Laser-quality oxide Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> ceramics. Comparative studies of its basic characteristics and laser ceramics of a known manufacturer

The effect of sintering aids of SiO₂, ZrO₂, B₂O₃, and MgO oxides on the optimum sintering temperature, ceramics grain growth, total volume of residual pores, and optical quality of obtained ceramics is studied. The best combinations of sintering aids are found; as a result, YAG:Nd (1 at%) samples of ceramics of high optical quality are obtained. An original method for characterizing laser properties of ceramics is developed. Comparative measurements of main laser characteristics of the obtained ceramics and ceramics of the Konoshima Chemical Corp. Ltd wellknown in the world practice, are performed.     

Formation of Mg2Ni nanofibres in a Mg-based metal matrix composite

A Mg–Mg₂Ni nano-eutectic and MgO-reinforced Mg-based metal matrix composite (Mg-MMC) was synthesized by in-situ reactions. When a Mg-rich sample containing 20?wt%?NiO is sintered at 420°C, MgO, Ni and Mg₂Ni are formed in the Mg matrix. When a sample molten at 550°C is furnace-cooled to room temperature, a lamellar two-phase Mg–Mg₂Ni eutectic is formed, with the Mg₂Ni lamella about 1.5?µm thick. Proeutectic Mg together with the MgO formed in situ act as heterogeneous nuclei for the growth of the Mg–Mg₂Ni eutectic. Quenching the molten sample to room temperature results in a lamellar two-phase eutectic containing Mg₂Ni nanofibres. In the eutectic grown at the Mg grain, the Mg₂Ni nanofibres with a diameter of about 25?nm are bent, whereas in the eutectic grown at the MgO grain, the Mg₂Ni nanofibres with a diameter of about 30?nm are long and straight, while the Mg phase of this eutectic is oxygen enriched. The differences in morphologies between the two Mg–Mg₂Ni eutectics are ascribed to the growth rates of Mg and Mg₂Ni, which differ in the eutectic grown at the Mg grain, but remain similar when growth takes place at the MgO grain.     

Superconducting and structural properties of YBaCuO thin films deposited by inverted cylindrical magnetron sputtering

Thin films of YBaCuO have been deposited onto Al₂O₃, MgO, SrTiO₃ and ZrO₂ substrates by inverted cylindrical magnetron sputtering. The main advantage of this preparation technique is the high reproducibility of the results allowing systematic studies of the film properties as a function of deposition parameters. Optimum preparation conditions were a low discharge voltage (100 V), a high oxygen partial pressure (2×10^–1 Torr) in an oxygen argon mixture, and substrate temperatures around 800°C. The films grow highly textured on all substrates yielding single crystalline growth on SrTiO₃ and MgO as proved by X-ray diffraction and channeling. The zero resistance values of the sharp transtions usually are around 90 K. highest critical current densities were determined in films grown on MgO and SrTiO₃ with values up to 5.5×10⁶ A/cm² at 77 K.     

Reactivity and oxygen diffusion property of resistive barriers for Bi-2223/Ag tapes

Reactivity of several oxide materials (OM) with BSCCO powder and oxygen diffusion through OM layer has been tested at temperature ≈840 °C in air. The OM (e.g.: BaZrO₃, SrCO₃, MgO and ZrO₂) showing the low or no reactivity with BSCCO have been mixed (10 wt.%) with precursor powder and used for single-core tapes. Bi-2223/Ag/OM/Ag single-core tapes with oxide barriers made of BaZrO₃, SrCO₃, ZrO₂ and Al₂O₃ have been also prepared by a standard powder-in-tube technique. The used OM in the direct contact with BSCCO influences the electrical properties of Bi-2223 phase differently. These is because the oxides react with BSCCO during the heat treatment and simultaneously affect the 2212→2223 phase transformation, the Bi-2223 grain growth and so also grain connectivity. SrCO₃ powder has been evaluated as the best material from the point of no destructive effect on 2223 phase transport current property. The oxide barrier controls the oxygen diffusion during the tape heat treatment and simultaneously the HTS phase formation kinetics, its purity and content within the superconducting core. For single-core Bi-2223/Ag/OM/Ag tapes, the highest current density was measured for Al₂O₃ due to only slightly reduced oxygen diffusion through the barrier.     

Optimised NASICON ceramics for Na+ sensing

NASICON dense ceramics were obtained from solid state reaction between SiO₂, Na₃PO₄·12H₂O and two different types of zirconia: monoclinic ZrO₂ and the yttria-doped tetragonal phase (ZrO₂)_0.97(Y₂O₃)_0.03. Higher temperatures were needed to obtain dense samples of the yttrium free composition (1265 °C). The electrical conductivity, at room temperature, of the yttria-doped samples sintered at 1230 °C (0.20 S/m) is significantly higher than the value obtained with the material prepared from pure ZrO₂. The impedance spectra show that the differences in conductivity are predominantly due to the higher grain boundary resistance of the undoped ceramics, probably due to formation of monoclinic zirconia and glassy phases along the grain boundary. Further improvement of the electrical conductivity could be achieved after optimization of the grain size and density of grain boundaries. A maximum conductivity value of about 0.27 S/m at room temperature was obtained with the yttria-doped samples sintered at 1220 °C for 40 h. Yttria-doped and undoped ceramics were tested as Na⁺ potentiometric sensors. The detection limit of the yttria-doped sample (10⁻⁴ mol/l) was one order of magnitude lower than the obtained with the undoped material. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

High-Temperature Behavior of SiO2 at Grain Boundaries in TZP

The chemical reaction between SiO₂ and tetragonal zirconia polycrystal (TZP) was directly observed using a TEM in-situ heating technique in order to understand the behavior of SiO₂ in TZP at high temperatures. Their dynamic interaction was recorded up to about 1400°C using a CCD camera-video system connected to the TEM. Most of SiO₂ phase dissolved into the ZrO₂ grains above 1300°C. On the other hand, during cooling from the high temperature to around 400°C, amorphous SiO₂ reprecipitated from the surface of ZrO₂ grains and formed a thin layer around the ZrO₂ grains. This result agrees well with the fact that silicon segregates in the vicinity of grain boundaries in SiO₂-doped TZP. In order to confirm the grain boundary segregation at high temperatures, we investigated grain boundaries in quenched specimens by high resolution electron microscopy (HREM), energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS). It was found that no amorphous phase was present between two adjacent grains in the quenched samples. EDS analysis revealed that silicon segregated at the grain boundaries and that the segregation layer was wider than that in as-sintered specimens. The electron energy loss near edge structure (ELNES) of O K-edge was measured from both grain boundary and grain interior in quenched specimen, and their spectra were interpreted by a first principles molecular-orbital (MO) calculation using the discrete-variational (DV)-X method.     

Phase composition, microstructure and electrical properties of alumina-zirconia eutectic composites

Oxide eutectic composites prepared by a directional solidification are noticeable for their good mechanical properties and high temperature stability in oxidizing environments. In this paper we study the influence of stabilizers (Y, Sc) on the phase composition, microstructure and electrical properties of Al₂O₃ - ZrO₂ eutectic composites. At a hypereutectic composition of the melt, we have prepared the composites with tetragonal ZrO₂, which possesses conductivities comparable to those of the composites containing cubic ZrO₂. An addition of Sc₂O₃ improves the ionic conductivity of the Al₂O₃ - (Y₂O₃)ZrO₂ eutectic composite at high temperatures. Both the microstructure and ionic conductivity do not change along the composite (grown at a constant growth rate). Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Influence of deposition conditions on the microstructure of oxides thin films

Thin films of ZrO₂, HfO₂ and TiO₂ were deposited on kinds of substrates by electron beam evaporation (EB), ion assisted deposition (IAD) and dual ion beam sputtering (DIBS). Then some of them were annealed at different temperatures. X-ray diffraction (XRD) was applied to determine the crystalline phase and the grain size of these films, and the results revealed that their microstructures strongly depended on the deposition conditions such as substrate, deposition temperature, deposition method and annealing temperature. Theory of crystal growth and migratory diffusion were applied to explain the difference of crystalline structures between these thin films deposited and treated under various conditions.     

Surface oxidation behavior of MgNd alloys

The oxidation kinetics of MgNd alloys oxidized in pure O₂ at high temperatures has been investigated. The results revealed two stages of the reaction: A fast initial oxidation was followed by a slow oxide growth with a parabolic kinetics. For MgNd alloys (Nd = 25 wt.%), the oxidation rate increased with the enhancement of the oxidation temperature. A sudden ignition was found for this alloys oxidized at 873 K up to about 80 min. Moreover, the increase of the Nd content would harm the oxidation resistance of the MgNd alloys. By Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis, it was found that a triplex structure of oxide film formed. The outer layer was composed of MgO, Nd₂O₃ and Nd(OH)₃, the middle layer mainly consisted of MgO and Nd₂O₃, and the inner layer was the transitional layer made of MgO, Nd₂O₃ and the content of the substrate. The protective oxidation was associated with the formation of the dense Nd₂O₃/MgO layer during isothermal oxidation process. The oxidation mechanisms for the formation of oxide film are discussed.     

Evaporation-coupled wetting of ZrO2 by molten Mg in Ar atmosphere

The wetting behaviors of molten Mg drops on polycrystalline ZrO₂ substrate surfaces were studied in a controlled Ar atmosphere at 948–1173 K using an improved sessile drop method. The ZrO₂ substrate is virtually not wetted by molten Mg at temperatures below 1173 K. The wetting and evaporation stages according to different variation behaviors of contact angle, contact diameter and drop height were identified. Six representative modes were proposed to describe the evaporation-coupled wetting behaviors during different stages. The competitions between surface oxidation, chemical reaction and drop evaporation were discussed to account for the mechanisms for various wetting behaviors at different temperatures. The chemical reaction leads to the formation of more wettable MgO phase at the interface; however, it yields only an inconspicuous improvement in the wetting due to enhanced Mg evaporation.     

Dry sliding wear behaviour of magnesium oxide and zirconium oxide plasma electrolytic oxidation coated magnesium alloy

Two types of PEO coatings, one consisting of magnesium oxide (MgO) and the other comprising zirconium oxide (ZrO₂) as the main phase composition were produced on AM50 magnesium alloy from alkaline and acidic electrolytes, respectively. The ZrO₂ coating was found to be spongy and thicker with a higher roughness, whilst the relatively more compact MgO coating was having contrasting features. In the dry sliding oscillating wear tests under two different loads viz., 2 N and 5 N, the ZrO₂ coating exhibited a very poor wear resistance. The MgO coating showed an excellent resistance to sliding wear under 2 N load; however, the load bearing capacity of the coating was found to be insufficient to resist the wear damage under 5 N load. The higher specific wear rates of the MgO coating under 5 N load and that of the ZrO₂ coating under 2 N and 5 N loads were attributed to the poor load bearing capacity and a three-body-abrasive wear mechanism.     

Implication of the role of oxygen anions and oxygen vacancies for methanol decomposition over zirconia supported copper catalysts

The interaction of methanol with Cu, monoclinic ZrO₂, and Cu/m-ZrO₂ catalysts has been investigated by temperature programmed desorption (TPD) and reaction (TPRS) with the aim of understanding the nature of the surface sites and the mechanism involved in methanol decomposition. A synergetic effect has been detected since the combination of copper and ZrO₂ significantly facilitates the methanol decomposition with the facile evolution of H₂ and CO species at much lower desorption temperature. In conjunction with DRIFTS and H₂-TPD measurements of the Cu/ZrO₂ sample reduced at elevated temperatures, methanol decomposition over Cu/ZrO₂ is suggested to occur primarily on ZrO₂ with the aid of the presence of oxygen anions and oxygen vacancies generated by species-spillover between copper and zirconia. The interface between copper and zirconia is also evidenced to be crucial to the decomposition of methanol, with the main role of metallic Cu being to provide sites for H₂ removal by efficiently recombining the hydrogen atoms formed during the dehydrogenation of species located on zirconia.     

Investigation of optical photorefractive properties of Zr:Fe:LiNbO3 crystals

A series of Zr:Fe:LiNbO₃ crystals with various levels of ZrO₂ doping were grown by Czochraski technique. The optical damage resistance and photorefractive properties were deeply explored. The results showed that the ability optical damage resistance increased remarkably when the concentration of ZrO₂ is over threshold concentration, but which is lower than that of traditional damage resistant additive MgO. While, the holographic storage properties can be greatly enhanced by proper level of ZrO₂ doping in Fe:LiNbO₃. In terms of ions' site occupation model, the photo-damage resistant ability enhancement and the change of the photorefractive properties were discussed.     

蓝宝石衬底上Gd2O3掺杂CeO2氧离子导体电解质薄膜的生长及电学性能

采用反应磁控溅射方法,在(0001)蓝宝石单晶衬底上,制备了纳米多晶Gd₂O₃掺杂CeO₂(GDC)氧离子导体电解质薄膜,采用X射线衍射仪(XRD)、原子力显微镜(AFM)对薄膜物相、结构、粗糙度、表面形貌等生长特性进行了表征,利用交流阻抗谱仪测试了GDC薄膜不同温度下的电学性能;实验结果表明,GDC薄膜为面心立方结构,在所研究的衬底温度范围内,均呈强(111)织构生长;薄膜表面形貌随衬底温度发生阶段性变化：衬底温度由室温升高到300℃时, 关键词： 2O₃掺杂CeO₂电解质薄膜')" href="#">Gd₂O₃掺杂CeO₂电解质薄膜 反应磁控溅射 生长特性 电学性能     

Study of phase transition in ZrO2 doped with Pb3O4

Electrical conductivity of ZrO₂ doped with Pb₃O₄ has been measured at different temperatures for different molar ratios (x=0, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06). The conductivity increases due to migration of vacancies, created by doping. The conductivity increases with increase in temperature till 180 °C and thereby decreases due to collapse of the fluorite framework. A second rise in conductivity at higher temperatures beyond 500-618 °C is due to phase transition of ZrO₂. DTA and X-ray powder diffraction were carried out for confirming doping effect and transition in ZrO₂.The addition of Pb₃O₄ to ZrO₂ shifted the phase transition of ZrO₂ due to the interaction between Pb₃O₄ and ZrO₂.     

Conductivity behaviour of a cubic/tetragonal phase stabilized nanocrystalline La2O3-ZrO2

La₂O₃ doped nanocrystalline zirconia (ZrO₂) was prepared by chemical co-precipitation method for the 3, 5, 8, 10, 15, 20 and 30 mol.% concentrations of La₂O₃. Structural studies were performed using X-ray diffraction (XRD). All the as-synthesized samples were found to be in monoclinic phase. As-synthesized samples were given heat treatment at higher temperatures for tetragonal/cubic structural phase stabilization. Sintering the samples at temperature 1173 K stabilized the tetragonal and cubic phases. A slight shift in the 100% peak of the cubic phase was observed towards the low diffraction angle indicating the substitution of the bigger La³⁺ ion into the ZrO₂ lattice. Grain sizes were found to lie between 10 and 13 nm. Electrical conductivity studies were performed on the cubic phase stabilized La₂O₃-ZrO₂ by complex impedance spectroscopy. The conductivity increases up to the dopant concentration 10 mol.% and then decreases with further increase in La₂O₃ concentration. Initial increase in conductivity is correlated to the stabilization of the cubic phase and the subsequent decrease in the conductivity with the dopant content is interpreted on the basis of the oxygen-ion movement model. Electrical conductivity has contributions from grain and grain boundary regions. But the grain boundary conductivity is slightly higher than the corresponding grain conductivity. Higher grain boundary conductivity shows higher diffusion coefficient for the atoms on the surface of the ZrO₂ grains. The possible mechanism of the oxygen ion conduction in the La₂O₃ stabilized zirconia (LSZ) is reported. The Barton, Nakajima and Namikawa (BNN) relation has been applied to the conductivity data and found that the d.c. and a.c. conductions have been correlated to each other by the same mechanism.     

Studies on chemical stability in CO2 and H2O and electrical conductivity of perovskite-type Ba3In2Zr1?xCexO8 (x?= 0, 0.5, 1)

We report the synthesis, structure, microstructure, chemical stability in H₂O and CO₂, and electrical transport properties of an oxide ion-conducting perovskite-related structure Ba₃In₂MO₈ (M = Zr, Ce, Zr_0.5Ce_0.5). Powder X-ray diffraction confirmed the formation of a simple cubic perovskite-like structure for Ba₃In₂ZrO₈ (a = 4.205(9) ?), Ba₃In₂CeO₈ (a = 4.234(1) ?), and Ba₃In₂Zr_0.5Ce_0.5O₈ (a = 4.285(8) ?). The increase in lattice constant is consistent with the Shannon’s ionic radius trend. Among the three samples investigated, Ba₃In₂ZrO₈ was found to be stable against reaction with pure CO₂ at elevated temperature, while the Ce and 1:1 Zr and Ce compounds were unstable at 600 °C. Ba₃In₂ZrO₈, Ba₃In₂CeO₈, and Ba₃In₂Zr_0.5Ce_0.5O₈ were found to be chemically unstable in H₂O at about 50 °C. The bulk electrical conductivity of the samples prepared at different temperatures was found to be nearly the same; the total conductivity (bulk + grain–boundary + electrode) seems to change with sintering temperature. Both Ba₃In₂ZrO₈ and Ba₃In₂CeO₈, prepared at 1,400 °C, exhibited comparable electrical conductivity of about 6 × 10⁻³ S cm⁻¹ at 800 °C, which is comparable to that of conventional Y₂O₃-doped ZrO₂ electrolyte. These compounds are very promising electroltes, provided that their chemical and mechnical stabitities are improved without losing any ionic conductivity.     

Fabrication of surface relief gratings on transparent dielectric materials by two-beam holographic method using infrared femtosecond laser pulses

Fabrication of surface relief-type gratings in transparent dielectrics, which are hard to machine, has been achieved by a holographic technique using two infrared femtosecond (fs) pulses from a mode-locked Ti:sapphire laser. The present method can be applied for a variety of transparent dielectrics, Al₂O₃ (sapphire), TiO₂, ZrO₂, LiNbO₃, SiC, ZnO, CdF₂, MgO, CaF₂ crystals, and SiO₂ glass. It is found that the grating formation is due primarily to laser ablation processes. Planar surface relief gratings can be fabricated by colliding two fs laser pulses on the surface of substrates which move at a constant speed, synchronized with the laser repetition rate. Received: 1 March 2000 / Published online: 7 June 2000     

Effect of the substrate crystalline structure on the magnetic, electrical, and crystallographic characteristics of La0.35Nd0.35Sr0.3MnO3 epitaxial films

A study has been made of the electrical resistivity ρ, magnetoresistance Δρ/ρ, and magnetization of La_0.35Nd_0.35Sr_0.3MnO₃ epitaxial films on ZrO₂(Y₂O₃), SrTiO₃, LaAlO₃, and MgO substrates. The first film can exist in four equivalent crystallographic orientations in the sample plane, while the other three have only one orientation. The maxima of ρ and Δρ/ρ of the first film are broadened considerably in the vicinity of the Curie point T _C compared to the three others, the magnitude of ρ itself being larger by 1.5 orders of magnitude, and a large negative magnetoresistance (|Δρ/ρ| ～ 10% in a field of 8.4 kOe) is observed at temperatures 80≤T≤200 K. In all films, the magnetic moment per molecule at 5 K is ～46% smaller than the pure spin value, due to the existence of magnetically disordered regions. The larger value of ρ of the film deposited on ZrO₂(Y₂O₃) is due to the electrical resistance of the boundaries separating regions with different crystallographic orientations, and the magnetoresistance is associated with polarized carriers tunneling through the boundaries coinciding with domain walls. The low-temperature magnetoresistance in fields above technical saturation is caused by the strong p-d exchange coupling within spin-ordered regions.