Densification of zirconia-hematite nanopowders
Introduction
During the mid-1980s research into the synthesis and properties of nanograined dense materials was initiated.1 Due to size and structural effects, special properties of nanomaterials can be expected. Size effects become important if the building blocks are reduced to critical length scales of physical phenomena (e.g. the mean free paths of electrons or phonons) which can influence the electrical, magnetic and optical properties of materials. Structural effects in nanomaterials are mainly ascribed to different atomic structures of the grain boundary region compared with the bulk of the grain. A solid material containing a high density of grain boundaries has a high defect concentration. When the volume fraction of defects becomes very high, the properties of the material are influenced and change.
It can be expected that distinctive properties of nanocrystalline materials emerge in composites rather than in single-phase materials. In addition to the grain boundary components of two different kinds of grains, the variation in atomic bonding, lattice mismatches and related relaxation in the structure of the interfaces are reflected in the macroscopic properties of these composites. Nano/nano dual-phase composites in which both phases are present as grains with typical dimensions <100 nm are expected to exhibit physical properties that deviate from their coarser-grained counterparts. If one phase is electron conducting and the other electrically isolating, special properties emerge due to Schottky-barrier space-charge formation in the isolator phase near dual-phase boundaries. The extent of the Schottky-barrier (Debije length) can be of the same order as the total grain dimension resulting in special electrical properties.
The only dense (>95% from theoretical density) nano/nano ceramic oxide composites that have been reported to date are the zirconia-alumina system.2, 3 Alumina as minor phase had an inhibiting effect on the growth of zirconia grains. In cases where the alumina phase was not percolative, the growth of alumina grains was also severely retarded.2, 3 Nanograined zirconia–alumina composites had a lower hardness but increased toughness compared to larger grained zirconia–alumina composites.4
In order to prepare dense nano/nano composite ceramics the starting powder should not only consist of small crystallites but should also have a low degree of agglomeration.5 One method to prepare these type of powders is to start with a single-phase powder in which the cations present in the minor phase of the final product are dissolved in the major phase. This system then transforms into a dual phase composite during sintering.5 Another method is to start with a dual-phase powder in which the nano-crystallites of the two phases are distributed homogeneously.
This study describes the densification or sintering behaviour of nano/nano yttria-doped tetragonal zirconia (Y-TZP) and α-Fe2O3 (hematite) composites. The synthesis of zirconia-hematite dual-phase powders and single-phase solid solutions of iron oxide in zirconia has been described previously, using either wet-chemical techniques6, 7 or high-energy ball milling.8 Densification of these powders has rarely been reported, however, and in most cases Fe2O3 was added in small amounts (up to 3 mol%) to Y-TZP in order to improve the mechanical properties of the compact.9, 10, 11 Larger amounts of Fe2O3 have been added to cubic yttria-stabilised zirconia (YSZ) to probe the change in electrical and ionic conductivity of the material. In those cases formation of a dual-phase compact was reported.12, 13 Upon addition of 8 mol% FeO1.5 to monoclinic zirconia followed by sintering at 1500 °C densification was improved and grain growth was inhibited by the presence of Fe3+ ions.14 When 10 mol% FeO1.5 was added to cubic zirconia and the powder compact was sintered at 1480 °C, densification was also improved by the presence of Fe3+ ions.15 Only one investigation describes the microstructure of a dual-phase zirconia-hematite compact, where freeze-drying as powder preparation method resulted in a better homogeneity for sintered zirconia-hematite compacts compared to ball-milling.12 This emphasises the importance of powder processing for obtaining the desired microstructure of the dual-phase zirconia/hematite composite.
The influence of the zirconia-hematite powder synthesis method and powder composition on the densification and microstructure of the resulting Y-TZP/α-Fe2O3 composite is described in this paper. Both single-phase solid solutions of Fe2O3 in zirconia and dual-phase Y-TZP/α-Fe2O3 powders have been prepared as starting materials to form dense composites. Different powder compaction techniques and sintering methods (pressureless sintering, sinterforging) were compared. In addition, the influence of the powder synthesis and densification parameters on the final grain size and homogeneity of the dense compact was investigated with several methods.
Section snippets
Sample preparation
Two different synthesis routes were used to prepare powders with different compositions (Y-TZP with FeO1.5 ranging from 27 to 43 mol%). One method was through co-precipitation (CP), in which a solution of ZrOCl2·8H2O (Merck), YCl3 and FeCl3·6H2O (Fluka) was slowly added to an excess of concentrated (25 wt.%) ammonia (pH ∼14) under vigorous stirring, resulting in immediate precipitation of mixed metal hydroxides. The second method was through a sequential precipitation (SP) method where two
Compaction and sintering
The elemental and phase compositions of the calcined powders are shown in Table 1. The number in the sample key gives the amount of FeO1.5 in mol%. The zirconia phase of the SP-powders was purely tetragonal, while for the CP-powders also cubic zirconia was present. No monoclinic zirconia was detected. The XRD spectra of the co-precipitated powders with 27 and 34 mol% FeO1.5 (CP27 and CP34) did not show any hematite phase, while all other powders clearly showed a two phase zirconia-hematite
Compaction and sintering
For the systems investigated, pressureless sintering of isostatically pressed powders (at 400 MPa) did not lead to a nano/nano composite (grain sizes <100 nm), and it seems unlikely that this goal can be achieved with this method. By using higher pressures (e.g. by magnetic pulse compaction) during green compact formation it may be possible to obtain dense nano/nano composites after pressureless sintering these type of powders at 1000 °C. Sinterforging, however, was more efficient in obtaining
Conclusions
Compacts made of co-precipitated zirconia-hematite powders showed the largest grain growth and phase dehomogenisation during sintering when compared to sequentially precipitated zirconia-hematite powders. Due to secretion of dissolved Fe3+ ions from a ‘metastable’ zirconia lattice, the ionic mobility of the whole system was increased during sintering, causing increased segregation between the two phases and grain growth within the co-precipitated compacts. A powder prepared by the SP-method
Acknowledgements
Mark Smithers is acknowledged for the EDX measurements and Herman Koster for the XRD measurements. We are indebted to Michiel Jak of the Delft University of Technology for performing the magnetic pulse compactions, Coen van Dijk of the University of Groningen for making many SEM-pictures and Dr. Schmitt of Hitachi Ratingen for the Hitachi-SEM pictures. We gratefully acknowledge the Chemical Sciences (CW) foundation of the Netherlands Organisation for Scientific Research (NWO) for financial
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Grain growth kinetics and growth mechanism of columnar Al <inf>2</inf> O <inf>3</inf> crystals in xNb <inf>2</inf> O <inf>5</inf> –7.5La <inf>2</inf> O <inf>3</inf> -Al <inf>2</inf> O <inf>3</inf> ceramic composites
2019, Ceramics InternationalCitation Excerpt :The grain growth rate and growth mechanism are obtained by studying grain growth kinetics [16,17]. Accordingly, the study of grain growth kinetics has received increased attention [9,12–20]. Among different systems, the grain-growth kinetics index (n) and growth activation energy (Q) vary.
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2015, Ceramics InternationalCitation Excerpt :They found that the solubility of iron in zirconia increases with yttrium content. Raming et al. [25] found that in fully crystalline tetragonal zirconia (Y-TZP), only small amounts of Fe3+ (<1 mol%) could be dissolved into the zirconia lattice. Stefanic et al.[19] found that the solubility limit of the Fe3+ cations into the zirconia decreases with increasing calcinations temperatures, inducing Fe3+ cations to segregate out of zirconia and form a α-Fe2O3 phase containing probably some Zr4+ cations [14].
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2009, Composites Science and TechnologyFunctional nano-composite oxides synthesized by environmental-friendly auto-combustion within a micro-bioreactor
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Present address: Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210-1178, USA.