Nanocrystalline Zr Research Articles

The complex structural and chemical nature of monolithic U-10Mo fuel and Zr barrier layer

Nanoscale microstructural characterization by advanced transmission electron microscopy techniques on a U-10Mo/Zr barrier layer monolithic fuel plate was performed to evaluate the microstructural evaluation after high burn-up. Gas bubble superlattice evolution, grain restructuring, and evolution of the Zr interaction layer is investigated through detailed electron microscopy characterization. The use of automated crystallographic orientation mapping to irradiated U-10Mo fuel highlights that the restructured ultra-fine grains are separated by high angle grain boundaries at a burn up of 4.42 × 1021 fissions/cm3. Additionally, advanced chemical analysis and multi-variable statistical analysis shows spatial clustering of solid fission product precipitates. Finally, characterization of a newly observed porous nanocrystalline Zr region in the barrier layer is studied. This work provides insights into the grain subdivision and restructuring process while using advanced microscopy techniques to analyze fission products in neutron-irradiated U-10Mo fuel.

Molecular dynamics simulation of grain size effect on friction and wear of nanocrystalline zirconium

In this study, molecular dynamics simulation was conducted to investigate the frictional behaviors between diamond tool and zirconium (Zr) substrates at the nanoscale. The effects of grain size on friction and wear were discussed under different sliding velocities. The simulation results showed that the friction forces had similar variation tendencies under different sliding velocities. Besides, the friction responses were stronger at high sliding velocities because of the atomic adhesion while the ploughing effect was more obvious at slower sliding velocity. Moreover, both the friction forces and the wear amounts increased with the decrease in the average grain sizes of the substrates. To explain this phenomenon, the internal mechanism was investigated by using the dislocation extract algorithm and the atomic displacement analyses. The results showed that the [0001]-oriented single crystalline substrate was prone to form continuous dislocation structures moving tangentially along the sliding direction due to the characteristic of Zr's slip systems, whereas grain boundaries conducted the deformation further into the polycrystalline substrates, increasing the contact areas and causing atomic accumulation in front, both resulted in stronger friction responses and wear. Accordingly, with the decrease in average grain sizes, the substrates experienced more severe subsurface damage and the deformation mechanism of nanocrystalline Zr had evolved from dislocation emission to grain boundary rotation and sliding.

Preparation of nanocrystalline Zr, La and Mg-promoted 10% Ni/Ce0.95Mn0.05O2 catalysts for syngas production via dry reforming reaction

The influence of the various promoters (Zr, La and Mg) on the physicochemical and catalytic characteristics of the 10% Ni/Ce0.95Mn0.05O2 solid solution catalyst were investigated in methane dry reforming at atmospheric pressure. The co-precipitation method was employed for the synthesis of the catalyst carrier. The catalysts were characterized by BET, XRD, H2-TPR and TPO analyses. The obtained results revealed that the addition of the promoters increased the BET surface area and the highest BET surface area was related to the catalyst promoted by La (58.99 m2/gr). The results of the TPR analysis showed that the broad peak related to the reduction of NiO species was shifted to the higher temperature, indicating the enhancement of the interaction between NiO particles and the support due to the addition of the promoter. The obtained results indicated that the addition of Mg improved the activity (CH4 conversion (%) = 67 at 700 °C) and stability and reduced the amount of deposited carbon. Furthermore smaller Ni crystalline size was related to the catalyst promoted by Mg (10.0 nm). The highest and the lowest amount of carbon deposition was observed on the 10Ni/Ce0.95Mn0.05O2 and 10Ni/Ce0.85Zr0.10Mn0.05O2 catalysts, respectively.

The corrosion of Zr(Fe, Cr)2 and Zr2Fe secondary phase particles in Zircaloy-4 under 350 °C pressurised water conditions

Using Scanning Transmission Electron Microscopy (STEM) coupled with Dual Electron Energy Loss Spectroscopy (DualEELS) and scanned diffraction, the corrosion and incorporation of Secondary Phase Particles (SPPs) in the oxide layer of Zircaloy-4 material has been investigated. This study focuses on mapping the corrosion of Zr2Fe and Zr(Fe, Cr)2 precipitates during the oxidation process and depicting their morphology as the oxidation front advances through the material. It has been found that Zr2Fe SPPs retain the same general shape as in their pre oxidation stage, and transform to a nanocrystalline homogeneous mixed oxide, with a strong crystallographic texture, but hitherto unknown structure. The Zr(Fe, Cr)2 Laves-phase SPPs however, oxidise in a notably more complicated manner. As the α-Zr around an SPP begins to oxidise, the SPP is completely encapsulated by the ZrO2 whilst much of the SPP remains initially unoxidised. But, on oxidation, significant elemental segregation takes place, usually leaving a Cr2O3-rich cap, a nanocrystalline Zr,Cr mixed oxide body and veins of well-crystallised metallic iron. Both forms of SPP have a different expansion on oxidation compared to the Zr, resulting in cracking of the ZrO2.

Atomistic simulation of the tension/compression response of textured nanocrystalline HCP Zr

Molecular dynamics virtual tension/compression tests were performed to study the deformation mechanisms in nanocrystalline (15–38nm grain size) 50 grains HCP Zr columnar samples with random [1−100] prismatic and [0001] basal textures. Two different embedded atom potentials were used to model atomic interactions and to describe the relationship between generalized stacking fault energies and mechanical predictions. Both potentials predict higher flow stresses in compression for both textures. Nanocrystalline Zr [0001] basal textured samples deform mainly by dislocations emission and glide along 〈11−20〉 {1−100} and by grain boundary sliding and migration; both potentials show the same deformation mechanisms for this texture. Prism [1−100] textured samples deform mainly by twinning. Both in tensile and compression tests {11−21} twins nucleate mainly at grain boundaries and sites with high stress concentration. These twins grow by a shuffle mechanism. Emission and sliding of partial dislocations, grain boundary gliding and migration were also observed for this texture with the main dislocations being of the 1/3 〈1−210〉 type. These basic deformation mechanisms were the same for both potentials tested. The observed quantitative differences between the predictions of both potentials are discussed in terms of some of their basic properties.

Thermal diffusivity of submicro- and nanocrystalline Zr–2.5% Nb and Zr–50% Nb alloys at high temperatures

Measurement data are given for the thermal diffusivity of Zr–2.5% Nb and Zr–50% Nb alloys both with the usual polycrystalline structure and with submicro- and nanocrystalline structures at high temperatures, which were determined in the automated mode using the dynamic technique of plane temperature waves.

Composites of amorphous and nanocrystalline Zr–Cu–Al–Nb bulk materials synthesized by spark plasma sintering

The fabrication of Zr70Cu24Al4Nb2 bulk metallic glass composite samples by spark plasma sintering (SPS) process has been successfully realized. The unique characteristics of bulk metallic glasses could lead to the possibility of future applications as new structural and functional materials. The densification of an amorphous Zr70Cu24Al4Nb2 powder was realized in a systematic study changing the sintering temperature in the SPS process leading to stable composites characteristic of amorphous and nanocrystalline structures. X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) analysis, transmission electron microscopy (TEM) as well as hardness tests were applied to determine the structural and mechanical properties of the sintered materials. A stable amorphous bulk metallic glass based on Zr70Cu24Al4Nb2 with a low fraction of crystallites could be fabricated applying a nominal sintering temperature of 400 °C. Higher sintering temperatures lead to composites with high fractions of nanocrystalline material with porosities below 0.5%.

Time-dependent creep behavior of amorphous ZrCu and nanocrystalline Zr thin films – A comparison

Nanoindentation time-dependent relaxation tests were performed on the amorphous ZrCu, nanocrystalline Zr and multilayer ZrCu/Zr thin films aiming to explore the different time-dependent behaviors of these materials under the similar load level at room temperature. There appears an interesting crossing phenomenon of the creep rate as a function of applied stress. In comparison with the ZrCu thin films, the Zr film shows higher load/stress sensitivity for the creep response, suggesting the operating of dislocation creep along various slip systems and some minor grain-boundary-sliding creep mechanism. Multilayered ZrCu/Zr thin films also exhibit higher creep response due to the presence of numerous interfaces.

Structural and mechanical properties of nanocrystalline Zr co-sputtered a-C(:H) amorphous films

The aim of this study was to investigate the effect of Zr as alloying element to carbon films, particularly in respect to film structure and mechanical properties. The films were deposited by magnetron sputtering in reactive (Ar+CH4) and non-reactive (Ar) atmosphere with different Zr contents (from 0 to 14at.%) in order to achieve a nanocomposite based films. With an increase of Zr content a broad peak was observed in X-ray diffraction spectra suggesting the presence of nanocrystalline (nc) ZrC phase for the coatings with Zr content higher than 4at.%. The application of Scherrer formula yielded a grain sizes with a dimension of 1.0–2.2nm. These results were supported by X-ray photoelectron spectroscopy showing typical charge transfer at ZrC nanograins and carbon matrix interface. The nc-ZrC phase was also observed by transmission electron microscopy. The hardness of the coatings was approximately independent of Zr content. However, the Young modulus increased linearly. The residual stress of the coatings was strongly improved by the presence of nc-ZrC phase embedded in the a-C matrix. Finally, the incorporation of H into the matrix led to denser and harder films.

Structural, optical, dielectric and electrical studies on RF sputtered nanocrystalline Zr doped MgTiO3 thin films

Mg(Zr0.05Ti0.95)O3 (MZT) thin films have been deposited onto amorphous SiO2 and platinized silicon (Pt/TiO2/SiO2/Si) substrates by RF magnetron sputtering at different oxygen mixing percentages (OMP). The effect of OMP and post annealing temperature at 600°C on crystal orientation, surface morphology, optical, electrical and dielectric properties of crystallined MZT films are investigated. A change of preferred orientation from (104) to (110) has been observed with increasing OMP from 0% to 100%. The preferred orientation growth has been explained on the basis of Lotgering model by calculating the orientation factor. The optical properties such as refractive index and bandgap were strongly influenced with O2 content. It is also observed that the refractive index and packing density decreases with increasing OMP, leading to an enhancement in the optical bandgap from 4.19 to 4.52eV. The dielectric constant and dielectric loss of the films ranged between 5.0–15.69 and 0.082–0.0135. The MZT thin films deposited at 100% OMP exhibited the high dielectric constant and low loss tangent. Further, the DC conductivity with frequency follows Jonscher law and the activation energy found to be 0.32eV for MZT films deposited in pure oxygen atmosphere. Nyquist plots of MZT films revealed two semi circular arcs and are explained on the basis of equivalent circuit model. A series of two parallel RC combination in series fit with experimental data. The I–V characteristics reveals that the leakage current density decreases from 1.136×10−8 to 4.69×10−9A/cm2 for OMP, 0–100%, respectively for electric field strength of 200kV/cm.

Low-Temperature Synthesis of Ultra-High-Temperature Coatings of ZrB2 Using Reactive Multilayers

We demonstrate a route to synthesize ultra-high-temperature ceramic coatings of ZrB2 at temperatures below 1300 K using Zr/B reactive multilayers. Highly textured crystalline ZrB2 is formed at modest temperatures because of the absence of any oxide at the interface between Zr and B and the very short diffusion distance that is inherent to the multilayer geometry. The kinetics of the ZrB2 formation reaction is analyzed using high-temperature scanning nanocalorimetry, and the microstructural evolution of the multilayer is revealed using transmission electron microscopy. We show that the Zr/B reaction proceeds in two stages: (1) interdiffusion between the nanocrystalline Zr and the amorphous B layers, forming an amorphous Zr/B alloy, and (2) crystallization of the amorphous alloy to form ZrB2. Scanning nanocalorimetry measurements performed at heating rates ranging from 3100 to 10000 K/s allow determination of the kinetic parameters of the multilayer reaction, yielding activation energies of 0.47 and 2.4 eV for Zr/B interdiffusion and ZrB2 crystallization, respectively.

Effects on structure and properties of Zr55Al10Cu30Ni5 metallic glass irradiated by high intensity pulsed ion beam

Abstract High intensity pulsed ion beam technology was used for the surface irradiation treatment of metallic glass Zr 55 Al 10 Cu 30 Ni 5 and W metal. The ion beam was mainly composed of C n + (70%) and H + (30%) at an acceleration voltage of 250 kV under different energy densities for different number of pulses. XRD analysis showed that the metallic glass remained amorphous in its main structure after HIPIB irradiation, without apparent presence of crystalline phases. SEM analysis concluded that there was no apparent irradiation damage on the surface of metallic glass at the low irradiation frequency (3 times and 10 times); “petal”-shaped irradiation damage appeared on the surface of metallic glass after multi-irradiation (100 times and 300 times), and the composition of the petal center included Fe and Cr, the composition of an ion diode cathode, in addition to the composition of Zr-based metallic glass. TEM analysis of irradiated metallic glass showed that a small amount of nanocrystalline Zr 2 Ni-type phase (face centered cubic) was produced only at 300-time irradiation. Cracks appeared on the surface of W after 100-time and 300-time irradiation; shedding phenomenon even appeared on the surface of W at the energy densities of 1.4 J/cm 2 and 2.0 J/cm 2 . The surface nano-hardness of irradiated metallic glass decreased.

Microwave absorbance properties of zirconium–manganese substituted cobalt nanoferrite as electromagnetic (EM) wave absorbers

Nanocrystalline Zr–Mn (x) substituted Co ferrite having chemical formula CoFe2−2xZrxMnxO4 (x=0.1–0.4) was prepared by co-precipitation technique. Combining properties such as structural, electrical, magnetic and reflection loss characteristics. Crystal structure and surface morphology of the calcined samples were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). By using two point probe homemade resistivity apparatus to find resistivity of the sample. Electromagnetic (EM) properties are measured through RF impedance/materials analyzer over 1MHz–3GHz. The room-temperature dielectric measurements show dispersion behavior with increasing frequency from 100Hz to 3MHz. Magnetic properties confirmed relatively strong dependence of saturation magnetization on Zr–Mn composition. Curie temperature is also found to decrease linearly with addition of Zr–Mn. Furthermore, comprehensive analysis of microwave reflection loss (RL) is carried out as a function of substitution, frequency, and thickness. Composition accompanying maximum microwave absorption is suggested.

Magnetic hardening of Zr2Co11:(Ti, Si) nanomaterials

Abstract The role of Ti and Si additions in the magnetic hardening of rapidly-quenched Zr 2 Co 11 -based nanomaterials has been investigated. Nanocrystalline Zr 17 − x Ti x Co 83 and Zr 18 Co 82 − y Si y are mainly composed of rhombohedral Zr 2 Co 11 and a small amount of orthorhombic Zr 2 Co 11 , hcp Co and cubic Zr 6 Co 23 . Ti addition decreases the mean grain size of the magnetic phases, and thus increases coercivity and energy product from 1.6 kOe and 1.9 MGOe for x = 0 to 2.6 kOe and 3.9 MGOe for x = 2, respectively. Si addition enhances the anisotropy field of the hard phase which increases the coercivity but slightly decreases the magnetization. This work shows that Ti has a positive effect on energy product through refinement of structure.

Grain size effects in the deformation of [0 0 0 1] textured nanocrystalline Zr

Molecular dynamics was used to simulate tensile testing of columnar grain samples of nanocrystalline Zr with random rotations around the [0 0 0 1] axis. For grain sizes smaller than a critical size the results show an inverse Hall–Petch effect. For larger grains the calculated Hall–Petch constant agrees well with the experimental data. Grain size effects on deformation mechanisms, including grain boundary sliding and dislocation emission, are also discussed.

Deformation crossover in nanocrystalline Zr micropillars: The strongest external size

For small-scale metallic single crystals, systematic studies have established that external sample size ( ϕ ) has an obvious influence on the apparent strength, following a “smaller is stronger” fashion. In the present nanocrystalline (NC) Zr micropillars, deformation crossover leads to the strongest external size emerging at a critical ϕ ≈ 400 nm , similar to that of bulk-scaled NC metals. Above the critical external size the NC Zr pillars failed via highly inhomogeneous single-shear offset, below which they deformed via relative homogeneous shear.

Reduction of the allotropic transition temperature in nanocrystalline zirconium: Predicted by modified equation of state (MEOS) method and molecular dynamics simulation

The temperature at which polymorphic phase transformation occurs in nanocrystalline (NC) materials is different from that of coarse-grained specimens. This anomaly has been related to the role of grain boundary component in these materials and can be predicted by a dilated crystal model. In this study, based on this model, a modified equation of state (MEOS) method (instead of equation of state, EOS, method) is used to calculate the total Gibbs free energy of each phase (β-Zr or α-Zr) in NC Zr. Thereupon, the change in the total Gibbs free energy for β-Zr to α-Zr phase transformation (ΔGβ→α) via the grain size is calculated by this method. Similar to polymorphic transformation in other NC materials (Fe, Nb, Co, TiO2, Al2O3 and ZnS), it is found that the estimated transformation temperature in NC Zr (β→α) is reduced with decreasing grain size. Finally, a molecular dynamics (MD) simulation is employed to confirm the theoretical results.

Hard nanocrystalline Zr–B–C–N films with high electrical conductivity prepared by pulsed magnetron sputtering

Zr–B–C–N films were deposited on silicon and glass substrates using pulsed magnetron co-sputtering of a single B4C–Zr target (at 15% or 45% of zirconium in the target erosion area) in nitrogen–argon gas mixtures. A planar unbalanced magnetron was driven by a pulsed dc power supply operating at a repetition frequency of 10kHz with a fixed 85% duty cycle. The total pressure was 0.5Pa and the substrate temperature was adjusted to 450°C during the depositions on the substrates at a floating potential. High-quality defect-free films, 3.5 to 4.1μm thick, with smooth surfaces (the average roughness Ra≤4nm) and good adhesion to substrates at low compressive stresses (less than 0.9GPa) were produced. Hard (37GPa) nanocolumnar ZrB2-type films of the Zr25B57C14N3 composition (in at.% without 1at.% of hydrogen) with a very low compressive stress (0.4GPa), high electrical conductivity (electrical resistivity of 2.3×10−6Ωm) and high oxidation resistance in air up to 650°C were prepared in pure argon at a 15% Zr fraction in the target erosion area. Hard (37GPa) nanocomposite Zr41B30C8N20 films with a low compressive stress (0.6GPa), even higher electrical conductivity (electrical resistivity of 1.7×10−6Ωm) and high oxidation resistance in air up to 550°C were deposited in a 5% N2+95% Ar gas mixture at a 45% Zr fraction in the target erosion area. Increasing the N2 fraction (>5%) in the gas mixture resulted in a significant decrease of the film hardness and in a rapid rise in their electrical resistivity and oxidation resistance in air at elevated temperatures due to a growing volume fraction of an amorphous phase with a high content of nitrogen (up to 52at.%) in the materials.

Effect of microstructure on the electrocatalytic activity for hydrogen evolution of amorphous and nanocrystalline Zr–Ni alloys

Rapidly quenched Zr2Ni amorphous and nanocrystalline ribbons were studied as electrocatalysts for hydrogen evolution in 6 M KOH. Linear polarization, potentiostatic hydrogen charge/discharge and EIS measurements at various potentials were carried out for the Zr alloys with different microstructure with the aim to extract information about the mechanism of hydrogen evolution and absorption and estimate the kinetic parameters of the hydrogen evolution reaction (HER). Though the melt-spun Zr67Ni33 alloys with varying microstructure do not show substantially different catalytic activity for HER, it could be clearly demonstrated that the nanocrystalline material reveals better catalytic performance than the entirely amorphous and nano-/amorphous alloys with the same chemical composition. It was found that all studied Zr–Ni alloys absorb hydrogen under the conditions of the hydrogen evolution experiments, as the amount of the absorbed hydrogen depends to a large degree on the alloys microstructure as well as on the applied potential during the HER experiment. The diffusion coefficient of hydrogen into the amorphous Zr67Ni33 alloy, as well as the thickness of the hydrided layer were found to be noticeably larger than those of the nanocrystalline alloy at the same conditions of hydrogen charging. Therefore the improved electrocatalytic properties of the nanocrystalline alloy could only be explained by its favorable microstructure (e.g. higher density of defects) and weaker hydrogen absorption into the nanostructured material under the conditions of the HER.

Nanocrystalline Zr 3Al made through amorphization by repeated cold rolling and followed by crystallization

The intermetallic compound Zr 3Al is severely deformed by the method of repeated cold rolling. By X-ray diffraction it is shown that this leads to amorphization. TEM investigations reveal that a homogeneously distributed debris of very small nanocrystals is present in the amorphous matrix that is not resolved by X-ray diffraction. After heating to 773 K, the crystallization of the amorphous structure leads to a fully nanocrystalline structure of small grains (10–20 nm in diameter) of the non-equilibrium Zr 2Al phase. It is concluded that the debris retained in the amorphous phase acts as nuclei. After heating to 973 K the grains grow to about 100 nm in diameter and the compound Zr 3Al starts to form, that is corresponding to the alloy composition.