Yttria-doped Zirconia Powders Research Articles

Is direct microwave heating well suited for sintering ceramics?

This paper presents a thorough analysis of direct microwave heating as a sintering process of ceramic materials. It questions why susceptor-assisted microwave heating is used in most experimental works, although direct microwave coupling is preferable for taking advantage of the possible beneficial effects of the microwave field on the sintering phenomena. This issue was investigated by conducting both experiments and numerical simulations. The experiments consisted of sintering alumina and yttria doped zirconia powder samples in a 2.45 GHz resonant cavity with automatic thermal monitoring, whereas the numerical simulations coupled electromagnetics, thermal transfer and sample deformation. Alumina and yttria doped zirconia are widely used materials and they exhibit different microwave field behaviours (transparent and absorbent, respectively), which are representative of most ceramic materials. The influence of the insulating material was discussed by considering different sintering cell designs. The very low coupling capacity of alumina made its direct heating very difficult. It was therefore necessary to apply a strong electric field to heat it. This situation promoted the absorption of microwave energy by other elements such as the insulating material, leading to heating instabilities and degradation of the insulation cell. In the case of zirconia, its coupling properties change abruptly with increasing temperature. It is poorly absorbent at low temperature, highly absorbent at intermediate temperature and it finally becomes reflective at the end of the sintering process. The consequences of this behaviour are (i) a very difficult control of direct heating (ii) a propensity to form damaging hot spots and (iii) the impossibility to reach high temperatures without forming plasma. Therefore, this experimental and numerical study showed that direct microwave heating is not suitable for conducting reliable and homogeneous sintering of classical ceramics. This explains why susceptor-assisted heating is most of time preferred.

Evaluation of fracture toughness of zirconia ceramics with heterogeneous yttrium distribution microstructures

Yttrium stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics containing 2 mol% and 2.5 mol% yttria were prepared by mixing 3 mol% yttria-doped zirconia powders with yttria-free zirconia powders. The microstructures of the sintered samples were studied using SEM, XRD, and Raman spectra. In addition, an ultra-sharp V-notch (ρ < 0.5 μm) was induced by femtosecond laser to measure its actual fracture toughness by single-edge V-notched beam (SEVNB) method. The results reveal that mixture-based Y-TZP with 2.5 mol% yttria has deeper phase transformation depth, and fracture toughness of 6.4 MPa.m1/2 is obtained. The heterogeneous distribution of the yttrium is taken account for the excellent properties.

Yttria stabilized zirconia synthesis in supercritical CO 2: Understanding of particle formation mechanisms in CO 2/co-solvent systems

The increasing interest of supercritical (SC) fluids for inorganic materials synthesis recently stimulated the development of innovative synthesis processes and strategies. The supercritical CO 2 aided sol–gel process, developed for preparing various ceramic oxide powders with attractive applications in cosmetics, chromatography, catalysis or solid oxide fuel cells, usually suffer from both reproducibility problems and poor knowledge of the key parameters defining the final powder characteristics. In the present work a specific effort has been put on the understanding of reaction mechanisms and process parameters like co-solvent polarity and ageing time of the starting solution, which appeared to play a crucial role for the control of powder characteristics. Two different reaction mechanisms have been proposed to explain the formation of tetragonal yttria-doped zirconia powders by a batch process in either CO 2/pentane or CO 2/isopropanol mixtures. The first mechanism corresponds to a CO 2 anti-solvent precipitation process while the other one is based on a condensation reaction as in the conventional sol–gel process. This improved understanding in particle formation allows better control of powder characteristics and reproducibility.

Mechanical properties and interfaces of zirconia/nickel in micro - and nanocomposites

The effect of processing parameters on the hardness and toughness of yttria-doped zirconia/nickel nanocomposites is reported. The experimental procedure consists in the deposition of nickel nanoparticles––previously obtained from nickel nitrate salts––on the surface of the zirconia powders. Slight changes in the proposed processing route (i.e. drying temperature of powders) gave rise to a noticeable decrease in the hardness (15–20%). However, no effect was observed on the toughness values in the studied composition range (0–30 vol% Ni). These results have been explained in terms of the nickel particle size distribution and agglomeration due to differences in the melting temperature of nickel nanoparticles, and because of epitaxial growth of nickel on the zirconia surface. For comparison purposes, samples obtained following a conventional wet-processing route starting from micrometric nickel and yttria-doped zirconia powders were prepared. In this case, the toughness is strongly dependent on the nickel fraction. This fact has been related to the weak bonding between metal and ceramic particles.

The synthesis and characterisation of mixed Y2O3-doped ZrO2and α-Fe2O3nanosized powders

Several wet chemical precipitation methods used to synthesise nanocrystalline composite powders containing zirconium oxide, yttrium oxide and iron(III) oxide are described. The crystallisation and phase composition of the precipitates were studied as a function of temperature. A co-precipitation method was compared with two different sequential precipitation routes. The amount of iron(III) oxide added varied from 27 to 42 mol% FeO1.5. The dried gel obtained after co-precipitation was extensively studied as a function of calcination temperature. After calcination for 2 hours at 600?700 °C about 10?16 mol% FeO1.5 was dissolved in the zirconia matrix. A part of the ferric oxide was present outside of the zirconia lattice, either as a crystalline hematite phase (when adding >40 mol% FeO1.5), or as a finely dispersed amorphous ferric oxide (when adding <34 mol% FeO1.5). After calcination at temperatures around 900 °C only a few mol% FeO1.5 were dissolved in the zirconia matrix. All sequential precipitation routes resulted in dual-phase yttria-doped zirconia and hematite powders after calcination. Only up to 4 mol% of ferric(III) oxide could be dissolved in the zirconia lattice when using a sequential precipitation route.

Effect of sintering temperature and holding time on the properties of 3Y-ZrO2 microfiltration membranes

Sintering behavior of supported and unsupported microfiltration membranes prepared from 3 mol% yttria doped zirconia powder was investigated as a function of temperature and holding time in non-isothermal and isothermal densification. Shrinkage that started at 1000°C showed the highest rate between 1200°C and 1300°C although the rate decreased above 1300°C. The activation energy of sintering was calculated at 735 kJ/mol, assuming the grain boundary diffusion mechanism for mass transport. Mean pore size decreased in unsupported membranes and increased in supported ones as the sintering temperature increased up to 1200°C. Dimensional shrinkage of unsupported membrane slabs showed an increase in shrinkage first in the lateral dimension and then in the thickness as the sintering temperature increased. Pore growth and lower hardness in supported membranes, can be explained due to the restricted lateral shrinkage in the supported membranes. Removal of porosity was pronounced above 1100°C and the density increased linearly as a function of holding time. Microhardness of membranes sintered above 1100°C increased as a function of sintering temperature and was higher in unsupported membranes. Samples sintered above 975°C had a100% tetragonal phase structure. Permeability of supported membranes increased as a function of sintering temperature due to pore growth despite a decrease in porosity.

Electrostatic and electrosteric stabilization of aqueous slips of 3Y–ZrO 2 powder

Yttria-doped zirconia powder (3Y-ZrO 2) was characterized and dispersed in distilled water. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the slip. Zeta potential, apparent viscosity and the mean particle size as a function of pH indicated the pH range of electrostatic stabilization. These results showed that electrostatic stabilization of the slip can be accomplished in low acidic and high basic range of pH. Dissolution of yttria from the powder surface in acidic pH was found to be high and fast, risky to the stability of the slip as well as the microstructure of the sintered body. Electrosteric stabilisation by addition of an anionic polyelectrolyte (PMAA-NH 3) shifted the isoelectric point (IEP) to lower pH. The state of dispersion was further investigated by particle size measurements of the solid phase in the slip. The optimum amount of dispersant is discussed in terms of zeta potential and viscosity. The adsorption of polyelectrolyte is considered at pH 4, native pH of suspension and at pH 10, below and above the IEP of powder and correlated with the dissociation rate of polymer and the net surface charge of particles at given pH. Higher adsorption of dissociated polymer to the positively charged surfaces justifies the higher optimum amount of polyelectrolyte at pH 4.

Detection of carbon residue on the surface of 3Y–ZrO2 powder and its effect on the rheology of the slip

The surface composition and rheology of two yttria doped zirconia powders (3Y-ZrO2) were investigated in aqueous suspensions using electron spectroscopy for chemical analysis (ESCA), infrared spectroscopy, potentiometric titration, electrophoretic mobility, and rheological measurements. Both physisorbed and chemisorbed carbon were detected on the surface of powders in different extent. The point of zero charge being different from the isoelectric point (iep) indicated the effect of carbon on the surface charge and likely the specific adsorption of cations inside the slipping plane. Infrared spectroscopy detected both physisorbed and chemisorbed carbon on the surface. Chemisorbed carbon also affected the rheology of the slips especially in acidic suspensions. Contact angle and surface energy measurements showed that powder with chemisorbed carbon has low surface energy and exhibited a hydrophobic surface.

X-ray diffraction study of compositionally homogeneous, nanocrystalline yttria-doped zirconia powders

The crystal structure of compositionally homogeneous, nanocrystalline ZrO2 - X mol% Y2O3 (X = 2.8, 4, 5, 6, 7, 8, 9, 10, 11, and 12) powders synthesized by a nitrate-citrate gel-combustion process has been studied by X-ray diffraction. By applying the Rietveld method, it was found that all the powders presented the tetragonal phase (P4 2/nmc space group). The axial ratio c/a f decreased with increasing Y2O3 content and became almost unity at 9 mol% Y2O3. However, powders in the compositional range of ZrO2 - 9 to 12 mol% Y2O3 exhibited the oxygen atoms displaced from their ideal sites of the cubic phase along the c axis, which is known as the t′′-form of the tetragonal phase. A conjunct analysis of crystallite size and microstrain of all the powders is also presented.

Synthesis of nanocrystalline zirconia powders for TZP ceramics by a nitrate–citrate combustion route

A nitrate–citrate combustion route to synthesise nanocrystalline yttria-doped zirconia powders for tetragonal zirconia polycrystal (TZP) ceramics is presented. This route is based on the gelling of nitrate solutions by the addition of citric acid and ammonium hydroxide, followed by an intense combustion process due to an exothermic redox reaction between nitrate and citrate ions. X-ray diffraction characterisation of powders showed the stabilisation of the tetragonal phase at room temperature because of their small crystallite size (about 10 nm). Dense ceramic samples prepared by uniaxial pressing and sintering in air were also studied.

5YSZ powders from gels: Densification and microstructure characterization

Five mol% yttria doped zirconia powders were prepared by controlled hydrolysis of alkoxides. Their sintering behavior was monitored by dilatometry to optimize the sintering schedule. The compact bodies were characterized by SEM and quantitative analysis of their X-ray patterns. Re-sintering under axial pressure slightly improved the final relative density of the zirconia powders at the expense of a grain size increase that, however, for most of them remains submicrometric.

Dispersibility of yttria-doped zirconia powders in aqueous media

Dispersibility of yttria-doped zirconia powders in aqueous media

Theoretically dense and nanostructured ceramics by pressureless sintering of nanosized Y-TZP powders

Nanosized yttria-doped tetragonal polycrystalline zirconia (Y-TZP) powder was prepared by the coprecipitation method using isopropanol solutions of organic and inorganic precursors. A isopropanol-washing step led to a powder mainly constituted by very soft agglomerates with 6 nm crystallite size. Such a weakly agglomerated powder compacted uniformly (average diameter pore size = 6 nm) with a homogeneous shrinkage to full density. The small particle size of the Y-TZP powder, the small pores in the green compacts, and its homogeneous pore size distribution led to theoretically dense and nanostructured (< 100 nm) Y-TZP bodies by pressureless sintering at 1000°C for 80–100 h or at 1070°C for 2 h, in spite of the relative low green density (~43% theoretical). Comparatively, a commercial powder consisting of spherical weak agglomerates resulted in a fully densified ceramic but at higher sintering temperature (> 1200°C) and with a grain size higher than 0,25 mm.

Characterization of yttria-doped zirconia powders produced by plasma-chemical method

Ultrafine non-doped and yttria-doped zirconia (ZY) powders have been produced by a single-step plasma synthesis method. The amount of yttria doping was varied between 0 and about 10 mol%. The phase composition, structural parameters and morphology of the as-prepared powders have been examined by X-ray diffraction, neutron diffraction and electron microscopy. The average particle size of the produced powders was found to be around 30 nm, changing little with composition. The particles were mostly spherical but a large number of faceted single crystals were also present, as revealed by high resolution transmission electron microscopy. The ZY ultrafine powders were mixtures of a cubic and a non-transformable tetragonal zirconia phases. The amount of the tetragonal phase decreased with the overall yttria content but with a composition that remained almost the same. Neutron diffraction revealed diffuse scattering located at the positions of tetragonal maxima forbidden for cubic structure. The origin of diffuse scattering is associated with oxygen vacancies, either due to relaxation around Y 3+ ions or due to ordering of vacancies. X-ray diffraction showed no diffuse scattering.

Short-range order of yttria doped zirconia powders studied by X-ray absorption (II)

The atomic structure of polymorphic zirconia powders doped with 6 mol.% yttria prepared by controlled hydrolysis of alkoxides has been studied by X-ray absorption spectroscopy. The short-range orders before and after heat treatment have been compared. The crystallization and incorporation of the yttrium atoms into the network have been monitored. Rietveld analysis of X-ray diffraction data has been used to determine the relative amounts of crystalline phases present in the sample.

Crystallite growth in yttria-doped superfine zirconia powders and their compacts: A comparison between Y-TZP and YSZ

Crystallite growth characteristics in Y-TZP (yttria-tetragonal zirconia polycrystals) and YSZ (yttria-stabilized cubic zirconia) powders and their compacts were compared. When holding at ≤ 1000 °C the crystallite size of Y-TZP is greater than YSZ both in powders and in compacted bodies, but the crystallites of YSZ grow faster than Y-TZP at elevated temperatures and the grain size of YSZ becomes larger when heated at 5 °C/min to above 1200–1250 °C or so, both in powders and in their compacts. It is proposed that this phenomenon results from the different crystallite surface or interface energy between different crystal phases, which affects the initial crystallite size in nucleation at crystallization temperature and crystallite/grain growth rates at higher temperatures.

Short-range order of yttria doped zirconia powders study by X-ray absorption (I)

The atomic structure of polymorphic zirconia powders doped with yttria 6 mol% prepared from ZrOCl 2 · 8H 2O and Y 2O 3 has been studied by X-ray absorption spectroscopy. The short-range orders before and after heat treatment have been compared. The crystallization and incorporation of the yttrium atoms to the network have been monitored.

Preparation and properties evaluation of zirconia-based/Al2O3 composites as electrolytes for solid oxide fuel cell systems

Yttria-doped zirconia powders containing 3 to 8 mol% Y 2 O 3 and 0 to 20 wt% Al 2 O 3 were prepared by both mixing commercial oxides and a coprecipitation method, and the mechanical and electrical properties have been examined as a function of the Al 2 O 3 content. The bending strength of the composite at room temperature increased with increasing Al 2 O 3 content. In the temperature range 500-1000°C the bending strength increased with Al 2 O 3 content up to 10 wt% and then decreased, the measured value at 1000°C (200 MPa) being higher than those at lower temperatures for cubic zirconia materials. Fracture toughness (K IC ) decreased with increasing Y 2 O 3 content in the Al 2 O 3 -free zirconia materials. Al 2 O 3 additions enhanced the fracture toughness and this was maximum (7 MPa m 1/2 ) for the composite ZrO 2 -3 mol% Y 2 O 3 /10 wt% Al 2 O 3 . The electrical conductivity of cubic ZrO 2 /Al 2 O 3 composites decreased monotonically with Al 2 O 3 content, but in tetragonal ZrO 2 /Al 2 O 3 composites hardly varied or apparently increased up to 10 wt% Al 2 O 3 . At 1000°C the highest electrical conductivity was 0.30 S cm -1 for ZrO 2 -8 mol% Y 2 O 3 , and this decreased up to 0.10 S cm -1 for the composite ZrO 2 -8 mol% Y 2 O 3 /20 wt% Al 2 O 3

Preparation and properties evaluation of zirconia-based/Al2O3 composites electrolytes for solid oxide fuel cell systems

The compaction behaviour of ultrafine yttria-doped zirconia powders (6–8 nm) without and with alumina additions (0 to 20 wt%) has been studied. From the pore size distribution and using isothermal and nonisothermal techniques, the sintering behaviour of zirconia compacts in the temperature range 800–1500 °C was studied. It was found that alumina additions (up to 10 wt%) enhanced the zirconia compacts' densification process and, above that alumina content, that process was retarded. Alumina additions did not affect the grain grown process in tetragonal zirconia samples. However, this was strongly hindered in the fully stabilized zirconia ones. The results were compared with those obtained in the same experimental conditions on a commercial zirconia powder.

A combustion synthesis process for synthesizing nanocrystalline zirconia powders

Materials with nanocrystalline features are expected to have improved or unique properties when compared to those of conventional materials. Methods for the practical and economical production of nanoparticles in large quantities are not presently available. A method based on combustion synthesis for preparing nanocrystalline powders was investigated in this work. Yttria-doped zirconia powders with an average crystallite size of 10 nm were synthesized. The characteristics of the powder (e.g., surface area and phase content) were found to depend strongly on the fuel content in the starting mixture and on the ignition temperature used in the process. The method is expected to be suitable for commercial fabrication of nanocrystalline multicomponent oxide ceramic powders.