Unique sharp photoluminescence of size-controlled sonochemically synthesized zirconia nanoparticles

The present study explores the features of tetragonally stabilized polycrystalline zirconia nanophosphors prepared by a sonochemistry based synthesis from zirconium oxalate precursor complex. The sonochemically prepared pristine zirconia, 3 mol%, 5 mol% and 8 mol% yttrium doped zirconia nanophosphors were characterized using thermo-gravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and photoluminescence spectroscopy (PL). The reaction mechanism of formation of zirconia nanophosphors is discussed in detail. The probable sonochemical formation mechanism is being proposed. Stabilization of tetragonal phase of pristine zirconia even at room temperature was effectively established by controlling the particle size using ultrasonic waves. Improved phase purity and good surface morphology of the nanophosphors is being achieved via sonochemical route. FE-SEM micrographs reveal that the nanoparticles have uniform spherical shape and size. The narrow particle size distribution (∼15–25 nm) of the zirconia nanoparticles was found from FE-SEM statistical analysis and further confirmed by TEM. Zirconia nanophosphors exhibit a wide energy band gap and which was found to vary with yttrium dopant concentration. The highlight of the present study is the synthesis of novel nanocrystalline ZrO₂ and Y-ZrO₂ phosphor which simultaneously emits extremely sharp as well as intense UV, violet and cyan light on exciting the host atom. The yttrium ion dopant further enhances the photoluminescence property of zirconia. These nanocrystalline phosphors are likely to have remarkable optical applications as light emitting UV-LEDs, UV lasers and multi color displays.     

Characterization of low temperature protonic conductivity in bulk nanocrystalline fully stabilized zirconia

We investigated the conductivity of samples of bulk nanometric cubic yttria stabilized zirconia (YSZ 8%) with a grain size of about 16 nm and a relative density above 98%. In an oxygen atmosphere the material showed a large grain boundary resistivity. However, when exposed to a moist atmosphere at temperatures below 150 °C it showed a high conductivity, several orders of magnitude higher than the corresponding extrapolated ionic conductivity. A fairly high conductivity was measured even at room temperature. The conductivity was strongly dependent on the water partial pressure and showed a distinct isotopic effect, suggesting a protonic conductivity mechanism. This effect was not observed in samples with grain size above 50 nm, suggesting the possibility that nanostructure can induce drastic modification in the conduction mechanism of ceramic electrolytes.     

Effect of Ni doping on the phase stability and conductivity of scandia-stabilized zirconia

The effect of Ni doping on the phase stability and conductivity of scandia-stabilized zirconia (SSZ) thick film was studied. A free-standing 10SSZ thick-film (10 mol% Sc₂O₃-stabilized zirconia, ~ 10 μm thick) that was previously in contact with a Ni layer during co-firing was fabricated. The 10SSZ thick-film showed a cubic phase in contrast to the rhombohedral phase shown for a bulk 10SSZ sample. The Ni content in the SSZ thick film was ~ 1.7 mol%. The effect of Ni on the cubic phase formation was also confirmed by the similar observation of the cubic phase in the Ni-doped bulk 10SSZ sample. The observed conductivity behavior also supported the XRD observation. Ni was found to hinder the transformation of the cubic phase to the rhombohedral on cooling in 10SSZ samples after a reduction treatment.     

Preparation and analysis of zirconia doped ceria nanocrystal dispersions

Crystalline and highly dispersible solid solutions with the general formula Ce_1−xZr_xO_2−δ, where δ represents any possible oxygen vacancies, and in the ceria rich region were prepared using an inorganic sol-gel technique. The particle size distributions of the transparent sols were measured using photon correlation spectroscopy and found to be les than 25 nm with narrow distributions. Compositional analysis using EDAX confirmed the correct zirconia doping. Lattice parameters, structural and crystallite size data for the dried sols and heated powders using X-ray diffraction confirmed a cubic structure stable to at least 1000°C with crystallite sizes varying from 34 Å to 55 nm, depending on calcination temperature and composition. Dispersions of the nanocrystals on silica were studied using high resolution TEM/EDAX, the structure and compositions of individual nanocrystals closely matching those of the bulk agglomerated powders, showing exceptional homogeneity.     

Structural and hydrophobic-hydrophilic properties of nanosilica/zirconia alone and with adsorbed PDMS

Two nanosilica A-300/zirconia (SZ) composites at zirconia content CZrO₂=5 and 20 wt.% were synthesized using a wet impregnation method with zirconium acetylacetonate as a precursor. The specific surface area of SZ is larger than that of A-300 because zirconia is composed of nanoparticles (crystallites of 4 nm in average size at CZrO₂=20 wt.%) smaller than those of the initial silica (d_av ≈ 11 nm). A-300 and SZ modified by polydimethylsiloxane (PDMS at molecular weight 1700 and 7960) in amounts of 5, 10, 15, 20 and 40 wt.% remained in the powder state with aggregates of primary particles smaller than those of A-300. SZ is more hydrophilic than silica but PDMS/SZ is more hydrophobic (maximum hydrophobic at C_PDMS 15-20 or 40 wt.%) than PDMS/A-300.     

Influence of the size effect on the electrical conductivity of partially stabilized zirconia

The volume component of the electrical conductivity of bulk ceramics of the partially stabilized zirconia (ZrO₂)_0.94(Y₂O₃)_0.05(Al₂O₃)_0.01 is found to increase by a factor of 1.7 with the grain size decreasing from 600–800 to 200–300 nm. The observed effect is explained by the action of the pressure produced by surface tension forces, which shifts equilibrium toward the point of the polymorphic transition to the cubic phase.     

The mechanism of ionic conductivity in stabilized cubic zirconia

The electron-density functional method (in the gradient approximation) and the pseudopotential method are used to study the mechanism of ionic conductivity in the cubic phase of zirconia stabilized with magnesium or yttrium. The oxygen-ion migration in the stabilized zirconia is shown to be a two-stage process, which consists in the formation of active oxygen vacancies and in oxygen-ion jumps from one active vacancy to another. The total activation energy of these processes is calculated to be 1.0–1.5 eV, which agrees with experimental data.     

Critical radius of zirconia inclusions in transformation toughening of ceramics

The paper presents a transformation toughening model of ceramics taking into account an energy barrier the overcoming of which results in phase transformation of zirconia inclusions. Methods based on experimental data analysis are proposed for estimating the energy barrier. The size range of zirconia inclusions in Al₂O₃ and WC matrices is defined depending on the energy barrier value, working temperature, and external load. It is shown that the introduction of an energy barrier enables an adequate estimation of the size range of inclusions at which transformation toughening occurs in ceramics. The elastic interaction of inclusions is shown to cause a decrease in their critical radii with the growing volume density, which agrees with experimental data.     

纳米NiO的制备及其谱学特性研究

以醋酸镍、氢氧化钠为原料,吐温80为分散剂,通过固相反应制备了纳米级NiO。 用X射线衍射仪、透射电子显微镜、傅里叶红外光谱、紫外-可见分光光度等方法对材料的粒径、晶格畸变率、形貌以及红外、紫外-可见光的吸收性能进行了表征。结果表明：制得的纳米NiO产物为球形、属立方晶系,粒径大小在9～30 nm左右;晶格畸变率随粒径的增大而减小;纳米NiO红外吸收峰出现在437 cm-1处,与普通粒径的NiO光谱纯(484 cm-1)相比,其吸收峰红移了47 cm-1,体现了纳米NiO的表面效应;不同粒径大小的NiO对紫外-可见光的吸收特性不同。普通粒径的NiO光谱纯在紫外-可见光区域没有吸收,颗粒尺寸越小吸收波长越短,10 nm NiO的紫外-可见光吸收峰位于309 nm处,直接跃迁的光学能带隙约为4.2 eV,比体相材料(3.65 eV)增加0.55 eV,表现出明显的量子尺寸效应。对纳米NiO的谱学特性研究表明该材料在光电领域具有潜在的应用价值。     

Experimental and theoretical investigation of oxygen diffusion in stabilised zirconia

Oxygen diffusion in stabilised zirconias is investigated by the simultaneous application of computer modelling and experimental techniques to yttria-stabilised zirconia. Using the Mott-Littleton method, migration pathways for oxygen ions have been calculated in perfect cubic zirconia. The oxygen migration occurs through a straight pathway, but not starting from the ideal lattice positions. The calculated activation energy of migration is about 0.2 v eV. Oxygen transport is investigated experimentally in YSZ containing 8-24 v mol% Y 2 O 3 as a function of stabiliser content by combining the stable isotope ( 18 O 2 ) method with ionic conductivity measurements. It was found that for a given temperature, diffusion and conductivity are highest for YSZ containing 8-10 v mol% yttria, but with differing activation energies which can be compared to the calculated values.     

Yttrium segregation and surface phases of yttria-stabilized zirconia (1 1 1) surface

Surface segregation of yttria-stabilized zirconia (YSZ) was studied via first-principles computations and thermodynamics. For the cubic YSZ (1 1 1) surface, yttrium can segregate only to a subsurface layer, and these segregation phases are terminated at the surface by defective oxygen layers with honeycomb structure. The segregation is independent of the bulk yttrium concentration at high oxygen partial pressures or low temperatures. At very low oxygen partial pressures and high temperatures there is no surface yttrium segregation and the surface is terminated by O–Zr. Our results provide a reasonable explanation for previous experimental work, and also a framework for extending our understanding of cation segregation in oxide surfaces.     

ESR study of color centers in yttria stabilized zirconia

The ESR of single crystals of yttria stabilized zirconia (YSZ), containing color centers due to reduction in a flowing H₂ atmosphere, has been measured at room temperature. The spin density is about 3 × 10¹⁷ cm^?3 and the strong ESR line displays axial symmetry with principal values g₆ = 1.989, g_⊥ = 1.860.The behaviour of the ESR spectra when the magnetic field is rotated about the 〈1 0 0〉 and 〈1 1 0〉 directions shows that the centers have axial symmetry, the symmetry axis being along the 〈111〉 and equivalent directions. The derivative peak to peak envelope width is g-dependent and varies from 18 Gauss at g = 1.989 to 65 Gauss at g = 1.860. The results are interpretable in terms of an electron trapped at an oxygen vacancy adjacent to an yttrium ion.     

Molecular dynamics simulation of zirconia melting

The melting point for the tetragonal and cubic phases of zirconia (ZrO₂) was computed using Z-method microcanonical molecular dynamics simulations for two different interaction models: the empirical Lewis-Catlow potential versus the relatively new reactive force field (ReaxFF) model. While both models reproduce the stability of the cubic phase over the tetragonal phase at high temperatures, ReaxFF also gives approximately the correct melting point, around 2900 K, whereas the Lewis-Catlow estimate is above 6000 K.     

Theoretical study of the initial stage of InN growth on cubic zirconia (111) substrates

An initial stage of InN growth on cubic zirconia (111) substrates has been investigated using first‐principles calculations based on density functional theory (DFT). We have evaluated adsorption energies of indium and nitrogen atoms on cubic zirconia (111) surfaces, and have found that the differences in the adsorption energies of the indium atoms at various adsorption sites were small, indicating that the migration of the indium atoms on zirconia (111) surfaces occurs readily. On the other hand, we have found that the differences in the adsorption energies of the nitrogen atoms at various adsorption sites were large, implying that the nitrogen atoms tend to stay at the stable site with the largest adsorption energy, which was identified as the O–Zr bridge site. These results suggest that the first layer of InN films is the nitrogen layer. In addition, we have found that the energetically favorable arrangement is comprised of InN(0001)//cubic zirconia (111) and InN $ [11\bar 20] $ //cubic zirconia $ [1 \bar 10], $ which is quite consistent with previously obtained experimental data. Furthermore, the hybridization effect between N 2p and O 2p plays a crucial role in determining the interface structure for the growth of InN on cubic zirconia (111) surfaces. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mesoporous silica templated zirconia nanoparticles

Nanoparticles of zirconium oxide (ZrO₂) were synthesized by infiltration of a zirconia precursor (ZrOCl₂·8H₂O) into a SBA-15 mesoporous silica mold using a wet-impregnation technique. X-ray diffractometry and high-resolution transmission electron microscopy show formation of stable ZrO₂ nanoparticles inside the silica pores after a thermal treatment at 550 °C. Subsequent leaching out of the silica template by NaOH resulted in well-dispersed ZrO₂ nanoparticles with an average diameter of ~4 nm. The formed single crystal nanoparticles are faceted with 110 surfaces termination suggesting it to be the preferred growth orientation. A growth model of these nanoparticles is also suggested.     

Evidence of the surface layer in tungstated zirconia

We characterized the structure of tungstated zirconia (WO_x–ZrO₂) by combining X-ray diffraction, Raman spectroscopy and High Resolution Electron Microscopy (HREM) together with molecular simulations. Our results indicate that the structure of this material consists of metastable tetragonal ZrO₂ nanoparticles (<20 nm in diameter) covered by a few-nanometers thick low-crystallinity surface layer formed by tungsten oxospecies (WO_x). Although the X-ray diffraction pattern matched the spectra of the tetragonal ZrO₂ bulk phase the lattice fringes of the ZrO₂ nanoparticles observed by HREM were locally distorted, presumably as a result of the interaction with the surface WO_x layer. The local interplanar distances of the surface layer were close to those present in different bulk tungsten oxocompounds, and its variability was also an indication of the WO_x–ZrO₂ interaction. Molecular simulations corroborated our structural assignment. The results presented here are a direct evidence for the presence of a surface WO_x layer in the case of WO_x–ZrO₂. PACS 68.35.Bs; 81.05.Ys; 82.65.Dp     

Size-dependent phase transformations in nanoscale pure and Y-doped zirconia thin films

Phase stability in nanoscale pure zirconia and 9.5 mol.% yttria-doped zirconia (YDZ) thin films was studied by in-situ transmission electron microscopy. Oxygen vacancies are found to play a significant role in determining the microstructure and phase evolution. Pure zirconia thin films of ～52 nm thickness were stabilized without any dopants at room temperature, whereas they transformed into a tetragonal phase upon heating to 400°C. On the other hand, 9.5% yttria doping enables stabilization of the cubic structure regardless of grain growth. Annealing of amorphous YDZ films in air (oxygen-rich) leads to tetragonal phase formation, whereas ultrahigh vacuum (oxygen-deficient) annealed samples display a cubic phase at high temperature. Detailed discussions on the effects of initial microstructure, oxygen deficiency, aliovalent doping and thickness are presented.     

Phase transformation of ZrO2 nanoparticles produced from zircon

This article focuses on the phase transformation of zirconia (ZrO₂) nanoparticles produced from zircon using a bottom-up approach. The influence of mechanical milling and thermal annealing on crystalline phase transformation of ZrO₂ nanoparticles was explored. It was found that the iron oxide, as an inherent impurity present in ZrO₂ nanoparticles, produced from zircon stabilises the cubic phase after calcination at 600°C. The stabilised cubic phase of ZrO₂ nanoparticles was disappeared and transformed into partial tetragonal and monoclinic phases after mechanical milling. The phase transformation occurred on account of the crystal defect induced by high-energy mechanical milling. The destabilisation of cubic phase into monoclinic phase was observed after the thermal annealing of ZrO₂ nanoparticles at 1000°C. The phase transitions observed are correlated to the exclusion of iron oxide from the zirconia crystal structure.     

Oxygen electrodes of zirconia electrolytes: fundamentals and application to analysis of oxygen containing gases

The kinetics of the oxygen exchange reaction and the reduction of NO at La_0.8Sr_0.2CoO₃−, La_0.8Sr_0.2MnO₃− and Ag-electrodes on stabilized zirconia (8mol% Y₂0₃=YSZ) has been studied by means of electrochemical methods (impedance, I-U characteristics). For La_0.8Sr_0.2CoO₃ electrodes the oxygen exchange was found to proceed via the bulk of the electrode with a rate limiting oxygen exchange at the electrode surface. Electrodes based on La_0.8Sr_0.2MnO₃ change their electrode characteristics with the applied potential. At low cathodic polarization the electrode reaction is limited to the three-phase boundary electrode/YSZ/gas. At high cathodic potentials oxygen vacancies are created and consequently additional oxygen is exchanged via the electrode bulk. Furthermore, a significant NO reduction was observed which indicate a reaction with the oxygen vacancies at the electrode surface. For Ag a rate limiting transport of oxygen atoms through the bulk of the electrode was found. As a consequence the oxygen concentration at the electrode surface remains nearly constant. In this context, the observed inactivity for the NO reduction of Ag-electrodes may be explained. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

The synthesis of iron–nickel alloy nanoparticles using a reverse micelle technique

Nanosized Fe_0.2Ni_0.8 particles were prepared by reducing their salts with sodium borohydride (NaBH₄) in cationic water-in-oil (w/o) microemulsions of water/cetyl-trimethyl-amonium bromide (CTAB) and n-butanol/isooctane at 25 °C. According to the TEM and X-ray diffraction analyses, the synthesized particles were around 4–12 nm in size. Due to their nanodimensions, the particles had a primitive cubic (pc) structure rather than the body-centered cubic (BCC) structure of the bulk material. An examination of the synthesis from the reverse micelle reveals that the morphology of the iron–nickel alloy nanoparticles depends mainly on the microemulsion's composition. The magnetization of the nanoparticles was much lower than that of the bulk material, reflecting the influence of the nanodimensions on the particles’ magnetizations.