Pure Monoclinic Zirconia Research Articles

Impact of radiation defects on the structural stability of pure zirconia

Optical spectroscopy and x-ray diffraction were used to study the behavior of polycrystalline samples of pure monoclinic zirconia irradiated by low energy ions. A microscopic model based on these experiments is proposed to explain the displacive phase transition observed in this material after irradiation. Defects, produced in the oxygen sublattice, induce important strain fields on a nanometric scale. This strain field can be handled as a secondary order parameter within the Landau theory approach, leading to a decrease of the phase transition temperature and thus quenching the high temperature tetragonal phase. The model also explains the consequences of the thermal annealing for restoring the ground state monoclinic structure.

Viscous flow sintering of bioactive glass-ceramic composites toughened by zirconia particles

In order to widen the application fields of a bioactive glass-ceramic (Bioverit 1 I), the possibility of preparing a glass-ceramic matrix biocomposite, toughened by zirconia particles, was investigated. Two kinds of biocomposites were prepared, using as reinforcing phase tetragonal yttria-stabilised zirconia (ZrO2–3%Y2O3) and pure monoclinic zirconia particles, respectively. The composites were prepared by viscous flow sintering. The sintered composites were characterised by optical and Scanning Electron Microscopy (SEM), Energy Dispersion Spectroscopy (EDS) and X-Ray Diffraction (XRD). A mechanical characterisation of the sintered samples was carried out by means of Vickers indentations, Young’s modulus measurements, fracture toughness and compressive strength determinations. The chemical stability of the samples was evaluated by soaking them in Ringer solution. # 2002 Published by Elsevier Science Ltd.

Tetragonal structure, anionic vacancies and catalytic activity of SO 42−-ZrO 2 catalysts for n-butane isomerization

An assessment of the influence of the crystal structure, surface hydroxylation state and previous oxidation/reduction pretreatments on the activity of sulfate-zirconia catalysts for isomerization of n-butane was performed using crystalline and amorphous zirconia supports. Different sulfation methods were used for the preparation of bulk and supported SO 4 2−-ZrO 2 with monoclinic, tetragonal and tetragonal+monoclinic structures. Activity was important only for the samples that contained tetragonal crystals. The catalysts prepared from pure monoclinic zirconia showed negligible activity. SO 4 2−-ZrO 2 catalysts prepared by sulfation of crystalline zirconia displayed sites with lower acidity and cracking activity than those sulfated in the amorphous state. Prereduction of the zirconia samples with H 2 was found to greatly increase the catalytic activity, and a maximum rate was found at a reduction temperature of 550–600 °C, coinciding with a TPR peak supposedly associated with the removal of lattice oxygen and the creation of lattice defects. A weaker dependence of catalytic activity on the density or type of surface OH groups on zirconia (before sulfation) was found in this work. A model of active site generation was constructed in order to stress the dependence on the crystal structure and crystal defects. Current and previous results suggest that tetragonal structure in active SO 4 2−-ZrO 2 is a consequence of the stabilization of anionic vacancies in zirconia. Anionic vacancies are in turn supposed to be related to the catalytic activity for n-butane isomerization through the stabilization of electrons from ionized intermediates.

Analysis of the monoclinic–tetragonal phase transition of zirconia under irradiation

Zirconia produced by the oxidation of zirconium alloys in nuclear reactors exhibits a phase transition under ionic irradiation, simulating a neutron irradiation. To understand the mechanism responsible for this irradiation driven phase transition, different kinds of projectiles were used to irradiate pure monoclinic zirconia samples. The evolution of these irradiated samples as a function of dpa has been studied using grazing X-ray diffraction. The Rietveld method has been applied on collected X-ray diffraction diagrams to study the phase produced under irradiation and the kinetics of its formation. Even at high dpa values, only the monoclinic and tetragonal phases were used to simulate X-ray diffraction diagrams. No amorphisation of zirconia was observed. The evolution of unit cells and short range strains in both phases under irradiation leads us to think that the irradiation driven transition is martensitic. Supposing that the inelastic stopping power in sub-cascades is responsible for the irradiation driven phase transition, we propose a model based on the Landau–Ginzburg effective hamiltonian to explain both the m→ t transition observed under irradiation and the t→ m transition measured during isochronal annealing after irradiation.

Investigation on the zirconia phase transition under irradiation

Zirconia, ZrO 2, produced by the oxidation of zirconium alloys in nuclear reactors, possesses a high stability under neutron irradiation. No amorphisation of yttrium-stabilised zirconia has been observed even at high dpa values (≈100 dpa). In pure monoclinic zirconia, a phase transition monoclinic → cubic (tetragonal) induced by irradiation has already been observed. The aim of this work is to study in detail the mechanism responsible for this transition. For that purpose, different kinds of irradiations with electrons (to study point defects) and low energetic ions (to study clusters due to collision cascades) have been performed on zirconia samples. A local probe (Raman spectroscopy) and a non-local probe (grazing X-ray diffraction) have been used to characterise the phase formed during irradiation, which is clearly the tetragonal phase. For the ionic implantation, the grazing X-ray diffraction permits to separate effects due to the ballistic collisions and the implantation peak. Using this method, it was possible to show that the profile of the tetragonal phase was only linked to the dpa profile. This result associated to the results obtained by the Raman spectroscopy (broadening of Raman peaks) shows that the phase transition may be induced by clusters formed near the collision cascades.

Structural properties and quasiparticle band structure of zirconia

We report ab initio calculations of the structural and quasiparticle properties of ${\mathrm{ZrO}}_{2}$, otherwise known as zirconia. The plane-wave pseudopotential method is used to compute the structural properties of the cubic, tetragonal, and monoclinic phases of zirconia. Oxygen vacancies in the cubic phase are also studied using a supercell approach. The structural parameters, including all internal degrees of freedom of all phases, are relaxed. Excellent agreement is achieved with experiment and with other ab initio calculations available. We compute the quasiparticle band gaps within Hedin's $\mathrm{GW}$ approximation using the method of Hybertsen and Louie, and confirm that the quasiparticle approach can be successfully applied to transition-metal oxides if the core-valence overlap is small. We predict the fundamental gap of pure cubic, tetragonal, and monoclinic zirconia to be 5.55 eV, 6.40 eV, and 5.42 eV, respectively. Within the $\mathrm{GW}$ approximation, the oxygen vacancy state in the cubic phase is found to be nondegenerate, fully occupied, and well separated from the valence and conduction bands, positioned 2.1 eV below the conduction band edge.

On Raman spectroscopy of zirconium oxide films

Using Raman spectroscopy, we studied samples of pure tetragonal and monoclinic zirconia powders, and oxide film developed at 400°C in steam (autoclave) on Zircaloy-4 with various tin contents. We analyze the absolute intensities of the bands and background, as well as the position of the bands. All these parameters bring some surprising evolution with tin content and oxidation time, relatively to the powders. We have shown that the films are mainly monoclinic, but that the second phase is not similar to the tetragonal phase observed in powders. It is probably a distorted tetragonal phase. The oxide films are not homogeneous through their thickness, being better crystallized near the surface. This prevents to compute directly any relevant tetragonal fraction from the band intensities. The presence of tin induces a high background, probably related to the existence of numerous oxygen vacancies. The absorption coefficient of the films at 514 nm is about 0.4 μm −1.

Raman study of phase transitions in pure and alloyed zirconia induced by ball-milling and a laser beam

High-temperature phases of ZrO 2 were obtained at room temperature by incorporating a few mole fractions of other oxides (MgO, Y 2O 3 or CoO) in a zirconia crystal lattice using high-energy ball-milling. Raman spectra obtained after an adequate period of ball-milling show the formation of ZrO 2MgO and ZrO 2Y 2O 3 cubic solid solutions and a ZrO 2CoO tetragonal solid solution. However, in the case of pure monoclinic zirconia, the transformation from a monoclinic to a tetragonal structure could not be completed by the use of ball-milling only. The spectra of fully stabilized solid solutions did not change essentially with laser power, while insufficiently milled samples underwent phase changes when the laser power increased. In pure milled zirconia, annealing effects were observed.

Acidity of sulfated zirconia as studied by FTIR spectroscopy of adsorbed CO and NH3 as probe molecules

The acid properties of pure and sulfated zirconias were studied by FTIR spectroscopy of adsorbed CO and NH3 as probe molecules. Whereas pure monoclinic zirconia shows only Lewis acidity, sulfation of tetragonal and monoclinic zirconia creates new bridging OH groups. Two types of Bronsted-acidic centers and two types of Lewis-acidic centers with enhanced acid strength were identified. A communication between the different types of Lewis-acidic sites and the related adsorbed sulfate molecules could be shown. The coordination of basic molecules such as CO onto Lewis-acid sites induces a decrease of the intrinsic Bronsted-acidity of the bridging OH groups. These effects are discussed on the basis of a model of the acidic centers that was previously proposed.

A procedure for achieving single crystal-like zirconia particles

The paper contains an improvement of a previously reported procedure aimed at obtaining pure phase monoclinic zirconia powders. The former method, implying the combination of calcination and grinding stages of hydrothermal precursors, determined the presence of surface roughness and chemical surface defects. By modulating duration and alternation of mechanical and firing treatments, the present procedure determines the formation of single crystal, smooth particles, with regularly coordinated surface cations.

A discussion on advanced methods of energy conversion- Magnetohydrodynamic power generation - New ceramics based on monoclinic zirconia

In m.h.d. generators the mechanism of electrical conductivity of the electrodes should be electronic with a value of about 100 mho/m at 2000 °K. In addition, the material should have good thermal shock resistance. Calcia- or yttria-stabilized zirconia has sufficiently good electrical conductivity and thermal shock resistance but since their conduction mechanism is ionic they are spoilt by the passage of direct current because of electrolysis. The conductivity of pure monoclinic zirconia is mainly electronic and at 2000 °K has a value of about 50 mho/m but this declines very sharply as the temperature falls. Also the phase change at 1500 °K; from a monoclinic to tetragonal form with a concomitant volume shrinkage of 9% is mechanically destructive and precludes the use of this material for making electrodes. A new ceramic material has been designed to overcome these difficulties. A combination of zirconia, a second ceramic (which may be an insulator) and a metal is used. The metal provides electrical conductivity below 1800 °K; the second ceramic material acts as a diluent to the zirconia and reduces the effect of the phase change at 1500 °K sufficiently to prevent mechanical failure of the material, whilst the zirconia itself provides the hot electrode surface and the electrical conduction at the high temperature.

Electrical Resistivity and Diffusionless Phase Transformations of Zirconia at High Temperatures and Ultrahigh Pressures

The resistivity of chemically pure monoclinic zirconia has been investigated at pressures to 45,000 bars and temperatures to 1200°C. Measurements were made on thin, prepressed wafers of situated between the opposed, cemented carbide Bridgman anvils of a uniaxial Griggs and Kennedy simple squeezer. Observation of discontinuous changes in electrical resistance at various temperatures under isobaric conditions have been used to define the phase diagram. The pressure dependence of the temperature of the reversible monoclinic ⇌ tetragonal phase transition in zirconium dioxide agreed with the slope of the P‐T equilibrium curve up to 15 kilobars as calculated from thermodynamics, i.e., . Activation energy for electrical conduction followed the Arrhenius equation and was found to be pressure dependent, decreasing from 3.56 ev at atmospheric pressure to an apparently constant value of 0.86 ev from 30 to 45 kilobars.