Nano-powders of Na0.5K0.5NbO3 made by a sol–gel method
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- Chowdhury, A., Bould, J., Zhang, Y. et al. J Nanopart Res (2010) 12: 209. doi:10.1007/s11051-009-9595-0
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Sodium potassium niobate (NKN) nano-particle powders were synthesised through the thermal decomposition of a sol–gel NKN precursor. Powders and gels were characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). Hydrated carbonate phases formed as a result of reaction with evolved vapours during organic decomposition, and by reaction of NKN powders with H2O and CO2 on exposure to air. The primary particle size of the powders increased from <50 to <250 nm as decomposition temperatures were raised from 500 to 950 °C.
KeywordsNano-powdersSol–gelSodium potassium niobateX-ray diffractionFourier transform infrared spectroscopyTransmission electron microscopySynthesis method
Over the past few years, environmental concerns have stimulated interest in developing lead-free ferroelectric and piezoelectric ceramic compositions as replacements for lead zirconate titanate. One of the most promising candidates in this category is a solid solution series based on sodium potassium niobate (NKN), NaxK1−xNbO3, modified by lithium and tantalum ions (Saito et al. 2004; Guo et al. 2005).
For the NaxK1−xNbO3 system (NKN), compositions around x = 0.5, Na0.5K0.5NbO3, lie in the vicinity of one of the system’s morphotropic phase boundaries and show the most favourable ferroelectric and piezoelectric parameters (Shirane et al. 1954; Haertling 1967; Egerton and Dillon 1959; Jaeger and Egerton 1962; Tennery and Hang 1968). Given the technological potential of NKN-based piezoceramics, it is important to develop appropriate ceramic fabrication techniques. Bulk ceramics are prepared traditionally using powders obtained from milling and calcining mixtures of oxides or compounds such as carbonates that decompose into oxides at high temperatures. These calcined powders are then compacted and sintered to form high-density ceramics. However, in the case of NKN, it is difficult to achieve high densities using conventional powder processing methods. There are also problems in avoiding loss of volatile Na2O and K2O vapours during both calcination and sintering.
Over recent years, a number of solution-based powder synthesis routes have been developed as alternatives to the mixed-oxide route, including co-precipitation and sol–gel methods (Smart and Moore 1996). These “soft chemistry” methods can result in smaller particle sizes and improved chemical uniformity compared to mixed-oxide routes. When applied to NKN, they could offer reductions in processing temperatures which would be expected to reduce the tendency for loss of alkali metal oxides, and to produce smaller particle size thereby enhancing densification kinetics.
Sodium niobate, NaNbO3, nanopowders have been reported from the reaction of hydrogen peroxide solution with sodium and niobium ethoxides (Cheng et al. 2006). Transmission electron micrographs of the powder heat treated at 500 °C for 1 h revealed primary particles around 15–30 nm in size. Lithium modified NaNbO3 has been made using Na2CO3 and Li2CO3 precursors along with ammonium niobium oxalate, NH4H2[NbO(C2O4)3] · 3H2O (Franco et al. 1999). The powders were of a high surface area, ~10 m2 g−1, with equivalent spherical diameters of ~130 nm. Powders of NaNbO3 (Nobre et al. 1996) have also been reported using methods based on a Pechini-type reaction route, involving citric acid and ethylene glycol reagents, giving high crystallinity and high surface area ~28 m2 g−1, with equivalent spherical diameters of ~46 nm.
In the present study, we report the synthesis and properties of Na0.5K0.5NbO3 (NKN) nanopowders produced via a sol–gel method involving ethoxides of sodium, potassium and niobium as precursors, and 2-methoxyethanol as solvent.
Precursor solutions were prepared from commercially available ethoxides of sodium [CH3CH2ONa], potassium [CH3CH2OK] and niobium [(CH3CH2O)5Nb] (Aldrich). The ethoxides were stored and handled under a dry N2 atmosphere in a re-circulating glove box (Saffron, UK). Chemicals were weighed and mixed in 2-methoxyethanol [CH3OCH2CH2OH] (Aldrich), followed by stirring for 2 h to give a yellow-coloured solution, referred to as the stock solution, with a concentration of 0.34 M (in terms of Nb content).
The stock solution was maintained at 60–70 °C, with slow stirring for 4 h. The sample was exposed to atmospheric moisture, but no deliberate addition of water was carried out. A sticky resinous gel formed after standing for a further 3 h at room temperature. The gel was transferred to an oven and dried at 120 °C for a period of 24 h to form a yellow powder. For each batch, ~0.5 g of dried gel powder was produced; this was ground into a finer powder using an agate mortar and pestle. The powder was calcined at different temperatures in order to study phase development using XRD (Philips APD 1700, Almelo, The Netherlands) with Cu–Kα radiation. Fourier transform infrared spectroscopy (FTIR) was carried out on samples of the NKN powder after calcination at different temperatures for dwell times of 30 min (Perkin Elmer Spectrum One FTIR spectrometer). Spectra were recorded over the wavenumber range 4,000–1,000 cm−1. The particle size and morphology were evaluated using transmission electron microscopy (TEM, Philips CM 200 FEGTEM, Eindhoven, the Netherlands) with an accelerating voltage of 200 kV. Unit cell parameters were calculated using a least squares refinement programme. For TEM investigations, powders were suspended in isopropanol, and a drop of this suspension was deposited on a holey carbon-coated film supported on a 400 mesh copper grid. Thermogravimetric analysis (TGA) was conducted in air (Stanton & Redcroft TGA 1000, London, England). The gel for this purpose was obtained by drying the sol at 60 °C for 4 h. The TGA furnace was run at 20 °C min−1 until 950 °C. This was the maximum working temperature deemed to avoid significant levels of alkali metal evaporation and consequent damage to the apparatus; the TGA sample was held at 950 °C for 20 min. Surface area measurements were performed using a 3-point BET technique (Quantachrome Instruments, Florida, USA).
Results and discussion
The TGA loss at <130 °C is consistent with evaporation of: ethanol (boiling point = 79 °C) derived from the metal ethoxides; methoxyethanol solvent (boiling point = 124 °C); and water present in the precursors, or absorbed from the atmosphere.
Wavenumbers of key peaks of the NKN gel samples after heating at progressively higher temperatures
Wavenumbers of the key peaks (cm−1)
~3,300 (very broad), 1,770, 1,610, 1,430, 1,310, 1,120, 1,070
~3,300 (very broad), 1,770, 1,630, 1,450, 1,310, 1,070
~3,300 (very broad), 1,630, 1,365, 1,070
~3,300 (very broad), 1,630, 1,365, 1,070
~3,300 (very broad), 1,654, 1,430, 1,070
~3,300 (very broad), 1,654, 1,520, 1,430, 1,215
~3,300 (faint), 1,430, 1,215
The small 1,770 cm−1 peak also existed in the 350 °C sample, Fig. 3; the other peaks were similar to those of the 250 °C sample. However, the 1,770 cm−1 peak was absent in the 450 °C sample. It is assumed that all organics had decomposed by ~450 °C.
At 650 °C there were several changes. The 1,365 cm−1 peak disappeared and there was a peak at 1,654 cm−1 as opposed to 1,630 cm−1 at lower temperatures, Table 1. At 750 °C, peaks at 1,520 and 1,215 cm−1 are developed. On increasing the decomposition temperature to 850 °C, the peak at ~1,630 cm−1 could not be distinguished, but 1,430 and 1,215 cm−1 peaks remained, together with OH stretch at high wavenumbers, indicating a hydrated phase. The 950 °C sample showed no evidence of secondary carbonated/hydrated phases.
The 1% mass change highlighted by TGA above 750 °C is most probably due to the final residual carbonate phase decomposing, but the FTIR spectra showed hydrated carbonate phases persisted in a NKN powder sample even after decomposition at 850 °C. It is probable that some of the hydrated carbonates detected in the high temperature FTIR samples are a consequence of a reaction between the NKN powders, after thermal decomposition, with moisture and carbon dioxide in the air during sample storage, prior to recording the FTIR spectra. The absence of peaks in FTIR patterns in the 950 °C sample may therefore be due to its larger particle size, as described below, and consequent lower surface area available to react with atmospheric vapours.
The peak changes described above are considered to mark a change from a system containing a mixture of organic residues and co-existing hydrated carbonates, <450 °C, to one where NKN co-exists with hydrated carbonate phases (450–850 °C), and finally single-phase NKN is present (950 °C). Variations in peak positions at temperatures above 550 °C signify slight changes in the composition of the constituent carbonate species are taking place.
The XRD pattern of the 650–950 °C samples showed splitting of some peaks, e.g. the pair of closely spaced peaks at ~22° 2θ and also the peaks at ~46° 2θ. The Na0.5K0.5NbO3 phase is known to be orthorhombic at room temperature (Tennery and Hang 1968). Initial crystallisation occurred at 450 °C, but peaks in this sample, and the 550 °C sample, were broad and no clear peak splitting was observed. The patterns for the 650–950 °C samples in Fig. 4 exhibited peak splitting, although continued peak broadening made it difficult to resolve closely spaced peaks.
A variation in the relative intensity of certain peaks was observed in the temperature range 650–950 °C. For alkali niobates, the relative intensity of pairs of peaks at ~22 °2θ and ~45 °2θ (Fig. 4) can be indicative of variations in the proportions of orthorhombic and tetragonal phases. For single-phase orthorhombic samples, the peak intensity ratio α may be expressed as α = (I110/I001 + I220/I002)/2, with a value of α = 1.8, whilst for a tetragonal NKN-based composition, α ~ 0.5 (Skidmore and Milne 2007). The α values for the 850 and 950 °C samples were only slightly lower than the expected value for an orthorhombic phase (Skidmore and Milne 2007), with experimental values of 1.5 and 1.4 for the 850 and 950 °C samples, respectively. Therefore, crystallisation to predominantly orthorhombic NKN phase is indicated to occur on heating the precursor gels to ≥850 °C. However, the peak ratio displayed by the 650 and 750 °C samples, α ~ 1, suggests that a significant amount of tetragonal phase may co-exist with the orthorhombic phase at intermediate temperatures. The tetragonal phase of NKN is thermodynamically stable above ~200 °C. Its presence in a metastable form in the 650 and 750 °C samples could be due to particle size effects. Compositional non-uniformities in the samples may also affect peak intensities at intermediate decomposition temperatures.
Estimated values of unit cell lattice parameters were obtained using a least squares refinement programme, giving values of: a = 5.660 Å, b = 5.655 Å and c = 3.946 Å (calculated for a sample decomposed at 850 °C) with standard deviations of ≤0.001 Å. These values compare to a = 5.695 Å, b = 5.721 Å and c = 3.974 Å for a standard orthorhombic NKN pattern (JCPDS, Joint Committee for Powder Diffraction file number 32-0822).
Thermal analysis and FTIR studies indicated that Na0.5K0.5NbO3 (NKN) precursor gels made from an ethoxide-based sol–gel route decomposed to produce a NKN hydrated carbonate phase at process temperatures up to 850 °C. There was also some evidence that hydrated carbonate phases secondary phases were produced by reaction of the NKN powders with atmospheric moisture and carbon dioxide. The NKN particle sizes varied from <50 nm in samples decomposed at 500 °C, to <250 nm for the highest temperature studied, 950 °C. The absence of secondary phases formed on exposure to air in powders produced at 950 °C is consistent with their increased particle size and lower surface area.