Metastable ZrO2 Research Articles

Rationally constructing metastable ZrO2 supported Ni catalysts for highly efficient and stable dry reforming of methane

The metastable Ni/ZrO2 catalysts were rationally constructed via reflux treatment and employed for dry reforming of methane. Reflux could suppress the transformation and improve the stability of the metastable ZrO2, as well as enrich its pore structure. Meanwhile, acidity-basicity, Zr3+ and oxygen vacancies were also promoted. Additionally, Ni dispersion was remarkably improved with increasing reflux time due to the abundant pores and enhanced metal-support interaction. These results illustrated the evaluation that prolonging reflux time could significantly proliferate the performance of Ni/ZrO2. Besides, reflux could strengthen the anti-carbon ability but the widened gap between CH4 dissociation and CO2 activation would cause more carbon deposition. The catalyst with reflux for 240 h showed the robust stability for 9000 min (CH4 conversion: 80 %; CO2 conversion: 88 %) at 750 °C. Furthermore, the mechanisms were discussed to unravel the origin of the accelerated performance by reflux and provide some references for designing efficient Ni-based catalyst.

Voltage-controlled magnetic anisotropy effect through a high-k MgO/ZrO2/MgO hybrid tunneling barrier

We investigated the voltage-controlled magnetic anisotropy (VCMA) effect in epitaxial magnetic tunnel junctions (MTJs) with a hybrid MgO/ZrO2/MgO tunnel barrier. A metastable cubic ZrO2(001) thin film was successfully grown on a MgO(001) layer, leading to the high dielectric constant of 26.5. Using the hybrid tunneling barrier, we achieved the large VCMA coefficient of −350 fJ V−1 m−1, which is 70% larger than that observed in the magnetic tunnel junction with the single MgO barrier. Introduction of crystalline high-k dielectric tunneling barrier can open up new pathways to improving the VCMA properties in MTJs for voltage-driven spintronic devices.

Effect of Heat Treatment on the Structure and Phase Composition of the Nanosized Powder Based on a ZrO2 Solid Solution

The nanosized 90 mol.% ZrO2–2 mol.% Y2O3–8 mol.% CeO2 powder was produced by hydrothermal synthesis in an alkaline environment and heat-treated in the range 400–1300°C. The powder properties were examined by X-ray diffraction (XRD), SEM and TEM, petrography, and BET. According to the XRD data, a low-temperature metastable cubic ZrO2 (F-ZrO2) solid solution formed after hydrothermal synthesis. According to the petrography and electron microscopy data, TZrO2 began to form already in the hydrothermal synthesis process. The F-ZrO2 → T-ZrO2 phase transformation was completed in the range 700–850°C. Some T-ZrO2 particles were characterized by a twin substructure. The T-ZrO2 unit cell volume monotonically increased from 133.58 · 10–3 nm3 to 137.09 · 10–3 nm3 and the degree of tetragonality from 1.0033 to 1.0140. No M-ZrO2 was found to form. The powder specific surface area decreased from 94 to 2 m2/g in the heat treatment process. The sizes of primary powder particles (5–10 nm) remained almost unchanged in heat treatment up to 1150°C. The Vickers hardness of the ceramics produced from the powder treated at 850°C was 3.1 GPa and critical fracture toughness factor KIc was 8.4 MPa · m1/2. The preservation of the tetragonal structure (T-ZrO2), which is capable of the martensitic T-ZrO2 → M-ZrO2 transformation, and the strength characteristics determined open ways for microstructural design of smart materials, including shape memory ones, in the ZrO2–Y2O3–CeO2 system.

A simple approach to achieve a metastable metal oxide derived from carbonized metal-organic gels.

A strategy for fabricating zirconium oxide doped nanoporous carbons derived from metal-organic gels (MOGs) is reported. For the first time, the achievement of metastable ZrO2-NPCs is demonstrated. The direct pyrolysis of MOGs provided new perspective in developing metal oxide-doped NPCs and circumvented the shortcomings of existing methods in synthesizing metastable ZrO2 doped NPCs.

Effect of drying conditions on the textural, structural and catalytic properties of Cr/ZrO2–SO4: n-hexane conversion

Aerogel and xerogel catalysts based on sulfated zirconia doped with chromium are prepared by sol–gel method. The effects of drying mode and calcination temperature are reflected in the characteristics of the final solids. The addition of chromium to aerogel sulfated zirconia not only provides an important development of the specific surface area from 100 to 213 m2 g−1; but also stabilizes a pure metastable ZrO2 tetragonal phase in a large calcination temperature domain. In addition, the aerogels develop active sulfate groups and preserve them on the surface even at high heating. Those characteristics contribute to the improvement of catalytic performances of sulfated zirconia in n-hexane conversion. However, in the case of xerogels, the increase of temperature treatment leads to the loss of sulfate active groups and porosity destruction which make them completely inactive in spite of the development of ZrO2 tetragonal phase.

Microstructure Evolution During Spark Plasma Sintering of Metastable (ZrO2–3 mol% Y2O3)–20 wt% Al2O3 Composite Powders

Metastable ZrO2 (3 mol% Y2O3)–20 wt% Al2O3 oxide composite powders are obtained by liquid nitrogen quenched plasma spraying of commercially available powders. During passage of the plasma, Al2O3, ZrO2, and Y2O3 are molten and rapidly solidified upon impact on the liquid nitrogen cooled substrate. Aluminum cations are nearly completely incorporated into zirconia leading to the stabilization of the cubic zirconia phase. Spark plasma sintering of these powders is successfully performed to obtain a dense composite with grain sizes in the range of 300–500 nm. Phase separation of the immiscible phases accompanied by homogeneous precipitation of Al2O3 occurs during sintering. Because of the short holding times applied during densification, complete phase separation may be suppressed and the metastable character is partially retained. The detected Al2O3 content in ZrO2 is still larger than the equilibrium solubility. Phase evolution during all phases of the process is recorded by means of X‐ray diffraction, scanning as well as transmission electron microscopy, and Raman spectroscopy.

Effect of Al2O3 on the properties of nanocrystalline ZrO2 + 3 mol % Y2O3 powder

We have studied the properties of nanocrystalline ZrO2〈3 mol % Y2O3〉 and 90 wt % ZrO2〈3 mol % Y2O3〉-10 wt % Al2O3 powders prepared via hydrothermal treatment of coprecipitated hydroxides at 210°C. The results demonstrate that Al2O3 doping raises the phase transition temperatures of the metastable low-temperature ZrO2 polymorphs and that the structural transformations of the ZrO2 and Al2O3 in the doped material inhibit each other.

Effects of Annealing Temperature on Microstructural Development at the Interface Between Zirconia and Titanium

The interfacial reaction layers in the Ti/ZrO2 diffusion couples, isothermally annealed in argon at temperatures ranging from 1100° to 1550°C for 6 h, were characterized using scanning electron microscopy and transmission electron microscopy, both attached with an energy‐dispersive spectrometer. Very limited reaction occurred between Ti and ZrO2 at 1100°C. A β′‐Ti(Zr, O) layer and a two‐phase α‐Ti(O)+β′‐Ti(Zr, O) layer were found in the titanium side after annealing at T≥1300°C and T≥1400°C, respectively. A three‐phase layer, consisting of Ti2ZrO+α‐Ti(O, Zr)+β′‐Ti (O, Zr), was formed after annealing at 1550°C. In the zirconia side near the original interface, β′‐Ti coexisted with fine spherical c‐ZrO2−x, which dissolved a significant amount of Y2O3 in solid solution at T≥1300°C. Further into the ceramic side, the α‐Zr was formed due to the exsolution of Zr out of the metastable ZrO2−x after annealing at T≥1300°C: the α‐Zr was very fine and dense at 1300°C, continuously distributed along grain boundaries at 1400°C, and became coarsened at 1550°C. Zirconia grains grew significantly at T≥1400°C, with the lenticular t‐ZrO2−x being precipitated in c‐ZrO2−x. Finally, the microstructural development and diffusion paths in the Ti/ZrO2 diffusion couples annealed at various temperatures were also described with the aid of the Ti–Zr–O ternary phase diagram.

Microstructural Evolution and Formation Mechanism of the Interface Between Titanium and Zirconia Annealed at 1550°C

The diffusional reaction between titanium and zirconia was carried out isothermally at 1550°C in argon. The distinct reaction layers in the reaction‐affected zone between Ti and ZrO2 were investigated using analytical scanning electron microscopy, analytical transmission electron microscopy, and electron probe microanalyses. In the metal side, there existed five reaction layers in a sequence of α‐Ti(O), Ti2ZrO+α‐Ti(O, Zr), Ti2ZrO+α‐Ti(O, Zr)+β′‐Ti (O, Zr), α‐Ti (O, Zr)+β′‐Ti (O, Zr), and β′‐Ti (Zr, O) after cooling. In the zirconia side, two reaction layers were found: near the original interface, β′‐Ti coexisted with fine spherical c‐ZrO2−x and Chinese‐script‐like c‐ZrO2−x, which dissolved a significant amount of Y2O3 in solid solution; further away from the original interface, the coarsened intergranular α‐Zr was excluded from metastable ZrO2−x, resulting in the lenticular t‐ZrO2−x and ordered c‐ZrO2−x. An attempt was made to determine and propose the microstructural evolution and formation mechanism of the reaction layers between titanium and zirconia isothermally annealed at 1550°C.

Applied Magnetic Field Rejects the Coating of Ferromagnetic Carbon from the Surface of Ferromagnetic Cobalt: RAPET of CoZr2(acac)2(OiPr)8

We present the results of the RAPET (reaction under autogenic pressure at elevated temperatures) dissociation of CoZr(2)(acac)(2)(O(i)Pr)(8) at 700 degrees C in a closed Swagelok cell under an applied magnetic field of 10 T. It produces a mixture of carbon-coated and noncoated metastable ZrO(2) nanoparticles, bare metallic Co nanoparticles, and bare carbon. The same reaction in the absence of a magnetic field produces spherical Co and ZrO(2) particles in sizes ranging from 11 to 16 nm and exhibiting, at room temperature, metastable phases: fcc for cobalt and a tetragonal phase for zirconia. The metastable phases of Co and ZrO(2) are manifested because of a carbon shell of approximately 4 nm thickness anchored to their surfaces. The effect of an applied magnetic field to synthesize morphologically different, but structurally the same, products is the key topic of the present paper.

Thermal behavior of the amorphous precursors in the ZrO 2–CrO 1.5 system

The amorphous precursors of the ZrO 2–CrO 1.5 system at the ZrO 2-rich side of the concentration range were prepared by co-precipitation from aqueous solutions of the corresponding salts. Thermal behavior of the amorphous precursors was monitored using differential thermal analysis, X-ray powder diffraction (XRD), Raman spectroscopy and Fourier transform infrared spectroscopy. The crystallization temperature of the amorphous precursors increased with an increase in the CrO 1.5 content, from 435 °C (0 mol% of CrO 1.5) to 822 °C (50 mol% of CrO 1.5). The results of phase analysis indicated an extended capability for the incorporation of Cr 3+ ions into the metastable ZrO 2-type solid solution obtained after crystallization of amorphous co-gels. Maximum solubility of Cr 3+ ions in the ZrO 2 lattice, obtained after crystallization of the amorphous precursors, was estimated at ∼26 mol%, but significantly decreased with further temperature treatment. The results of XRD and Raman spectroscopy showed that the incorporation of Cr 3+ ions partially stabilized only the tetragonal ZrO2 polymorph. However, precise determination of lattice parameters, performed using the whole-powder-pattern decomposition method, showed that the axial ratio c f/ a f in the t-ZrO 2-type solid solutions with Cr 3+ content ≥20 mol% approach 1. The incorporation of Cr 3+ ions caused a decrease in the unit-cell volume of the t-ZrO 2-type solid solutions, but the rate of the decrease turned out to be smaller than the rate obtained after the incorporation of Fe 3+ ions. The reason for such behavior was discussed.

Reconstructive phase formation of ZrO 2 nanoparticles in a new orthorhombic crystal structure from an energized porous ZrO(OH) 2· xH 2O precursor

A high energy amorphous ZrO(OH) 2· xH 2O precursor (porous) is obtained by a controlled hydrolysis of dispersed Zr 4+ cations in an aqueous solution in reaction with NH 4OH at 5 °C followed by aging and drying of the recovered gel at room temperature. At 20–40 °C in air, the excess H 2O evaporates leaving behind a porous ZrO(OH) 2· xH 2O powder with Φ ∼37% porosity. X-ray diffraction characterizes its amorphous structure with two halos at 20.8 and 36.7 nm −1 wavevectors. On heating, a reconstructive ZrO 2 phase transformation occurs from a refined amorphous state at 500 °C to form a new orthorhombic structure with a=0.3340, b=0.5535 and c=0.6364 nm lattice parameters. Average crystallite size is D∼15 nm. This irreversibly transforms to m-ZrO 2 at D⩾23 nm at 800 °C or above. A schematic energy level diagram for the precursor and the metastable ZrO 2 polymorphs is proposed with the observation of the phase transformations.

Fabrication of Metastable Oxide Ceramics with Oxygen Vacancy

Metastable ZrO 2 -x particles were successfully prepared by inert gas condensation (IGC) process. To control oxygen vacancies in metastable ZrO 2 -x, samples were annealed between 100°C and 400°C for 2 h under low oxygen partial pressures (PO 2 = 10-15 Pa). Therefore, metastable ZrO 2 -x samples with various oxygen vacancies (0.26 < x < 0.81) were successfully fabricated by IGC method; also, the particle sizes were less than 10 nm.

Fabrication of metastable ZrO2-x single nano-sized particles

Metastable ZrO2−x particles were successfully prepared by inert gas condensation (IGC) process. To control oxygen vacancies in metastable ZrO2−x, the samples were annealed between 100 and 400 °C for 2 h under low oxygen partial pressures (PO2=10−15 Pa). After annealing, metastable ZrO2−x samples with various oxygen vacancies (0.26<x<0.81) were successfully fabricated; also, the particle sizes were less than 10 nm.

Synthesis and structural and optical properties of metastable ZrO2 nanoparticles with intergranular Cr3+/Cr4+ doping and grain surface modification

Nanoparticles of stabilized ZrO2 in a single cubic (c) phase are obtained with an intergranular doping of 4 to 20 at.% Cr3+/Cr4+ additives by a chemical method using a high energy amorphous precursor with polymer molecules of sucrose and polyvinyl alcohol. In the polymer, the metal cations disperse and rearrange in a specific network structure with local symmetry probably similar to that in c-ZrO2. On heating at 250 to 800°C in air, the polymer network decomposes and burns out spontaneously (with a strong exothermic peak over 350 to 500°C in thermal analysis) in a refined microstructure in 10 to 20 nm diameter particles of near spherical shape. Those are identified to be of c-ZrO2 by x-ray diffraction. A modified microstructure of 15 to 30 nm crystallites of dispersed tetragonal (t) and/or monoclinic (m) phases in stabilized c-ZrO2 develops on a prolong heating at 900 to 1000°C from a polymer precursor for 2 h or longer. Particles in t-ZrO2 are in acicular shape, as long as 450 nm with aspect ratio φ ∼5 to 20, while in the shape of platelets in m-phase in an average 300 nm size. It is found that the Cr3+/Cr4+ additives promote formation of c-ZrO2 by a controlled decomposition and combustion of precursor in small particles at 250 to 800°C temperature. Part of the additives form a thin amorphous surface layer in individual c-ZrO2 grains so that it prevents them to grow or transform in the equilibrium m-ZrO2 bulk structure as long as the temperature lies below 900°C. The x-ray diffraction in light of the optical spectrum reveals that part of the Cr4+ cations occupy Zr4+ sites in a distorted c-ZrO2 lattice.

Stabilized nanoparticles of metastable ZrO2 with Cr3+/Cr4+ cations: preparation from a polymer precursor and the study of the thermal and structural properties

Abstract Nanoparticles of stabilized ZrO2 in a single cubic phase by 5–20 at.% (of total metal cations) Cr3+/Cr4+ addition are obtained through a chemical method using a polymer matrix made of sucrose and polyvinyl alcohol. On heating at 250–900 °C in air, the polymer network decomposes and burns out leaving behind a dispersed microstructure in 10–25 nm diameter particles of cubic ZrO2 in spherical shape. A modified microstructure comprises of 10–14 nm crystallites of dispersed tetragonal phase, or both tetragonal and monoclinic phases, in cubic phase appear on a prolong (2 h or larger) heating of precursor at 900–950 °C. Particles in tetragonal ZrO2 are in acicular shape, while the monoclinic phase is in the shape of platelets. The Cr3+/Cr4+ additive facilitates formation of cubic phase in small particles on a controlled decomposition and combustion of precursor. It stabilizes small particles by inhibiting their growth by forming a thin amorphous surface layer over them.

Metastable ZrO2 crystal structures in electron beam evaporated thin films

Electron diffraction evidence for the existence of metastable ZrO2 in thin zirconia films grown by electron beam evaporation is presented. X-ray powder diffraction results show that the films are at the boundary of the amorphous/crystalline state of the matter. Micro-regions of unknown crystalline structures of ZrO2 are formed during the growth and are observed by transmission and reflection electron diffraction. Two CaF2-type zircon oxides exist inside the films. They are the known high temperature cubic phase with lattice parameter a = 5.09 ± 0.08 Å and an isostructural analogue with a = 5.40 ± 0.10 Å. Series of hexagonal and cubic close packed structures, believed to be related to the packing of clusters, are also detected. The existence of metastable zirconia structures is explained by Ostwald's rule of stages. A comparison of some of the metastable oxide structures of ZrO2 films with their isostructural analogues observed in SiO2 and SnO2 films is included.

Synthesis of nanometric zirconia powders by hydrothermal reactions of Zr (IV) peroxide precursors

Hydrothermal reaction of a Zr(IV) peroxide precursor in the temperature range 110-180°C was used to produce nanocrystalline zircon ia powders. Cubic metastable ZrO2 is formed with or without the presence of is formed with or without the presence of 2O3 or CeO2 as stabilising agents. After firing 2h at 1000°C the pure ZrO2 powder remains cubic while the doped powders transforms to tetragonal ZrO2. The influence of Zr concentration and treatment temperature on the crystallite sizes are presented. The kinetic constant for the nucleation of cubic zirconia phase was also calculated according to Avrami's kinetic equation.

Effect of SiC and ZrO 2 particles on the mechanical properties of NiAl

Composites of NiAl reinforced with silicon carbide and tetragonal zirconia particles were fabricated by hot-pressing, and the effects of SiC and ZrO 2 content on the room temperature mechanical properties were investigated. Results showed that SiC is not an efficient ceramic for the enhancement of mechanical properties of NiAl. By contrast, improvements in flexural strength and fracture toughness, to 735 MPa and 11.3 MPa√m, respectively, can be reached by incorporating 15 vol% of zirconia. These increases in performance may result from the low thermal expansion mismatch between the ceramic reinforcement and the intermetallic, and from the stress-induced transformation of the metastable ZrO 2 particles.

Structural Changes of Scandia‐Doped Zirconia Solid Solutions: Rietveld Analysis and Raman Scattering

Powder X‐ray diffractometry (XRD) and Raman scattering measurements were used to study the structural changes of compositionally homogeneous metastable ZrO2 solid solutions induced by ScO1.5 doping. The crystal structures of monoclinic, tetragonal, cubic, and rhombohedral (Sc2Zr7O17, ß‐phase) solid‐solution phases have been refined by using the Rietveld analysis of the XRD data at room temperature of arc‐melted ZrO2‐XScO1.5 (X= 0, 2, …, 20, and 22 mol%) samples. The results can be interpreted as indicating that the structures of the monoclinic and tetragonal phases approach those of the tetragonal and cubic phases, respectively, by ScO1.5 doping. Raman scattering, as well as XRD, was useful to investigate the phase assemblage. Moreover, we could obtain Raman spectra of the ß‐phase probably for the first time.