Zr-rich Regions Research Articles

Effect of nucleating agents on structure and crystallization behavior of high-aluminum glass-ceramics system

In this work, a transparent and high strength spinel glass-ceramics containing excess Al2O3 (26.5 mol%) were prepared successfully. The influence of the substitution amount of TiO2 by ZrO2 on the structure, nucleation process, crystallization behavior and properties of glasses and glass-ceramics were investigated respectively. The results showed that ZrO2 would play different roles under different concentrations and the existence form of Al in glass network would also be influenced. Moreover, the nucleation process and crystallization behavior of glasses were studied and the mechanism of TiO2/ZrO2 induced crystallization was proposed. It is concluded that TiO2/ZrO2 as nucleating agents can induce glass to form Ti-rich phase droplets or Zr-rich inhomogeneous regions. Finally, the properties of the glasses and glass-ceramics with different nucleating agents were compared and analyzed. It is found that high Al2O3 content contributes to the improvement of microhardness, and could strengthen the role of ZrO2 in refining grains.

Formation of a zirconium oxide crystal nucleus in the initial nucleation stage in aluminosilicate glass investigated by X-ray multiscale analysis

Understanding the nucleation mechanism in glass is crucial for the development of new glass-ceramic materials. Herein, we report the structure of a commercially important glass-ceramic ZrO2-doped lithium aluminosilicate system during its initial nucleation stage. We conducted an X-ray multiscale analysis, and this analysis was used to observe the structure from the atomic to the nanometer scale by using diffraction, small-angle scattering, absorption, and anomalous scattering techniques. The inherent phase separation between the Zr-rich and Zr-poor regions in the pristine glass was enhanced by thermal treatment without changing the spatial geometry at the nanoscale. Element-specific pair distribution function analysis using anomalous X-ray scattering data showed the formation of a liquid ZrO2-like local structural motif and edge sharing between the ZrOx polyhedra and (Si/Al)O4 tetrahedra during the initial nucleation stage. Furthermore, the local structure of the Zr4+ ions resembled a cubic or tetragonal ZrO2 crystalline phase and formed after 2 h of annealing the pristine glass. Therefore, the Zr-centric periodic structure formed in the early stage of nucleation was potentially the initial crystal nucleus for the Zr-doped lithium aluminosilicate glass-ceramic.

Microstructure and First Hydrogenation Properties of Ti16V60Cr24−xFex + 4 wt.% Zr Alloy for x = 0, 4, 8, 12, 16, 20, 24

In body-centered cubic (BCC) alloys of transition elements, elemental addition or substitution in the vanadium-based alloys can be beneficial for improving the hydrogen storage properties and for reducing the production cost. In this context, the current study focused on the effect of the substitution of Cr by Fe in Ti16V60Cr24−xFex + 4 wt.% Zr alloys where x = 0, 4, 8, 12, 16, 20, 24. The microstructure of each alloy was composed of a matrix having a chemical composition close to the nominal one and a Zr-rich region. From X-ray diffraction patterns, it was found that the matrix has a BCC structure, and the Zr-rich regions present the C14 Laves phase structure. The lattice parameter of BCC phases decreased linearly with x, in accordance with Vegard’s law. The measurement of the first hydrogenation at 298 K under 3 MPa of hydrogen revealed a decrease in the maximum hydrogen capacity: 3.8 wt.% for x = 0, 3.1 wt.% for x = 4 and around 2 wt.% for x = 8 to 24. The XRD patterns after hydrogenation showed a BCT phase for all alloys, with a C14 phase for x = 4, 8, 12 and with C14 and C15 for x = 16, 20 and 24.

Corrosion behavior of Mg-Zn-Zr-RE alloys under physiological environment – Impact on mechanical integrity and biocompatibility

Benefits of RE addition on Mg alloys strength and corrosion resistance are widely reported but their effects on biodegradability and biocompatibility are still of concern. This paper investigates the effect of RE additions on biodegradability of Mg-Zn alloys under simulated physiological conditions. In this context, two commercial Mg-Zn-Zr-RE alloys, namely ZE41 and EZ33, with same RE addition but different concentrations are studied in Hank's Balanced Salt Solution (HBSS) at 37 °C and with pH of 7.4. Weight-loss, hydrogen evolution, real-time in-situ drop test, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization are deployed to study corrosion characteristics. The mechanical integrity of both alloys is assessed by mechanical testing post immersion. Furthermore, in vitro biocompatibility is evaluated by indirect cytotoxicity tests using NIH3T3 cells. Results reveal that although both alloys showed similar microstructure, size and distribution of precipitates played a significant role on its corrosion response. EIS and open circuit potential results show stable film formation on EZ33, while ZE41 showed passive layer formation followed by its deterioration, over the analyzed time period. Using real-time drop test, it was shown in ZE41 alloy that both T-phase and Zr-rich precipitates acted as micro cathodes, resulting in an unstable surface film. In EZ33, Zr-rich regions did not influence corrosion response, resulting in better corrosion resistance that was corroborated by post-immersion surface morphology investigations. The higher degradation observed in ZE41 alloy resulted in higher drop in flexural and tensile strength compared to EZ33 alloy. In addition, cytotoxicity tests on NIH3T3 cells revealed that cell viability of EZ33 increased with increasing incubation time, contrary to ZE41, owing to its lower biodegradation behavior and despite higher concentrations of REs. Present results show that an increase in RE concentration in EZ33, relative to ZE41, had a positive effect on corrosion rate that subsequently controlled alloy mechanical integrity and biocompatibility.

Investigation of constituent redistribution in U-Pu-Zr fuels and its dependence on varying Zr content

This contribution investigates fuel constituent segregation and fuel-cladding chemical interactions (FCCI) in three U-Pu-Zr fuel pins irradiated to ∼11% burnup in Experimental Breeder Reactor-II as part of the X441 DP-1 experiment. In examined pins, Zr content ranged from 6 to 10 and 14 wt.%, while Pu concentration was constant at 19 wt.% Pu. The primary goal of the investigation was to determine the role Zr content plays in fuel performance and this manuscript provides assessment of both constituent redistribution and FCCI as a function of axial position. FCCI was observed in all pins, though no cladding failure was noted. Optical and scanning electron microscopy (SEM) results show that variation in Zr content alters the number and relative size of constituent redistribution zones in the fuel. For example, larger Zr-rich central regions and smaller U-rich intermediate regions were observed in 14 wt.% Zr fuels. More importantly, all examined fuels have four or more discrete constituent redistribution regions, where each region has dissimilar morphological features. The existence of four to six distinctive regions deviates from traditionally accepted three region redistribution model , highlights complexity of constituent redistribution in this metal fuel , and identifies the need to conduct additional studies using state-of-the-art instrumentation.

Microstructure and Corrosion of Cast Magnesium Alloy ZK60 in NaCl Solution.

In this work, the effects of the microstructure and phase constitution of cast magnesium alloy ZK60 (Mg-5.8Zn-0.57Zr, element concentration in wt.%) on the corrosion behavior in aqueous NaCl (0.1 mol dm−3) were investigated by weight-loss measurements, hydrogen evolution tests, and electrochemical techniques. The alloy was found to be composed of α-Mg matrix, with large second-phase particles of MgZn2 deposited along grain boundaries and a Zr-rich region in the central area of the grains. The large second-phase particles and the Zr-rich regions were more stable than the Mg matrix, resulting in a strong micro-galvanic effect. A filiform corrosion was found. It originated from the second-phase particles in the grain boundary regions in the early corrosion period. The filaments gradually occupied most areas of the alloy surface, and the general corrosion rate decreased significantly. Corrosion pits were developed under filaments. The pit growth rate decreased over time; however, it was about eight times larger than the general corrosion rate. A schematic model is presented to illustrate the corrosion mechanism.

Metal organic framework UiO-66 and activated carbon composite sorbent for the concurrent adsorption of cationic and anionic metals

A composite sorbent for the simultaneous removal of both Hg2+ and SeO32− from aqueous media was produced from the solvothermal synthesis of a zirconium metal organic framework, UiO-66, in the presence of activated carbon. The composite sorbent has a large surface area of 1051 m2 g−1 with crystalized porous structures and has strong thermal stability up to 600 °C. The contaminant uptake of the sorbent follows a Langmuir adsorption isotherm with maximum sorption capacity of 205 mg g−1 and 168 mg g−1 for Hg2+ and SeO32−, respectively. Scanning electron microscopy-energy dispersive spectroscopy results show that the Se regions overlap exclusively with Zr-rich regions suggesting that SeO32− adsorption depends entirely on the exposed UiO-66 surface. In addition, X-ray photoelectron spectroscopy spectra of Se 3d and Hg 4f showed the association of SeO32− and Hg2+ on the UiO-66 and carbon surfaces, respectively. The sorbent could facilitate the development of a single process for the simultaneous removal of cationic Hg and anionic Se as well as other similar ionic metals with opposite charges from aqueous media.

Icosahedra clustering and short range order in Ni-Nb-Zr amorphous membranes

Crystalline Pd/Pd-Ag membranes are widely used for hydrogen separation from CO2 and other gases in power generation applications. To substitute these high cost noble metal alloy membranes, the Ni-Nb-Zr amorphous alloys are being developed that exhibit relatively high permeability of hydrogen between 200–400 °C. Atom probe tomography (APT) experiments performed on these ribbons revealed nm-scale Nb-rich and Zr-rich regions (clusters) embedded in a ternary matrix, indicating phase separation within the Ni-Nb-Zr amorphous alloy. Density functional theory (DFT) simulations have predicted that these clusters are composed of icosahedral coordination polyhedra. The interatomic distances and correlation lengths of the short range order of these alloys were determined by neutron total scattering which match well with our DFT based molecular dynamics (DFT-MD) simulations.

Microstructural and Chemical Evolution and Analysis of a Self-Activating CO2-Selective Cu–Zr Bimetallic Methanol Steam Reforming Catalyst

The microstructure of the CO2-selective self-activating and self-stabilizing Cu–Zr bimetallic compound Cu51Zr14 has been studied by a combination of high-resolution electron microscopy and energy-dispersive X-ray spectroscopy both before and after entering the CO2 selective state in methanol steam reforming. Prior to catalysis, the phase composition of the catalyst is characterized by a microstructural mixture of Cu51Zr14 and metallic Cu. The structure appears in a distinct needle-like morphology with a characteristic microstucture of small Cu particles embedded in the intermetallic matrix. In contrast, entering the CO2-selective state goes along with oxidative decomposition—investigated by differential thermal analysis (DTA), thermogravimetry (TG), and mass spectrometry (MS)—and therefore massive structural and compositional changes of the Cu51Zr14 compound both in the near-surface and bulk regions. The final state is then composed of a structurally very heterogeneous sample with Zr-rich and Cu-rich regions within the material bulk with a characteristic lamellar structure. Most importantly, the catalytically relevant surface regions are drastically corroded and depleted in Zr and are characterized by a majority of Cu in intimate contact with oxidized ZrO2 exhibiting a well-ordered, predominantly tetragonal structure. This newly created Cu–ZrO2 interface is believed to be the most significant descriptor steering the CO2 selectivity. In due course, this new method for self-adjustment of the microstructure starting from well-defined intermetallic compounds in the catalytic reaction mixture might pave the way for a more systematic approach of controlled oxidative decomposition of intermetallic compounds acting as promising catalyst precursors.

Effect of dopant nature on structures and lattice dynamics of proton-conducting BaZrO3

The influence of the acceptor-dopants on the local and long range structures and lattice vibrations of barium zirconate has been studied with Density Functional Theory (DFT) based periodic approach. Structural details of BaZr1 − xMxHxO3 (M = In and Sc, and x = 0.125, 0.25 and 0.375) with protonic defects in three different oxygen configurations are presented and discussed in relation with the results for Y-doped barium zirconates, obtained previously with the same methodology. The oxide symmetry and the short and long range structural arrangements in the M-rich and Zr-rich regions were established nearly independent on the dopant chemical nature, concentration and arrangement in the cell. The OH bond formation is however strongly influenced by the dopant nature, which is associated with the differences in dopant–oxygen bond character. The computed lattice vibrations in zirconates with Y, In and Sc dopants showed that the OH vibrations vary by 250–1500 cm− 1 depending on the acceptor-dopant.

Heterogeneous nucleation of β-type precipitates on nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy

Zirconium (Zr) is an important alloying element to Mg-Zn-based alloy system. In this paper, we report the formation of the β-type precipitates on the nanoscale Zr-rich particles in a Mg-6Zn-0.5Cu-0.6Zr alloy during ageing at 180°C. Scanning transmission electron microscopy examinations revealed that the nanoscale Zr-rich [0001]α rods/laths are dominant in the Zr-rich core regions of the as-quenched sample after a solution treatment at 430°C. More significantly, these Zr-rich particles served as favourable sites for heterogeneous nucleation of the Zn-rich β-type phase during subsequent isothermal ageing at 180°C. This research provides a potential route to engineer precipitate microstructure for better strengthening effect in the Zr-containing Mg alloys.

Development of Nanoscale Inhomogeneities during Drying of Sol–Gel Derived Amorphous Lead Zirconate Titanate Precursor Thin Films

The structural evolution of sol-gel derived lead zirconate titanate (PZT) precursor films during and after physical drying was investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), selected area electron diffraction (SAED), and time-resolved X-ray diffraction (XRD). Films were deposited from initial 0.3 mol/dm(3) precursor sols with varying hydrolysis ratios. Zr-rich grains of 1-10 nm size, embedded in a Pb-, Zr-, and Ti-containing amorphous matrix were found in as-dried films. The Zr-rich regions were crystalline at hydrolysis ratios [H(2)O]/[PZT] < 27.6, and amorphous at ratios > 100. X-ray diffraction analysis of PZT and zirconia sols revealed that the crystalline nanoparticles in both sols are identical and are probably composed of nanosized zirconium oxoacetate-like clusters. This study demonstrates that time-resolved X-ray diffraction combined with electron energy loss spectroscopy mapping is a powerful tool to monitor the nanoscale structural evolution of sol-gel derived thin films.

Corrosion of heat treated magnesium alloy ZE41

This paper investigates the effect of heat treatment upon the corrosion morphology and mechanism of ZE41 alloy. The results of optical and scanning electron microscopy (SEM) together with potentiodynamic polarisation reveal the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The corrosion of the heat-treated alloy is significantly altered due to changes in the microstructure, specifically the Zr-rich regions and the grain boundary T-phase.

Evaluation of Experimental Resume of BaZrxTi1-xO3 with Perspective to Ferroelectric Relaxor Family: An Overview†

In this paper we report the mechanism and nature of relaxor ferroelectric behavior in the environmental friendly lead free Barium Zirconate Titanate (BZT) system. A revised complete (Structure:Property) phase diagram of Ba(ZrxTi1-x)O3, with compositions 0.00≤ × ≤1.00 has been developed based on their electrical properties. Two different kinds of relaxor behaviors have been observed in the BZT system; one is dominated by polar Ti-rich regions and another by non-polar Zr-rich regions. All the BZT relaxor compositions are characterized by dielectric properties with and without bias, pyroelectric and thermal expansion measurements in the wide range of temperatures. The structure of the BZT compositions has been evaluated by XRD, Neutron diffraction study. Further the local structure of the BZT compositions has been probed by micro-Raman spectra. Although the global structure of BZT relaxors is cubic as observed in Neutron diffraction studies, the local symmetry is non-cubic as evident in the micro-Raman spectra of BZT relaxors as well as in their thermal strain measurement, dielectric and pyroelectric behavior.

Behavior of U-Zr Alloy Containing Simulated Fission Products during Anodic Dissolution in Molten Chloride Electrolyte

To investigate the distribution of fission product elements in the electrorefining of spent metallic fuel, electrorefining tests were carried out using U-Zr alloy containing simulated fission products. An intermediate region was found between undissolved alloy and Zr-rich regions formed by the anodic dissolution of uranium. The composition of this region determined by wavelength-dispersive X-ray spectroscopy (WDS) analysis corresponded to that of δ-phase (63–78 at% Zr) in the U-Zr binary phase diagram. This region is expected to form in the course of the anodic dissolution sequence that begins from an undissolved U-Zr alloy and ends with a Zr-rich region. From the results of chemical and WDS analyses of anode residue samples, it was predicted that noble metals would stay in the anode residue as long as zirconium remains even after most of the uranium in the alloy was dissolved. Analytical results indicated no large difference betweenthe noble metal behavior in simulated material and in irradiated EBR-II driver fuels. Decontamination factors of uraniumfrom molybdenum and palladium were defined as ratios between their concentrations in the initial alloy loaded into an anode and those in cathode products, and evaluated from the analytical results. Although there was some scattering of data, decontamination factors of around 70 for molybdenum and 10 for palladium would be expected.

Behavior of U-Zr Alloy Containing Simulated Fission Products during Anodic Dissolution in Molten Chloride Electrolyte

To investigate the distribution of fission product elements in the electrorefining of spent metallic fuel, electrorefining tests were carried out using U-Zr alloy containing simulated fission products. An intermediate region was found between undissolved alloy and Zr-rich regions formed by the anodic dissolution of uranium. The composition of this region determined by wavelength-dispersive X-ray spectroscopy (WDS) analysis corresponded to that of δ-phase (63–78 at% Zr) in the U-Zr binary phase diagram. This region is expected to form in the course of the anodic dissolution sequence that begins from an undissolved U-Zr alloy and ends with a Zr-rich region. From the results of chemical and WDS analyses of anode residue samples, it was predicted that noble metals would stay in the anode residue as long as zirconium remains even after most of the uranium in the alloy was dissolved. Analytical results indicated no large difference betweenthe noble metal behavior in simulated material and in irradiated EBR-II driver fuels. Decontamination factors of uraniumfrom molybdenum and palladium were defined as ratios between their concentrations in the initial alloy loaded into an anode and those in cathode products, and evaluated from the analytical results. Although there was some scattering of data, decontamination factors of around 70 for molybdenum and 10 for palladium would be expected.

Corrosion of magnesium alloy ZE41 – The role of microstructural features

Magnesium alloy ZE41, used extensively in the aerospace industry, possesses excellent mechanical properties albeit poor corrosion resistance. This paper investigates the mechanism of corrosion and the interaction between the grain boundary intermetallic phases, the Zr-rich regions within the grains and the bulk Mg-rich matrix. The results of optical and scanning electron microscopy (SEM) together with energy-dispersive X-ray (EDX) and atomic force microscopy (AFM) potential map measurements have shown the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment, indicating that the Zr-rich regions play a distinct role in the early stages of corrosion in this alloy.

EXAFS study of lead-free relaxor ferroelectricBaTi1−xZrxO3at the ZrKedge

Extended x-ray absorption fine structure experiments at the Zr $K$ edge were carried out on perovskite relaxor ferroelectrics $\mathrm{Ba}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Zr}}_{x}{\mathrm{O}}_{3}$ (BTZ) $(x=0.25,0.30,0.35)$, and on $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_{3}$ for comparison. Structural information up to $4.5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ around the Zr atoms is obtained, revealing that the local structure differs notably from the average $Pm\overline{3}m$ cubic structure deduced from the x-ray diffraction. In particular, our results show that the distance between Zr atoms and their first oxygen neighbors is independent of the Zr substitution rate $x$ and equal to that measured in $\mathrm{Ba}\mathrm{Zr}{\mathrm{O}}_{3}$, while the x-ray cubic cell parameter increases linearly with $x$. Furthermore, we show that the Zr atoms tend to segregate in Zr-rich regions. We propose that the relaxor behavior in BTZ could be influenced by the random elastic fields generated by this particular chemical arrangement.

Designed synthesis of large-pore mesoporous silica–zirconia thin films with high mixing degree and tunable cubic or 2D-hexagonal mesostructure

Mesoporous oxide thin films with a mixed silica/zirconia framework were reproducibly synthesised for the first time by a two-step dip-coating method. Different synthesis strategies were developed for the Si-rich and Zr-rich regions. These optical quality films are highly organised and oriented, stable, and exhibit amorphous mixed frameworks presenting good dispersion of both cations at the molecular level. Si inclusion in zirconia matrices modifies the inorganic network, improving the thermal stability of the mesostructure. A ‘library’ of varied compositions [0 ≤ fSi = [Si]/([Si] + [Zr]) ≤ 0.3; 0.7 ≤ fSi = [Si]/([Si] + [Zr]) ≤ 1] and tunable mesostructures (cubic Im3m, 2D-hexagonal p6m, local hexagonal, lamellar) has been established with F127, P123 and Brij 58 as templates, opening the way to using different PEO-based polymers to fine-tune the pore sizes.

Microstructure Feature of Bulk Glassy Cu&lt;SUB&gt;60&lt;/SUB&gt;Zr&lt;SUB&gt;30&lt;/SUB&gt;Ti&lt;SUB&gt;10&lt;/SUB&gt; Alloy in As-cast and Annealed States

The microstructures of a bulk glassy Cu 60 Zr 30 Ti 10 alloy in as-cast state and annealed at glass transition temperature (T g ), i.e., 440°C, have been studied using an electron transmission microscope (TEM) and a high-resolution transmission electron microscope (HRTEM). Nanocrystals with a size of 4 nm were observed in the cast rod with a diameter of 4 mm. The nanocrystal was determined to be CuZr, which is primitive cubic structure with a lattice parameter of a = 0.3262 nm by analyzing its nano-beam electron diffraction pattern, suggesting that such nanocrystals have a positive effect on mechanical properties. Obviously distinguished interval dark and bright regions were observed in the glassy Cu 60 Zr 30 Ti 10 alloy annealed for 10 min at T g . Nanocrystalline phase with a size of ∼10 nm was recognized after the annealing treatment. The concentration distribution of Cu, Zr and Ti in the alloy annealed for 10 min at T g was examined with a three-dimensional atom probe field ion microscopy (3DAP-FIM). It was found that the Cu and Zr elements fluctuated along depth direction while the Ti element remained almost unchanged. It is therefore interpreted that the annealing-induced nanocrystalline phase results from nucleation and growth at the sites of the Cu-rich and/or Zr-rich regions as well as from growth of the cast-in nanocrystals.