Densification of zirconia-hematite nanopowders

Abstract

The densification of dual-phase yttria-doped tetragonal zirconia polycrystals (Y-TZP) and α-Fe₂O₃ (hematite) composite powders is described. Different powder synthesis methods, different forms of dry compaction processes, and two sinter methods (pressureless sintering and sinterforging) were compared. The homogeneity and average grain sizes of the sintered compacts were determined with SEM/EDX. Compacts produced from homogeneous powders that were prepared by the co-precipitation (CP) method showed large-scale phase segregation and grain growth during sintering. Compacts made from the less homogeneous sequentially precipitated (SP) powders showed far less phase segregation and relatively small grain growth during sintering. Dense (>96%) nano/nano zirconia-hematite composites were made with average grain sizes of 80 nm by sinterforging at 1000 °C and 100 MPa.

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% FeO_1.5 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% FeO_1.5 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.