Nanocolumnar Grains Research Articles

Nanostructured LSC Thin Film Electrodes with Improved Electrochemical Performance and Long-Term Stability

Nanostructured La0.6Sr0.4CoO3-δ (LSC) thin film electrodes exhibit exceptionally high oxygen surface exchange properties, surpassing those of conventional microscale electrode structures, which are desirable for application in solid oxide cells (SOC) [1-2]. On the other hand, the LSC nanostructures also tend to undergo significant morphological changes at typically high temperatures required for SOC operation, leading to rapid degradation in performance. Here, towards the goal of improving the long-term stability of electrochemical performance of nanostructured LSC thin films, a systematic investigation of the effect of processing temperatures on long-term stability was carried out [3]. By varying the deposition temperature (500 °C to room temperature), the as-grown characteristic nanostructures of LSC thin films prepared using pulsed laser deposition can be tuned from highly dense nanocolumnar grains to nanofibrous structures with high porosity. Variations in the deposition temperature also resulted to differences in the proportion of surface-bound/lattice-bound Sr and Co2+/Co3+ at the surfaces of the as-grown LSC thin films; however, prolonged annealing at 700 °C in air essentially transforms the surfaces to a final state with mostly lattice-bound Sr and Co3+. Nevertheless, LSC films with initially nanofibrous structures are found to be less prone to the grain sintering effect occurring at high temperatures and exhibit less degradation of the electrode polarization resistance as compared to well-dense films. Using lower deposition temperatures, cation interdiffusion occurring at LSC/GDC interfaces is also significantly suppressed, thus leading to better interfacial stability as compared to those prepared at higher deposition temperatures. These results highlight the relationship between characteristic nanostructures of thin film electrodes and electrochemical performance and provide guidance on designing electrodes with improved long-term stability.[1] J. Januschewsky, M. Ahrens, A. Opitz, F. Kubel and J. Fleig, Adv. Funct. Mater., 2009, 19, 3151–3156.[2] J. Hayd, L. Dieterle, U. Guntow, D. Gerthsen and E. Ivers-Tiffee, J. Power Sources, 2011, 196, 7263–7270.[3] K. Develos-Bagarinao, O. Celikbilek, R. A. Budiman, G. Kerherve, S. Fearn, S. J. Skinner and H. Kishimoto, J. Mater. Chem. A, 10, 2445-2459 (2022).

Nanostructured LSC Thin Film Electrodes with Improved Electrochemical Performance and Long-Term Stability

Nanostructured La0.6Sr0.4CoO3- δ (LSC) thin film electrodes exhibit exceptionally high oxygen surface exchange properties, surpassing those of conventional microscale electrode structures, which are desirable for application in solid oxide cells (SOCs). Here, towards the goal of improving the long-term stability of electrochemical performance of nanostructured LSC thin films, a systematic investigation of the effect of processing temperatures on long-term stability was carried out. By varying the deposition temperature (from 500 °C to room temperature), the as-grown characteristic nanostructures of LSC thin films prepared using pulsed laser deposition can be tuned from highly dense nanocolumnar grains to nanofibrous structures with high porosity. From the comparison of the properties of the as-grown films and those annealed at 700 °C for 300 h, it was revealed that LSC thin films prepared at lower deposition temperatures are less prone to the grain sintering effect and cation interdiffusion and exhibit better interfacial stability and improved long-term performance.

Superhardness Induced by Grain Boundary Vertical Sliding in (001)-textured ZrB[formula omitted] and TiB[formula omitted] Nano Films

Superhardness up to 50 and 60 GPa has been achieved recently in orientationally (001)-textured ZrB2 and TiB2 films composed of nano columnar grains, which almost double those in common TMB2 (TM=Zr, Ti). But the atomic origin, especially the role of shear stresses under indentation on grain boundaries (GBs), remains largely unknown. In this work, we report the identification of the experimentally observed GBs as Σ13 [001] tilt GB by first-principles calculations. The calculated formation energies of the GBs and TM vacancies on the GBs explain well the experimental results. Under shear stresses beneath indentors, the GB motion transforms the accumulated shear stresses parallel to (001) plane into a vertical sliding of the nano grains in [001] direction, while maintaining the GB structure unchanged. This special mode of shear plastic deformation correlates their hardness to the compressive strengths in [001] direction, instead of to the shear strengths on (001) plane as in bulk crystals, with the calculated compressive strengths well accounting for the measured superhardness in these (001)-textured nano ultra-high temperature ceramics.

High-speed epitaxial growth of SrTiO3 transparent thick films composed of close-packed nanocolumns using laser chemical vapor deposition

(100) SrTiO3 transparent thick film was synthesized via high-speed epitaxial growth of close-packed nanocolumnar grains grown on (100) MgAl2O4 single-crystal substrate using laser chemical vapor deposition. The (100) epitaxial SrTiO3 film synthesized at a total chamber pressure of 0.4 kPa and a deposition temperature of 1113 K was optically transparent with a relative transmittance of 95% at a wavelength of 800 nm, and the deposition rate of the film was 20 μm h−1. TEM observation revealed that the SrTiO3 transparent thick film was composed of close-packed nanocolumnar grains.

Plastic anisotropy and tension-compression asymmetry in nanotwinned Al–Fe alloys: An in-situ micromechanical investigation

The mechanical strength of commercial Al alloys rarely exceeds 700 MP. Recent studies show that nanotwinned Al alloys and other nanocolumnar metals exhibit superb mechanical behaviors but a discrepancy between tensile and hardness measurements often emerges and their orientation dependent deformation mechanisms remain unclear. Here, we inspect the mechanical response of columnar nanotwinned Al–Fe alloys with emphasis on response of grain boundaries to in-situ tension and compression tests along both in-plane and out-of-plane directions inside a scanning electron microscope. Our studies reveal ultra-high out-of-plane tensile and compressive stress, exceeding 1.8 GPa, and an in-plane tensile and compressive stress of 1.1 and 1.6 GPa, respectively. Post-mortem TEM analyses were performed to elucidate the orientation-dependent plastic anisotropy and tension-compression asymmetry in columnar nanotwinned Al–Fe alloys. This study provides an important forward step towards the understanding of deformation mechanisms in high-strength nanotwinned Al alloys and metals with nanocolumnar or non-equiaxed grains.

Oblique angle deposition of nickel thin films by high-power impulse magnetron sputtering.

Background: Oblique angle deposition is known for yielding the growth of columnar grains that are tilted in the direction of the deposition flux. Using this technique combined with high-power impulse magnetron sputtering (HiPIMS) can induce unique properties in ferromagnetic thin films. Earlier we have explored the properties of polycrystalline and epitaxially deposited permalloy thin films deposited under 35° tilt using HiPIMS and compared it with films deposited by dc magnetron sputtering (dcMS). The films prepared by HiPIMS present lower anisotropy and coercivity fields than films deposited with dcMS. For the epitaxial films dcMS deposition gives biaxial anisotropy while HiPIMS deposition gives a well-defined uniaxial anisotropy.Results: We report on the deposition of 50 nm polycrystalline nickel thin films by dcMS and HiPIMS while the tilt angle with respect to the substrate normal is varied from 0° to 70°. The HiPIMS-deposited films are always denser, with a smoother surface and are magnetically softer than the dcMS-deposited films under the same deposition conditions. The obliquely deposited HiPIMS films are significantly more uniform in terms of thickness. Cross-sectional SEM images reveal that the dcMS-deposited film under 70° tilt angle consists of well-defined inclined nanocolumnar grains while grains of HiPIMS-deposited films are smaller and less tilted. Both deposition methods result in in-plane isotropic magnetic behavior at small tilt angles while larger tilt angles result in uniaxial magnetic anisotropy. The transition tilt angle varies with deposition method and is measured around 35° for dcMS and 60° for HiPIMS.Conclusion: Due to the high discharge current and high ionized flux fraction, the HiPIMS process can suppress the inclined columnar growth induced by oblique angle deposition. Thus, the ferromagnetic thin films obliquely deposited by HiPIMS deposition exhibit different magnetic properties than dcMS-deposited films. The results demonstrate the potential of the HiPIMS process to tailor the material properties for some important technological applications in addition to the ability to fill high aspect ratio trenches and coating on cutting tools with complex geometries.

Deformation behavior of electroplated gold composed of nano-columnar grains embedded in micro-columnar textures

Micro-mechanical property anisotropy of Au micro-pillars composed of nano-columnar grains embedded in micro-columnar textures was evaluated by micro-compression tests. The micro-pillars having dimensions of 10×10×20μm3 were fabricated from constant-current electroplated Au film by focus ion beam milling. The long-axis of the nano-columnar grains was parallel to the long-axis of the micro-columnar textures, and the long-axes were parallel to growth direction of the electroplated Au film. The deformation behavior changed from brittle fracture to multiple slip deformation and the yield stress varied from 650 to 300MPa when the compression direction changed from perpendicular to parallel to the long-axis of the micro-columnar textures.

Influence of pulse frequency and bias on microstructure and mechanical properties of TiB2 coatings deposited by high power impulse magnetron sputtering

Abstract The high plasma density and large fraction of ionized species created in a high power impulse magnetron sputtering (HiPIMS) discharge add new measures to control the sputtering process. We have studied the sputtering of TiB2 coatings by HiPIMS from a compound target in an industrial system. How the degree of ionized species effects coating microstructure and mechanical properties has been investigated by varying the pulse frequency between 200 Hz and 1000 Hz while keeping the average power constant at 2 kW. The coatings have a B/Ti atomic ratio ≥ 2.5 and a microstructure exhibiting 001 textured nanocolumnar grains with an amorphous B tissue phase in grain boundaries. Lower frequencies provide higher degree of ionization, which does, however, increase the compressive residual stress in the coatings. This results in harder coatings and the highest hardness of 49 GPa is measured for the coating deposited at 200 Hz (− 3.8 GPa residual stress). A change in texture from random orientation to 001 texture is achieved when going from regular dc sputtering to HiPIMS at a floating bias. Superhard (H = 43 GPa) TiB2 coatings with a relatively low compressive stress of about − 1 GPa can be deposited by HiPIMS at 1000 Hz using floating bias.

High frequency reciprocating sliding wear behavior and mechanisms of quaternary metal oxide coatings

Wear resistant coatings based on quaternary metal oxides were studied with comparisons made to more traditional ternary metal oxides. While much is known about ternary metal oxide wear behavior and mechanisms from room to higher temperatures, little is known about quaternary oxides; for instance, the role of the fourth element in determining the coating crystalline state and defect structure and how they control tribological properties. To this end, the system (ZnTiZr)xOy was deposited by atomic layer deposition (ALD) and compared to previously studied wear resistant nanocrystalline ALD ZnTiO3 coatings. X-ray diffraction determined that both as-deposited and ex situ 550°C annealed ternary and quaternary coatings exhibit ZnTiO3 phase or solid solution Zn(Ti,Zr)O3 phase only, respectively. However, the coatings have completely different growth structures where the ternary ZnTiO3 coatings exhibit textured (104) nanocolumnar grains and the quaternary Zn(Ti,Zr)O3 coatings are predominantly amorphous with some nanocrystalline grains. High frequency reciprocating sliding on these coatings revealed that the growth structure did not influence the wear rate (~1×10−6mm3/Nm). However, the crystal structure-dependent deformation mechanisms were different as revealed by FIB–SEM and HRTEM analyses inside worn surfaces. It was determined that both coatings show surface deformation due to ductile layering/smearing with no evidence of brittle fracture. Wear reduction of the ZnTiO3 coating was due to nanoscale sliding-induced plastic deformation when (104) stacking faults were sheared along the sliding direction resulting in an intrafilm shear velocity accommodation mode. Conversely, wear reduction in the quaternary Zn(Ti,Zr)O3 coating was a result of a tribofilm/mechanically mixed layer, composed of amorphous transferred silica from the sliding Si3N4 counterface and refined nanocrystalline Zn(Ti,Zr)O3 grains from the coating. Both wear surfaces contained solid cylindrical rolls or roll-ups often found in ceramic–ceramic reciprocating sliding. New insights revealed that the rolls originate in the tribofilm, with similar composition and structure, and evolve to the surface with continued sliding.

Asymmetric grazing incidence small angle x-ray scattering and anisotropic domain wall motion in obliquely grown nanocrystalline Co films

Strong asymmetries have been observed in grazing incidence small angle x-ray scattering (GISAXS) in situ patterns obtained from 30 nm-thick nanocrystalline Co films prepared by oblique sputtering (15°–75° off-sample normal). These asymmetries have been qualitatively simulated by a simple model consisting of an ensemble of 8 nm-wide inclined Co nanocolumns. It is found that narrow inclined features appear in the diffuse background resembling those characteristic of faceted systems, which can be used to obtain straightforward non-destructive estimations of buried nanocolumnar grains inclination, even for oblique angles below 45°, when the stronger and broader asymmetric features of the pattern are not yet fully formed. Furthermore, using magneto-optical microscopy, a marked change in the magnetic domain’s nucleation and growth process has been observed in the sample prepared at 75°, with the stronger GISAXS asymmetries. Easy axis magnetization reversal starts by a random and homogeneous nucleation of small (∼μm) elongated domains aligned with the nanocolumn’s long axis and proceeds through the preferred propagation of head-to-head domain walls (DWs) along the applied field direction. This peculiar magnetic behavior indicates that the strongly anisotropic nanostructuring created by the oblique growth process is equivalent, from a magnetic point of view, to an array of self-assembled buried nanowires. These results show how GISAXS and magneto-optical microscopy can be combined as a powerful tool for correlating the morphology and magnetism of thin nanostructured systems.

Investigation of obliquely evaporated nanocolumnar cobalt films

Cobalt films 100nm thick were thermally evaporated on NaCl crystals at an incidence angle of 45° in a vacuum of approximately 10−5mbar. The morphological structure of the films consisted of closely packed nanocrystalline grains, typically a few tens of nanometers in size. The alignment of nanocolumnar grains in the direction perpendicular to the incidence plane was observed. The magnetic structure was composed of domains running predominantly in the direction parallel to the incidence plane. The domains were about 1μm wide and substantially magnetized in the plane of the film. Because of the stripe and regular character of the magnetic domains, exchange coupling between the grains is found to be strong and homogeneous. The films were mainly composed of the hexagonal close-packed (HCP) phase of cobalt. The alignment of magnetic domains in the direction parallel to the incidence plane means that the crystallographic contribution to the magnetic anisotropy is dominant over the shape anisotropy.

TiO2/CaF2 composite coating on titanium for biomedical application

Porous and nanostructured TiO2/CaF2 composite coatings were successfully prepared on titanium by plasma electrolytic oxidation. Simulated body fluid (SBF) immersion test was employed to evaluate the in-vitro bioactivity of these composite coatings. The results reveal that the TiO2/CaF2 composite coatings show porous and nanostructured surface with pores of 100–500nm and nanograins of 50–200nm. Fluoridated hydroxylapatite (FHA) can form on the surface of the TiO2/CaF2 composite coatings immersed in SBF for 28 days. The newly formed FHA consists of highly ordered nanocolumnar grains, which vertically grow on the coating surface. The ability to induce the FHA formation might be beneficial to the biological performances of the TiO2/CaF2 composite coating.

Nanocrystalline orientation and phase mapping of textured coatings revealed by precession electron diffraction

Nanocrystalline orientation and phase mapping were performed to characterize texture and grain boundary tilt angles in the transmission electron microscope (TEM) using precession electron diffraction (PED). Texture analysis and grain boundary angle studies were performed on atomic layer deposition (ALD) ZnO/Al2O3/ZrO2 nanolaminate coatings. It was determined that the ZnO layer nanocolumnar grains, with random rotational orientation, exhibited fiber texture with predominately <0001> grains in the out-of-plane direction with respect to the substrate. The ZnO tilt angles were in the range of 10–20° signifying a large area fraction of predominately low angle grain boundaries. The nanolaminate coating has been shown to mitigate tribological (energetic) losses by decreasing friction and wear during sliding, hence is a potential solid lubricant material. The TEM-PED technique has future wide scale applicability in studying energy-related nanomaterial grain orientations and phases with a spatial resolution down to 1–2 nm, in which more traditional techniques, such as electron-backscatter diffraction, cannot resolve.

Elastic properties in different nano-structured AlN films

A study of the transverse acoustic phonons on nano-structured AlN films has been carried out by using high-resolution micro-Brillouin spectroscopy. Dense films have been deposited by radio frequency (r.f.) magnetron sputtering under ultra high vacuum at room temperature. Films with different morphologies were prepared and investigated by transmission electron microscopy and Brillouin Spectroscopy (equiaxed nano-sized, nano-columnar grains and amorphous phase). Results show a dependence of the transverse modes on the nano-structure. The nano-columnar film exhibits two transversal modes as expected for the AlN würtzite while the equiaxed nano-sized and the amorphous films only exhibit one isotropic transverse mode as expected in amorphous materials. One important result is that the sound propagation velocity in the AlN amorphous phase is higher than the one in the non-textured nano-crystalline phase. This phenomenon has, however, already been observed in ferroelectric ceramics.

Electrically pumped ultraviolet ZnO diode lasers on Si

Electrically pumped ZnO quantum well diode lasers are reported. Sb-doped p-type ZnO/Ga-doped n-type ZnO with an MgZnO/ZnO/MgZnO quantum well embedded in the junction was grown on Si by molecular beam epitaxy. The diodes emit lasing at room temperature with a very low threshold injection current density of 10 A/cm2. The lasing mechanism is exciton-related recombination and the feedback is provided by close-loop scattering from closely packed nanocolumnar ZnO grains formed on Si.

Characterization studies of pulse magnetron sputtered hard ceramic titanium diboride coatings alloyed with silicon

The composition, structure and mechanical properties of pulsed-DC unbalanced magnetron sputtered Ti–Si–B thin films—hard coatings with the potential for excellent thermal stability and oxidation resistance—are investigated and reported in this paper. Fully dense, hard (19–37 GPa) Ti–Si–B coatings were deposited at substrate bias voltages ( V s) ranging from floating potential to −150 V which resulted in substrate temperatures of ∼90–135 °C. We found that variation of substrate biasing conditions critically affected film composition, structure and resultant mechanical properties. For instance, concentration of Si in films decreased from 18.4 at.% to 3.8 at.% as V s was increased from floating potential to −150 V; composition profile analysis of the near-surface region of films (0–10 nm) revealed them to be rich in Si with significant differences among specimens produced at different substrate bias conditions. Variation of substrate biasing conditions provided coating structures that ranged from completely amorphous at floating substrate potential to nanocrystalline at V s = −50 to −100 V and crystalline nanocolumnar at V s = −150 V. We found that each of the structures obtained exhibited different specific values of hardness and elastic modulus, which is also in a good agreement with results reported for other coatings possessing similar micro- and nano-structures. Film structure was analyzed in detail by conventional and analytical transmission electron microscopy. Coatings that exhibited the highest values of hardness (37 GPa) were found to possess features such as crystalline nanocolumnar grains a few nanometres in diameter and disordered intergranular regions of different chemical composition, thus qualifying as nanocomposite films. Results of this work allowed relationships to be drawn between deposition parameters and Ti–Si–B coating composition, structure and mechanical properties. Qualitatively similar relationships are also expected for other biased plasma-assisted physical vapour deposited transition-metal-based ceramic coatings alloyed with Si (e.g. Ti–Si–N, Cr–Si–N, Cr–Al–Si–N).

Preparation of Immobilized Nano-Columnar TiO2 Grains by Chemical Vapor Deposition

Nano-columnar TiO2 grains are prepared and immobilized by chemical vapor deposition using TiCl4, H2 and O2 at low temperature. The structure of TiO2 is analyzed by X-ray diffraction (XRD), the morphology is observed by scanning electron microscopy (SEM) and the adhesion is estimated by measuring the critical load in scratch test. Results show that the structure of TiO2 films depend on the deposition temperature changing from amorphous, anatase, rutile, and both anatase and rutile phases as prepared at temperatures of 200, 300, 400 and 500 degrees C, respectively. The nano-columnar TiO2 grains are formed in both rutile and anatase phases, while it could be only rutile phase by increasing TiCl4 flow rate. The morphologies of TiO2 changes from smooth to nano-columnar grains as the deposition temperature increased from 200 to 400 degrees C. Excellent adhesion strength of crystalline TiO2 was obtained and it could be improved by increasing the TiCl4 flow rate in range of 0.3-0.6 sccm, where the critical load of TiO2 increases from 17 to 21 N.

Temperature dependence of structure and mechanical properties of Ti–Si–N coatings

Characterization of structure and mechanical properties of Ti–Si–N coatings deposited by a high-density plasma assisted vapor deposition technique at ∼250 °C was carried out and compared to Ti–Si–N coatings deposited at ∼700 °C. The nanoscale structure of Ti–Si–N coatings was probed by combining transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS). The structural characterizations showed that the present Ti–Si–N coatings consists of nanocolumnar B1-TiN grains interdispersed within an amorphous silicon nitride (a-Si:N) matrix, independent of the deposition temperature. Significant Ti atom dissolution within the a-Si:N matrix was observed, and increasing the deposition temperature from ∼250 to ∼700 °C decreases the Ti dissolution limit. The influence of the extent of the TiN/a-Si:N phase separation on the mechanical properties of Ti–Si–N coatings is probed by instrumented nanoindentation. The extent of phase separation was found to significantly influence the mechanical properties, with the hardness of Ti–Si–N coating deposited at ∼700 °C reaching ∼40 GPa. The present results illustrate the sensitivity of mechanical properties of ceramic nanocomposite coatings on the detailed nanoscale structure.