Nanocrystalline Tetragonal Zirconia Research Articles

Effect of applied stress on microstructural evolution during the sintering of nanocrystalline tetragonal zirconia below 1000 °C

Microstructural evolution during the sintering of tetragonal ZrO2 nanoparticles was investigated using stress-free sintering and sinter forging. The green body prepared via gel casting had a densely packed structure with a narrow pore size distribution. Special attention was paid to the evolution of pore structures in the initial and intermediate stages of sintering. Pore coarsening, which is considered an important issue in the nanoceramics densification, was not remarkable in sinter forging. The accelerated densification rate, coupled with the retention of finer pore size than that in stress-free sintering throughout densification, lowered the sintering temperature requirement. This temperature reduction effectively inhibited grain growth, allowing samples prepared via pressure-assisted sintering to maintain a nanometric structure.

Synthesis and Thermal Stability of Nanocrystalline Tetragonal Zirconia by Hydrolysis with Ethylene Diamine.

Nanocrystalline zirconia with high surface area and pure tetragonal crystalline phase was prepared using ethylene diamine (EDA), which acts as both precipitating agent and dispersant of the zirconium precursor. The yttria stabilized zirconia (Y-TZP) is a very attractive material due to its excellent biocompatibility, high fracture toughness, high strength and low wear rates. So zirconyl chloride octahydrate (ZrOCl2 · 8H2O) and yttrium chloride hexahydrate (YCl3 · 6H2O) in different molar ratios were used as starting solution. The detailed effects of various process parameters such as reaction time, concentration of the precursor solution, amount of ethylene diamine, and calcination temperature on the structural properties of the zirconia powders were investigated. The preparation conditions significantly affected the structural stability, crystal size, and crystal phase of the final material. Increases in the reaction time and amount of ethylene diamine led to a substantial increase of the crystal growth rate, the specific surface area, and the tetragonal content of the zirconia.

Combustion synthesis and properties of nanocrystalline zirconium oxide

Nanocrystalline tetragonal zirconia powders have been synthesized by aqueous combustion using glycine (Gly) as a fuel and zirconyl nitrate (ZN) as an oxidizer. The effect of the fuel-to-oxidant molar ratio on the structural and morphological properties of nanocrystalline zirconia powders was studied. Thermodynamic modeling of the combustion reaction showed that the increase in the Gly:ZN molar ratio leads to the increase in theoretical combustion temperature, heat of combustion and amount of produced gases. Powder properties were correlated with the nature of combustion and results of thermodynamic modelling. The increase in the Gly:ZN molar ratio produces more agglomerated powders characterized by a lower degree of uniformity, a lower specific surface area and a slightly bigger crystallite size. On the other hand, the presence of hard agglomerates suppresses the volume expansion, stabilizing tetragonal zirconia, as confirmed by Rietveld refinement. The absence of cubic zirconia was confirmed by FTIR and Raman Spectroscopy. The increase in the calcination temperature led to more agglomerated, compact and less uniform powders. The nanocrystalline nature of zirconia is the reason for the formation of bigger crystallites, the increase in the relative amount of monoclinic phase and sample sintering after calcination at high temperature. The highest measured specific surface area of zirconia was 45.8 m2·g−1, obtained using a fuel-lean precursor.

The role of local structural distortions in the stabilisation of undoped nanocrystalline tetragonal zirconia

Nanopowders of pure or very lightly doped zirconia were studied by means of total scattering and pair distribution function analysis, with the aim of understanding how the size of the particles (if a size limit of about 20 nm is not exceeded) tends to stabilise the tetragonal polymorph at room temperature. Total scattering and PDF analyses, together with Rietveld refinements, showed that the tetragonal model is indeed applicable to the average structure of these nanocrystalline zirconia samples, and provided comparable results. However, all the samples, with no influence from the dopant content, showed a similar local distortion for r < 10 Å. In this region, the data were fitted with two slightly different tetragonal structures, with a tetragonal distortion that decreases with the Rmax used, giving rise to a structure closer to the average tetragonal one (given by Rietveld as well as by average PDF refinements). In the first coordination shell, however, an orthorhombic distortion fits very well both in intensity and in position. This distortion may be responsible of the increased RMS strain local level, and therefore of the stabilisation of the high temperature structure at room temperature.

A Sonochemical‐Assisted Synthesis of Pure Nanocrystalline Tetragonal Zirconium Dioxide Using Tetramethylethylenediamine

Pure nanocrystalline zirconia has been successfully synthesized by sonochemical‐assisted route from zirconyl nitrate and tetramethylethylenediamine (TMED) as starting materials. Here, TMED for the first time was used as precipitating agent to generate nanocrystalline tetragonal zirconia via a sonochemical‐assisted route. The effect of solvent and concentration of TMED on the morphology and crystallite size was studied. It was found that these parameters have significant influence on the morphology and crystallite size of the products. The products were characterized by SEM, UV–vis absorption and TEM images, XRD patterns, PL, FTIR, and EDS spectroscopy.

Formation of oxide phases from anion-modified zirconium hydroxide during mechanochemical activation

We have studied the effect of tungstate and molybdate anions on the formation of zirconia polymorphs from amorphous anion-modified zirconium hydroxide during mechanochemical processing. The results demonstrate that the effect of anion additives on the formation of metastable zirconia from amorphous zirconium hydroxide during mechanochemical processing differs in a number of important points from that in thermochemical processes. In thermal processes, anion additives favor the formation and stabilization of nanocrystalline tetragonal zirconia. By contrast, in mechanochemical synthesis, anion additions inhibit the formation of metastable zirconia.

Structural and morphologic characterization of zirconia–silica nanocomposites prepared by a modified sol–gel method

In this study, we have synthesized zirconia–silica nanocomposites with different ZrO2 content (50% and 30%) using a modified sol–gel method, based on TEOS–Zr(IV)–NO3−–polyalcohols–H2O sols that successfully gelified to homogenous gels. The gels obtained at room temperature have been dried and then thermally treated at up to 150°C when a redox reaction took place between the organic polyalcohol and zirconia nitrate, evidenced by DTA analysis, leading to the formation of zirconia nanoparticles precursors in the pores of silica matrix. The obtained precursors have been characterized by thermal analysis and FTIR spectroscopy, and further annealed at higher temperature in the range 500–1200°C. XRD analysis of the resulted composites have evidenced the presence of nanocrystalline tetragonal zirconia as single crystalline phase up to 1200°C in the case of 30% ZrO2 samples, and up to 1000°C for the 50% ZrO2 samples. In the later samples, after being annealed at 1200°C, several crystalline phases have been present. For all nanocomposites we have obtained a homogenous dispersion of zirconia within the silica matrix, evidenced by Zr and Si elemental maps, performed by SEM-EDX analysis.

Preparation of mesoporous nanocrystalline tetragonal zirconia for catalytic methane combustion

Mesoporous nanocrystalline tetragonal zirconia (ZrO2) materials were synthesised via combining the soft templating method with the solid–liquid method. In brief, mesostructured zirconium hybrids were first prepared via the soft templating method and then ground with solid copper nitrate salts followed by low temperature heat treatment and further high temperature calcination in air. Finally, mesoporous nanocrystalline tetragonal ZrO2 materials were obtained after etching in acid solution. The characterisation results indicate that the obtained mesoporous ZrO2 materials have higher specific surface areas in the range of 100–300 m2 g−1 and pore diameters centred at 4·0–5·0 nm. The pore walls are composed of tetragonal ZrO2 nanocrystalline particles with an average size of 1–15 nm. These mesoporous nanocrystalline tetragonal ZrO2 materials as catalysts demonstrate good catalytic activity toward methane combustion, characterised by lower conversion temperatures (650–700°C) and active energies (75–90 kJ mol−1) than those of the reported ZrO2 materials, meaning their potential applications as catalysts or catalyst supporters in methane combustion.

Nickel Stabilized Zirconia for SOFCs: Synthesis and Characterization

Nanocrystalline tetragonal zirconia is commercially very significant material which finds extensive use as an anode material in SOFCs, as a catalyst oxygen sensor and structural material. Nanocrystalline zirconia powders for high performance anode of SOFCs have been synthesized by co-precipitation route. This technique is very helpful for the promotion of the stabilization of tetragonal phase of ZrO<SUB>2</SUB> in nano level at moderate temperature. The main objective of this paper is to stabilize the t-ZrO<SUB>2</SUB> through precipitation route using NH<SUB>4</SUB>OH solution. The processing features and the microstructural characteristics of NiO-ZrO<SUB>2</SUB> have been investigated by DSC-TG, XRD, SEM, and IR spectroscopy.The concentration of nickel-salt plays an important role for the enhancement of stabilized tetragonal phase at moderate temperature. From XRD results it was found that the t-ZrO<SUB>2</SUB> was more stabilized with 20 mol% nickel-salt concentration compared with 40 mol% Ni-salt at the same temperature.

Synthesis of nanocrystalline cubic zirconia using femtosecond laser ablation

We report on the synthesis of nanocrys- talline zirconia in liquid using femtosecond laser ablation. Nanocrystalline cubic zirconia has been prepared by femtosecond laser ablation of zirconium in ammonia, while nanocrystalline tetragonal and monoclinic zirconia was synthesized in water. The physical and chemical mechanisms of the formation of nanocrystalline metastable zirconia are discussed. The intrinsic properties of femtosecond laser ablation in liquid and OH -1 may be responsible for the synthesis of cubic zirconia. It is suggested that the femtosecond laser pulse can create higher tempera- ture and pressure conditions at a localized area in the liquid than the nanosecond laser pulse and the cooling is also faster in the femtosecond laser ablation process, which determined the difference between the products synthesized with femtosecond and nanosecond-pulsed laser ablation.

Effect of microscale shear stresses on the martensitic phase transformation of nanocrystalline tetragonal zirconia powders

For the first time, the effect of microscale shear stress induced by both mechanical compression and ball-milling on the phase stability of nanocrystalline tetragonal zirconia (t-ZrO 2) powders was studied in water free, inert atmosphere. It was found that nanocrystalline t-ZrO 2 powders are extremely sensitive to both macroscopic uniaxial compressive strain and ball-milling induced shear stress and easily transform martensitically into the monoclinic phase. A linear relationship between applied compressive stress and the degree of tetragonal to monoclinic (t → m) phase transformation was observed. Ball-milling induced microscale stress has a similar effect on the t → m phase transformation. Furthermore, it was found that even very mild milling condition, such as 120 rpm, 1 h (0.5 mm balls) was enough to induce phase transformation. Surfactant assisted ball-milling was found to be very effective in de-agglomeration of our nanocrystalline porous ZrO 2 particles into discrete nanocrystals. However, the t → m phase transformation could not be avoided totally even at very mild milling condition. This suggests that the metastable t-ZrO 2 is extreme sensitive to microscale shear stress induced by both mechanical compression and ball-milling. The findings presented in this work are very important in further understanding the stress-induced phase transformation of nanocrystalline t-ZrO 2 powders in a water free atmosphere and their further stabilization in industrially relevant solvents.

Grain core and grain boundary electrical/dielectric properties of yttria-doped tetragonal zirconia polycrystal (TZP) nanoceramics

Bulk samples of nanocrystalline tetragonal zirconia polycrystal (TZP), with 3 mol.% Y 2O 3, were fabricated over a range of average grain sizes (16–70 nm) by partial sintering. The samples were measured using AC-impedance spectroscopy over a range of temperatures, and porosity-corrected electrical results were interpreted in terms of microstructural models. Whereas the conventional brick layer model (BLM) significantly overestimated the specific grain boundary conductivity at the nanoscale, our recently developed nano-Grain Composite Model (n-GCM) allowed accurate determination of local grain boundary and grain core conductivities, grain boundary dielectric constants, and electrical grain boundary widths. Grain core effective dielectric constants were also separately measured on microcrystalline samples over a range of temperatures for use in the n-GCM analysis. It was found that TZP exhibits an enhanced local grain boundary conductivity at the nanoscale, but the enhancement is insufficient to improve the total conductivity. Rather, total conductivity decreased with decreasing grain size. Results were compared with those for nanocrystalline yttria-stabilized zirconia (8 mol.% Y 2O 3, YSZ).

Surface Enthalpy, Enthalpy of Water Adsorption, and Phase Stability in Nanocrystalline Monoclinic Zirconia

A fundamental issue that remains to be solved when approaching the nanoscale is how the size induces transformation among different polymorphic structures. Understanding the size‐induced transformation among the different polymorphic structures is essential for widespread use of nanostructured materials in technological applications. Herein, we report water adsorption and high‐temperature solution calorimetry experiments on a set of samples of single‐phase monoclinic zirconia with different surface areas. Essential to the success of the study has been the use of a new ternary water‐in‐oil/water liquid solvothermal method that allows the preparation of monoclinic zirconia nanoparticles with a broad range of (BET) Brunauer–Emmett–Teller surface area values. Thus, the surface enthalpy for anhydrous monoclinic zirconia is reported for the first time, while that for the hydrous surface is a significant improvement over the previously reported value. Combining these data with previously published surface enthalpy for nanocrystalline tetragonal zirconia, we have calculated the stability crossovers between monoclinic and tetragonal phases to take place at a particle size of 28 ± 6 nm for hydrous zirconia and 34 ± 5 nm for anhydrous zirconia. Below these particle sizes, tetragonal hydrous and anhydrous phases of zirconia become thermodynamically stable. These results are within the margin of the theoretical estimation and confirm the importance of the presence of water vapor on the transformation of nanostructured materials.

Low Temperature Synthesis and Characterization of CaO and MgO- Stabilized Nanocrystalline Tetragonal Zirconia by Citrate Gel Process

The citrate gel method, similar to the polymerized complex method, was used to synthesize homogenous tetragonal zirconia at 800oC and 1000oC. Nanocrystalline tetragonal single phase has been fully stabilized with 3, 7, 10 mol% CaO and 10, 15 mol% MgO at 800oC, respectively. In addition, the XRD analysis showed the absence of monoclinic phase after addition of 7 and 10 mol% CaO into zirconia-based solid solutions, which have been fully stabilized both 800oC and 1000oC. The crystallite sizes of the t-ZrO2 with 3, 7 and 10 mol% CaO at 1000oC were 32, 28 and 29nm, respectively. For ZrO2- x mol% MgO (x=3, 10, 15) solid solution, the crystallite sizes of samples at 800oC were less than 29nm, however it was increased up to 69nm at 1000oC. The prepared gel and subsequent heat-treated powders were characterized by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) to get detail information regarding to differentiation of polymorphs of zirconia as well as formation of powders.

Metastabilization of Tetragonal Zirconia by Doping with Low Amounts of Silica

Nanocrystalline tetragonal zirconia was obtained from ZrOCl2 via the modified forced hydrolysis method combined with aging of the hydrous amorphous precipitate in the mother liquor at 100 °C for 48 h (pH = 9.3). The role of the precipitation and aging temperatures in the metastabilization of the tetragonal ZrO2 polymorph is discussed in terms of the structural and textural data of the resultant oxide. The influence of low concentrations of silica (0.01 – 0.35 wt. % Si), spontaneously leached from the glass vessel or intentionally introduced to the parent solution, was shown to be a vital factor, controlling the phase composition of the final prepared zirconia. Using the concepts of zirconium aquatic chemistry, this effect was explained by incorporation of silicates into hydrous zirconia protostructures.

CHARACTERIZATION OF ZIRCONIA POWDER SYNTHESIZED VIA REVERSE MICROEMULSION PRECIPITATION

ABSTRACT A two-reverse-microemulsion precipitation technique was applied to synthesize nanocrystalline tetragonal zirconia using H2O solution/CTAB/hexanol as the microemulsion system. Two solutions of reverse microemulsion, one containing Zr4+ aqueous droplets and the other aqueous ammonia droplets with the same water/surfactant ratio, were prepared separately and mixed together to form a slurry of nanosized ZrO2 precursors, which filled the matrix of the surfactant, CTAB. The precursors were recovered, calcined to form nanocrystalline zirconia powder, and then characterized by using a transmission electron microscope to determine the particle size, a scanning electron microscope to examine the microstructure of the zirconia powder, and an X-ray diffractometer to determine the crystal phase and crystallite size. It is concluded that the primary particle size of the precursor determines the transformation temperature of the precursor and the crystal structure of the calcined zirconia.

Synthesis of nanocrystalline tetragonal zirconia by inorganic metathesis reaction

Nanocrystalline t-ZrO 2 particles (plate-like morphology with crystallite size of ca. 13 nm) have been prepared by a metathesis reaction between ZrO(NO 3) 2 and NaOH at 150 °C. A reaction scheme comprising hydroxylation and condensation of the zirconium center was proposed to explain the observation. Fine and uniform t-ZrO 2 particulates with smaller crystallite size (ca. 11 nm) can be produced by carrying out the same reaction in molten 1,12-diaminododecane.

Energy Crossovers in Nanocrystalline Zirconia

The synthesis of nanocrystalline powders of zirconia often produces the tetragonal phase, which for coarse‐grained powders is stable only at high temperatures and transforms into the monoclinic form on cooling. This stability reversal has been suggested to be due to differences in the surface energies of the monoclinic and tetragonal polymorphs. In the present study, we have used high‐temperature oxide melt solution calorimetry to test this hypothesis directly. We measured the excess enthalpies of nanocrystalline tetragonal, monoclinic, and amorphous zirconia. Monoclinic ZrO2 was found to have the largest surface enthalpy and amorphous zirconia the smallest. Stability crossovers with increasing surface area between monoclinic, tetragonal, and amorphous zirconia were confirmed. The surface enthalpy of amorphous zirconia was estimated to be 0.5 J/m2. The linear fit of excess enthalpies for nanocrystalline zirconia, as a function of area from nitrogen adsorption (BET) gave apparent surface enthalpies of 6.4 and 2.1 J/m2, for the monoclinic and tetragonal polymorphs, respectively. Due to aggregation, the surface areas calculated from crystallite size are larger than those measured by BET. The fit of enthalpy versus calculated total interface/surface area gave surface enthalpies of 4.2 J/m2 for the monoclinic form and 0.9 J/m2 for the tetragonal polymorph. From solution calorimetry, the enthalpy of the monoclinic to tetragonal phase transition for ZrO2 was estimated to be 10±1 kJ/mol and amorphization enthalpy to be 34±2 kJ/mol.

Quantification of Chemical Pressure in Doped Nanostructured Zirconia Ceramics

The influence of the substitution of Zr4+ by oversized rare-earth R3+ (R ≃ Sc, Yb, Y, Gd, Sm) was investigated in nanocrystalline tetragonal R2O3-doped zirconia (ZrO2) ceramics. Three batches of samples were investigated as a function of various contents of oversized trivalent dopants, i.e., (i) scandia and (ii) ytterbia, and as a function of different dopant ionic radii, for a constant R2O3 content of 3 mol %. Quantification of internal pressure was performed from frequency shifts of Raman spectra and from compressibility data from XRD.

Fracture toughness of nanocrystalline tetragonal zirconia with low yttria content

Nanocrystalline tetragonal zirconia samples are shown to reach toughnesses between 16 and 17 MPa·m 1/2, provided the conventional levels of 3 mol% yttria additive are lowered to 1 or 1.5 mol%. For 1 and 1.5 mol% compositions, maximum toughness occurs just below the critical grain sizes of 90 and 110 nm, respectively—i.e. just before spontaneous transformation to the monoclinic phase occurs. Moving away from the critical grain size, towards smaller grains, toughness decreases monotonically up to a factor of 5. These results indicate a critical parameter for maximizing toughness in nanocrystalline zirconia ceramics is proximity to the phase transformation boundary (critical grain size). Combining the present results with data from the literature, a linear relationship between yttria content and inverse critical grain size is found. A power law relationship between grain size and toughness increment is also obeyed by the present data.