Preparation of nanosized mixed oxide ceramic powders using polyvinyl alcohol and polyhydroxy organic compounds C. Koh, S. Tahir, A. Sen, A. Pathak,* and P. Pramanik *
A novel chemical technique has been developed for the preparation of nanosized ceramic powders by thermolysis of polymeric based aqueous precursor solutions of metal complexes, via the formation of mesoporous carbon precursors. T he precursor solutions consisted of metal ions complexed with suitable ligands, namely polyvinyl alcohol and polyhydroxy organic compounds such as mannitol and sorbitol. T he particle sizes of the ceramic powders, which were produced by calcination of mesoporous carbon rich precursor powders at temperatures below 800 K, ranged between 10 and 60 nm depending on the preparation conditions. T he resulting powders were found to have a narrow particle size distribution. BCT /504 Dr Koh and Dr T ahir are in the Department of Chemistry, King’s College L ondon, Strand, L ondon W C2R 2L S, UK and Ms Sen, Dr Pathak, and Professor Pramanik (
[email protected]) are in the Department of Chemistry, Indian Institute of T echnology, Kharagpur 721 302, India. Manuscript received 31 July 2001; in nal form 6 March 2002. © 2002 IoM Communications L td.
INTRODUCTION The performance of products or devices made of ceramic materials is often limited by both the highly brittle nature and large statistical variation in properties of ceramics. Any attempt to generate ceramics with properties which go beyond the present limitations has to focus on generating microstructures with new types of atomic arrangement or new chemical compositions. Nanosized ceramic materials (1 mm), for example high elastic modulus, high hardness, high compressive strength, refractoriness, and low relative density.1 A unique feature of ne ceramic materials is that a large fraction of their atoms reside in the grain boundaries. Atoms located within a certain distance of a boundary (i.e. within the grain boundary thickness) are displaced from their normal sites in a perfect crystal owing to the presence of atoms across the boundary.2 The properties of ne or nanosized ceramic materials are strongly in uenced by their surface structure and/or interfacial volume,3 – 6 which represents a signi cant fraction of their total volume. The surface atoms in a small crystallite/particle have lower density, a lower coordination number, a larger interatomic distance, and also lower symmetry.7 A modi cation in the coordination number of the surface atoms results in a modi ed electronic structure and related physical/chemical and catalytic properties of ne ceramic materials. Thus a 114
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change in the interfacial volume in nanosized crystallites/ particles will clearly cause a signi cant change in their electronic structure, depending on the chemistry of the constituent atoms. This introduces scope for producing a new generation of catalysts following the invention of the molecular sieve. Important solution based chemical synthesis methods commonly used for the preparation of ne grained ceramics include solvent vaporisation, aerosol, precursor compound, hydrothermal, coprecipitation,combustion,and sol–gel and other gel techniques. Details of the state of the art in powder synthesis are available in various review articles.6 – 1 5 It is a decade since the present authors became involved with the development and improvement of novel chemical routes to provide methods that are less cumbersome, more versatile, and also adaptable for large scale preparation of nanosized powders of various ceramic systems.1 6 – 2 3 Early work involved the thermolysis of an aqueous metal ion– polymer based precursor solution, using the polymerpolyvinyl alcohol (PVA). In subsequent studies the polymerPVA was partially replaced by sucrose in the precursor solution so as to circumvent the graphitisation tendencies associated with evaporated PVA.2 1 – 2 3 The present paper reports on the use of polyhydroxy organic compounds such as sorbitol and mannitol, in combination with PVA, as new polymeric reagents for modifying the metal ion–polymeric based precursor solution method for the synthesis of nanosized ceramic powders with good particle size distribution. The prepared powders have been characterised in terms of particle size and size distribution. The synthesis of various chromite, aluminate, tungstate, vanadate, molybdate, and phosphate systems has been investigated in order to establish the versatility of the new polymeric reagents in the aqueous precursor solution.
EXPERIMENTAL PROCEDURES Metal ions in the form of nitrates, formates, or acetates were rst dissolved in aqueous solution in accordance with the desired stoichiometries. Next, appropriate complexing agents, ethylenediaminetetraacetic acid (EDTA), triethanolamine (TEA), diethanolamine (DEA), were introduced into the aqueous solutions so as to have the metal ions in homogeneous solution through complex formation. The addition of the complexing agents helped avoid segregation and precipitation of the metal ions from the homogeneous solution during preparation. Appropriate amounts of an aqueous solution of polyhydroxy organic compound (sorbitol or mannitol) and PVA were then introduced into the starting aqueous solution of the complexed metal ions to obtain the respective precursor solutions. The resulting homogeneous aqueous precursor solutions were then evaporated at around 200°C. Rapid evaporation of the entire solution resulted in a uVy mass, rich in mesoporous carbon, which on subsequent calcination at temperatures varying from 500 to 800 K yielded the desired ne ceramic powders. The preparation conditions for the DOI 10.1179/096797802225003307
Koh et al.
1
X-ray diVractograms (Cu K a radiation) of virgin precursor powders of a CoCr2O4 and b CuCr2 O4 prepared using sorbitol–PVA as polymeric mixture
synthesis of the various aluminates, chromites, tungstates, vanadates,molybdates,and phosphates are given in Table 1. The virgin ceramic precursors and their corresponding calcined powders were characterised by room temperature X-ray diVraction studies (Philips PW1710). Average crystallite sizes of the powders, obtained after calcination at their respective crystallisation temperatures, were determined by X-ray line broadening using the Debye–Scherrer2 4 formula applied to the diVerent dh k l lines. Average particle sizes (average diameters of smallest visible isolated particle/ crystallite agglomerates) were determined by transmission electron microscopy (Philips TM300).
RESULTS AND DISCUSSION From the experimental studies it was observed that the nal ceramic powders and precursors obtained using the polymeric reagent containing mannitol were similar to those obtained through the use of the mixture containing sorbitol. X-ray diVractograms of representative chromite and tungstate precursors and their calcined powders are shown in Figs. 1 and 2 respectively. From the XRD studies it was observed that most of the samples indicated crystallinity in the virgin precursor (uncalcined) state, while some were found to be amorphous to X-rays at room temperature. Crystallinity in the virgin precursors was attributed to the presence of constituent metal ions with better catalytic properties which promoted faster burning of carbonaceous material in the precursors. The exothermic process generated high in situ temperatures and facilitated the production of crystallites of the desired ceramic phase in the virgin precursor state and, accordingly, the corresponding calcination temperature required for obtaining the nal carbon Table 1
2
Nanosized mixed oxide ceramic powders
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X-ray diVractograms (Cu K a radiation) of precursor powders of ZnWO4 system prepared using sorbitol–PVA as polymeric mixture: a virgin; b after calcination for 2 h at 450°C
free ceramic materials was reduced. However, because of the lower catalytic activity of the metal ions in some of the precursors, the in situ heat generated during combustion of the carbonaceous material was not suYciently high for the crystallites to form in the virgin precursors. As a result, these precursors appeared amorphous to X-rays and therefore required higher calcination temperatures to obtain the carbon free nal ceramics than the rest of the samples. Bright eld transmission electron micrographs for representative powders after calcination of the precursors at their crystallisation temperatures are shown in Figs. 3a and 4a. Corresponding selected area electron diVraction patterns of the same samples show distinct rings, characteristic of an assembly of randomly oriented nanocrystallites (Figs. 3b and 4b). The minimum calcination temperatures required to obtain carbon free powders, the average crystallite sizes calculated using the Scherrer formula, and average TEM particle sizes of the prepared ceramic powders are given in Table 2 for the various metal ion systems. The methods developed involved the complete evaporation of aqueous precursor solutions consisting of the desired metal ions complexed with a chelating agent, and dispersed in the polymeric network provided by the mixture of mannitol or sorbitol and PVA. The aqueous solution of sorbitol or mannitol in the presence of small amounts of PVA showed characteristics similar to those of a polymeric reagent. PVA easily decomposes exothermally at low ignition temperature (
