Nanostructured Zirconia is finding application in engineering materials, barrier coatings, technical ceramics, sensors, catalysts and fuel cells. Elsa Antunes, Nuno Neves and João Calado present InnovNano’s Emulsion Detonation Synthesis method.
Zirconia is one of the most important technical ceramic materials; its excellent mechanical properties ensure it has a broad range of applications. The material is found in natural state in the form of baddeleyite (monoclinic phase). Pure undoped zirconia exhibits the following phase transformations: the monoclinic phase, from room temperature to 1175 °C, tetragonal phase, between 1175 and 2370 °C and cubic phase from 2370 °C to its melting point (2750 °C). Zirconia also has another two phases: orthorhombic I and II at low temperatures and high pressures (above 4 GPa), which is being investigated as a possible candidate for a super hard material.
However, the sintering temperature of zirconia, 1400 to 1500 °C, means that it is difficult to produce ceramic parts with undoped zirconia; during the cooling a volume change (4-5 %) occurs, inherent in tetragonal-monoclinic transformation, which leads to the formation of some cracks on the final pieces. To produce fired ceramic pieces, zirconia material must be stabilised with several oxides (dopants): MgO, CeO2; CaO, Y2O3, etc., the most prevalent stabiliser being Y2O3.
The common method used for the industrial production of yttria doped zirconia powder is the simultaneous hydroxide co-precipitation of zirconia and yttria salts. The hydroxide is then filtered, washed, calcined and milled to the required final narrow size distribution.
It is well known that dynamic shock induces chemical reactions. Emulsion Detonation Synthesis (EDS) is an industrial process patented by INNOVNANO, where material synthesis occurs at very high pressures (1-10 GPa). High pressures are commonly used to modify phase diagrams, and produce novel micro and nanocrystalline structures with different properties contributing to the development of super-hard materials.
This work is focused on physics and mechanisms related to high pressure synthesis and its effect on the mechanical properties of 2 and 3% mol yttria doped zirconia, obtained by EDS method. Dynamic shock induces a high concentration of defects and solid state amorphisation which cause higher chemical reactivity and sinterability in zirconia. Consequently, high density sintered pieces can be achieved at lower temperatures (~1250°C) by conventional sintering contributing to the avoidance of grain overgrowth and improving both the mechanical properties and life time of ceramic products. Through the EDS method, it’s also possible to prepare a tetragonal zirconia phase with a lower amount of yttria, well distributed throughout the zirconia nanostructure, improving the fracture toughness without compromising the bending strength.
Consequently, high density sintered pieces can be achieved at lower temperatures (~1250°C) by conventional sintering contributing to the avoidance of grain overgrowth and improving both the mechanical properties and life time of ceramic products. Through the EDS method, it’s also possible to prepare a tetragonal zirconia phase with a lower amount of yttria, well distributed throughout the zirconia nanostructure, improving the fracture toughness without compromising the bending strength.
Due to simultaneous high pressure and temperature, EDS is a promising synthesis method for the production of nano composites and doped materials with high chemical homogeneity and excellent mechanical properties, opening a wide range of opportunities in hard ceramic materials.
The EDS method is a novel gas phase route to obtain nanometric-sized ceramic materials in the form of multiple crystalline structures, composites, solid solutions and doped compounds. This method comprises four main stages:
a) Preparation of a water/oil (w/o) emulsion containing selected precursors.
b) Detonation reaction of the (w/o) emulsion with formation of a gaseous plasma.
c) Condensation to form a nanomaterial.
d) Fast cooling rate (quenching).
The nanomaterials obtained by this method produce high chemical and crystalline phase homogeneity, complementary properties customizable to the final application. As a result of high pressure synthesis (1-10 GPa), nanomaterials also exhibit several singular phenomena like: nanocrystallinity, amorphisation and a high concentration of defects that have a remarkable effect on their final behavior; in some cases, high pressure can lead to completely new crystalline structures (orthorhombic phase at 4 GPa, for example, in zirconia).
The challenge of the most important and demanding engineering ceramics is to achieve a higher combination of strength-toughness properties. Ongoing research at INNOVNANO continues to explore the application of high pressure synthesis in enhancing the mechanical properties of ceramic parts prepared from tetragonal nanostructured zirconia, obtained by the EDS method. As dynamic shock induces a high concentration of defects, solid state amorphisation and a very small nanocrystal size in the zirconia structure, lower yttria content (< 3%mol) could be used to have thermodynamically stable zirconia. The prime focus of our research is to investigate the influence of lower yttria content on the mechanical properties of the final sintered pieces.
Powder Synthesis
Commercial INNOVNANO nanocrystalline yttria-doped zirconia powders (YZ) with 99.9 % purity were prepared using two different amounts of yttria (2 and 3 mol %). In this synthesis method, high pressure (1 to 3 GPa) and high temperatures (500 °C to 3000 °C) are combined with ultrafast quenching (up to 108 K s-1 to 109 K s-1) in a single reaction step. We believe that chemical homogeneity of the starting nanopowder could improve the sintered targets properties and consequently enhance their final mechanical properties.
Results and discussion
a. Powder Characterisation
The physical and chemical properties of the starting powders and the nomenclature are summarized in Table 1.
|
Powder |
Aggregate Size (µm) |
Crystallite Size (nm) |
Specific Surface Area (m2/g) |
Purity (%) |
Y2O3 (%mol) |
|
3YSZ |
0.25 |
16 |
16 |
>99.8 |
3 |
|
2YSZ |
0.22 |
10 |
24 |
>99.8 |
2 |
The physical properties of the two investigated samples are very similar. The specific surface area of 2YSZ zirconia is higher than 3YSZ zirconia because the aggregation level (opening aggregates) is lower with comparison with 3YSZ sample (Fig 2).
The XRD analysis of the starting materials confirms the presence of two crystalline phases: monoclinic and tetragonal. The percentage of monoclinic phase is higher in 2YSZ due the lower yttria content of this sample. The broadening of peaks in XRD patterns confirmed the small crystallite size presented on the table 1. The 2YSZ sample is more amorphous than the 3YSZ sample.
Scanning electron microscope is used to analyse the morphology and the particle size of the nanostructured zirconia powders. Fig. 3 shows a powder with primary particle size around 50 nm and the morphology is nearly spherical.
b. Sintering behavior and densification of nanostructured zirconia
Both samples exhibit a very good sinterability, however the 2YSZ samples has a higher sinter activity due the small crystallite size and high specific surface area. Accordingly with the charts, the maximum shrinkage rate is at approximately 1250 °C for 3YSZ and at 1150 °C for the 2YSZ zirconia sample.
The green density of the compacts uniaxially pressed at 70 MPa is around 48% of theoretical density for both samples. The densification studies are performed in a furnace in atmospheric conditions. The final results of relative density with different sintering temperatures are presented on the Fig. 4. These results show that the 2YSZ zirconia is fully dense at 100ºC below the sintering temperature of 3YSZ sample. Probably the high specific surface area and the higher amorphism of the 2YSZ sample are key to enhanced sinterability of this powder. It is worth noting, that both samples have a higher sinterability in comparison with other commercially available powders and this behaviour could be explained by the high concentrations of defects and amorphism induced by high pressures during the synthesis process. Also, the nanostructure provides a significant contribution to the accelerated sinter reactivity of the starting materials.
The mean grain size of the 2YSZ ceramic is about 120 nm and 250 nm for the 3YSZ sample. These preliminary results suggest that low content of yttria leads to lower sintering temperature and smaller grains.
The XRD analysis of polished surface revealed only the presence of tetragonal phase on the final sintered pieces of both samples. A uniform dispersion of Y and Zr elements are observed on the final ceramic pieces stem from the initial very good distribution of these elements in the starting powder materials (Figure 7).
The bending strength and the hardness increase with the decreasing of grain size, however the hardness also increases with the increasing of yttria content. The hardness values is similar for both samples, this suggest that has a good balanced between grain size and yttria content. The grain size of 2YSZ sample is half of the grain size of 3YSZ and this is the principal reason for the higher value of bending strength exhibited by 2YSZ zirconia.
The results presented on table 2 show that 2YSZ has an unexpected mechanical behavior, higher bending strength and a value of fracture toughness almost the double of 3YSZ. These results reveal that tetragonal (t) phase of 2YSZ is very stable, despite of low yttria amount. The good t-phase stability could be explained by the small grain size (120nm). The high t-phase stability is resultant from the excellent atomic homogeneous dispersion of yttria in zirconia due the high pressures reached on EDS method.
Conclusions
This work presents the mechanical performance of 2YSZ and 3YSZ zirconia synthesized by EDS process. The 2YSZ exhibits unbelievable mechanical performance in comparison with 3YZ due its excellent t-phase stability, small grain size and lower amount of yttria.
Lower yttria content zirconia enables high fracture toughness due the higher tetragonal transformability. Due the synthesis conditions of EDS method, it’s possible to prepare zirconia powder with low amount of yttria well dispersed in zirconia structure with improved fracture toughness and bending strength.
This study shows that EDS is a valuable synthesis method for the production of nanostructured zirconia with well dispersed stabilizers. The high pressures reached during the synthesis allow to obtain materials with high concentration of defects and elevated amorphism level. These special characteristics become nanostructured materials more sinter activity and the sintering temperatures decrease approximately to 1250ºC.
At low temperatures, very little grain growth can occur due to a large faction of smooth/low-mobility boundaries. Reducing the grain size in yttria-doped zirconia has several beneficial effects on the stability of tetragonal phase and therefore on the ageing and mechanical properties. Small grain size has a high surface tensile strength which increases the t-phase stability and consequently improving the ageing resistance. The good t-phase stability of zirconia powders produced by EDS method is one of the fundamental conditions to decrease the percentage of dopant in order to improve the fracture toughness of final ceramics.
The results demonstrated, that EDS method is suitable to produce high performance zirconia material and opening a wide range of opportunities on the technical ceramic world.
Biography
Elsa Antunes earned the Chemical Engineer title from University of Coimbra (2003). She joined INNOVANO in 2003 and participates on the development of the Emulsion Detonation Synthesis process. At this moment, she is head of Advanced Ceramics Department, with the responsibility of R&D of products and its applications.
