annealing). As can be seen, 2 h annealing at 800 °C for the sol–gel prepared samples is sufficient for the phase formation—there were not detected any other phases on the diffractograms of samples PP, PCI and PCII. This 2 h annealing at 800 °C is a part of gel decomposition in the used sol–gel processes and it usually is not long enough to prepare multi-component oxides using solid state reaction (e.g. in superconducting cuprates preparation). Grain size of 500 °C annealing product is probably so small that the following 2 h annealing at 800 °C is sufficient to decompose present carbonates and, consequently, to enhance Ca2PbO4 formation. High level of homogeneity achieved during sol-gel methods could also enhance the earlier phase formation. Quality of the sol–gel samples after 2 h annealing was also examined by Raman analyses. Figure 2 shows Raman spectra of the samples PP, PCI and PCII. There are not visible any bands belonging to any potential calcium containing phases (oxide, carbonate, hydroxide) indicating Pb losses. Raman analyses were performed at several points of each sample to confirm chemical homogeneity. All samples were then finally annealed for 24 h at 800 °C. The temperature was chosen as a sufficient for Ca2PbO4 formation [4, 7, 15]. Higher temperature that could enhance the solid state reaction was not applied in order to minimize eventual Pb losses. XRD diffraction analysis of all the samples showed Ca2PbO4 as the main phase present but, unfortunately, there were very weak reflections belonging to CaO. It corresponds to Pb losses and the present CaO agrees with the phase equilibrium of Fig. 2 Raman spectra of the sol–gel prepared samples before the final annealing
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PbO(PbO2)–CaO [5]. Figure 3 shows the XRD results of the sol–gel prepared sample PCI (as a representative of the sol–gel samples that showed equal XRD patterns) in comparison with the solid state prepared sample SS after the final annealing at 800 °C for 24 h. Pb losses after the final annealing were actually confirmed by XRF analysis—Table 1. The highest decrease of Ca:Pb stoichiometric ratio from 2:1 to 2:0,85 was found for the solid-state prepared sample SS. The sol–gel prepared samples after the final annealing did not show such high difference in Ca:Pb ratio probably due to earlier formation of Ca2PbO4 (PbO arising from decomposed gel has a smaller grain size and thereby higher reactivity). However, the level of Pb losses was the only difference we could find
Fig. 3 XRD patterns for samples PCI and SS after 24 h annealing at 800 °C indicating higher content of CaO in the solid state prepared sample
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Table 1 Percents by mass of Ca and Pb measured by XRF and re-counted Ca:Pb ratio Sample
Mass %
Re-counted Ca:Pb ratio
Ca
Pb
Ca
Pb
SS
30.99
68.25
2
0.850
PP
28.82
70.85
2
0.949
PCI PCII
29.15 29.45
70.30 70.40
2 2
0.931 0.923
among the sol–gel samples. XRF analysis of the samples PCI, PCII and PP showed following values of Ca:Pb ratio: 2:0.93; 2:0.92 and 2:0.95, respectively. However, appropriate amount of CaO (corresponding to this Ca:Pb ratio values) was below the detection limit of XRD analysis (Fig. 3). Lattice parameters of prepared Ca2PbO4 were also established. Calcium plumbate crystallizes on orthorhombic crystal system, space group Pbam. Calculated lattice parameters were the same for all the samples—a = 9.75 nm; b = 5.84 nm; c = 3.38 nm and did not differ from values mentioned in literature [3, 4]. The sol–gel prepared samples of Ca2PbO4 differ from the SS sample also in crystallinity and grain size. Figures 4 and 5 show SEM micrograph of sample PP (as a representative of sol-gel prepared samples) and solid state prepared sample SS, respectively. Grains of the SS sample were partially sintered into random conglomerates whereas the sol–gel prepared samples had regularly distributed grains with similar size. This contrast was confirmed by grain size measurement. The sample PP had a small distribution interval with median value in order of 1 lm. Grain size measurement for the SS sample showed widespread distribution interval with 30% volume fraction in
Fig. 5 SEM micrograph of the solid state prepared sample SS (magnification 10,0009)
size of micrometers (probably non-sintered part of grains) and rest volume in size of 10 lm (partially sintered portion of the sample). The distribution curves of PP and SS samples are given on Figs. 6 and 7.
Fig. 6 Grain size distribution curve for the sol–gel sample PP
Fig. 4 SEM micrograph of the sol–gel prepared sample PP (magnification 10,0009)
Fig. 7 Grain size distribution curve for the solid state sample SS
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The median value of PCI and PCII samples was established to be 5 and 3 lm, respectively. It confirmed the sol– gel methods as more suitable way to prepare Ca2PbO4 precursors for following preparation of superconducting cuprates.
4 Conclusions Ca2PbO4 was prepared by solid-state reaction and by different sol–gel methods. Samples without any other detectable phases were prepared by all three sol–gel methods after 2 h annealing at 800 °C. Quality of the samples was confirmed by Raman spectroscopy. All samples (sol–gel prepared as well as from solid state reaction) exhibited Pb losses after final annealing at 800 °C for 24 h. The Pb losses were the most visible for the solid state prepared sample where CaO reflections appeared at XRD patterns besides. Probably smaller grain size of arising intermediate in sol–gel processes can suppress Pb losses. Moreover, the samples prepared by chosen sol–gel methods—water-soluble polymer method and polymerized complex route in two variations—had better crystallinity and smaller distribution interval of grain size. In respect to our aim—to use Ca2PbO4 as precursor for superconducting cuprates preparation—the sol–gel samples with shortened time of annealing are more suitable for the next solid state preparation of mixed cuprates. Further shortening of Ca2PbO4 preparation time could decrease Pb losses and will be of our interest in the future. Acknowledgement This work was financially supported by MSMT CR as the project No. 604613730.
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