Nanocrystalline Counterparts Research Articles

Unravelling the development of preferred crystallographic orientation in dual-phase amorphous/nanocrystalline Zr-V thin films

Dual-phase amorphous/nanocrystalline alloys have been proven to present enhanced properties compared to their amorphous or nanocrystalline single-phase counterparts. These dual-phase materials are typically composed of nanocrystalline cores embedded in an amorphous matrix. In contrast, a new kind of dual-phase metallic films has recently been reported in which cone-shaped crystalline regions evolve as a result of competitive growth between the amorphous and crystalline phases with increasing film thickness. Here, we demonstrate that, unlike typical dual-phase alloys, Zr-V thin films exhibiting crystalline/amorphous competitive growth develop a preferred crystallographic orientation upon growth. Relying on pole figure observations, we uncover the development of a preferred orientation of the (110) crystallographic planes in the growth direction of the film with increasing film thickness. High-resolution transmission electron microscopy analysis reveals that the crystalline regions are formed by nano-branches which grow (110)-oriented. Based on this result, together with scanning and transmission electron microscopy observations, we associate the development of the thickness-dependent preferred orientation to the impingement between cone-shaped crystalline regions. This process triggers a selective growth mechanism that favors the selection of nano-branches growing near the direction of film growth. Our results and analyses provide a step forward in understanding the growth kinetics of dual-phase alloys presenting complex microstructures.

Improvement of oxidation resistance and alumina regrowth ability of Ni-30Cr-5Al alloy without and with Y: Nanocrystallization effect

Nanocrystallization of Ni-Cr-Al alloys decreases critical Al content (NAl*) for Al2O3 scale formation down to a level much lower than practical alloy Al content (NAl), and this is particularly prominent for the alloys with NAl≤ NAl*. In this work, oxidation of coarse-grained (CG) Ni-30Cr-5Al with NAl > NAl* and its nanocrystalline (NC) counterpart has been studied, with the focuses on the following issues: (i) nanocrystallization effect on Al2O3 steady-state growth and regrowth; (ii) nanograin growth effect on Al2O3 adhesion; and (iii) role of Y played in NC alloys by comparing the differences in oxidation of the NC alloys without and with Y.

Magnetic properties of Mn/Co substituted nano and bulk Ni–Zn ferrites: A comparative study

The present work shows the comparative studies on the magnetic properties of Ni0.4Zn0.6-xCoxFe2O4, (Ni–Zn–Co) and Ni0.4Zn0.6-xMnxFe2O4 (Ni–Zn–Mn) with x = 0.00 to 0.25 in steps of 0.05, ferrites synthesized by sol-gel autocombustion and conventional ceramic methods. Rietveld refinement data confirms the formation of single phase - spinel crystal structure in all the samples. The role of ferromagnetic dopants like Co and Mn on the structural, microstructural, magnetic characteristics such as Curie temperature (Tc), saturation magnetization and coercivity of NiZnFe2O4 ferrite are investigated. The influences of crystallite size obtained by using two different synthesis methods on different magnetic properties are highlighted. Continuous Tc increases have been noticed in both instances along with increasing dopant concentrations. However, compared to Ni–Zn–Mn ferrites, Ni–Zn–Co ferrites display greater Tc values. The experimental observed magnetic moments measured from vibrating sample magnetometer (VSM) are fitted with the theoretical data. The saturation magnetization values exhibit a marginal increase in the bulk form of the material as compared to its nanocrystalline counterparts. On the basis of distribution of cation and exchange interactions between magnetic cations at its tetrahedral and octahedral sublattices, the typical mechanisms accountable for the observed trends were elucidated. In addition, the dc-resistivity and dielectric loss are measured to show the application aspects of the materials in various potential device applications.

Nanocatalytic Medicine of Iron-Based Nanocatalysts

Iron can be found in all mammalian cells and is of critical significance to diverse cellular activities within human bodies. Widespread applications and the underlying chemical and biological funda...

Tensile deformation behavior and strengthening mechanism in graphene nanoplatelet reinforced bimodal grained aluminum nanocomposite synthesized by spark plasma sintering and hot rolling

Present research work demonstrates utilization of a unique combination of consolidation techniques, spark plasma sintering and hot rolling, to fabricate bimodal Al-graphene nanoplatelet (GNP) nanocomposite, possessing excellent amalgamation of strength and ductility. Bimodal Al-GNP nanocomposites comprised of nanocrystalline and microcrystalline Al (10, 20, 30, 40, 50 wt%) with 0.5 wt% functionalized GNPs were synthesized. Rolled Al-GNP nanocomposite containing 30 wt% microcrystalline Al exhibited 54% higher yield strength than rolled microcrystalline Al-GNP and 103% higher elongation than rolled nanocrystalline Al-GNP. A theoretical calculation of yield strength of the bimodal nanocomposite was carried out considering the different strengthening mechanisms viz. load transfer mechanism, grain refinement, and coefficient of thermal expansion mismatch. The load transfer mechanism contributed maximum (75%) towards strengthening in rolled 30 wt% microcrystalline bimodal Al-GNP nanocomposite. Microcrystalline Al compact fractured in ductile mode and the nanocrystalline counterpart exhibited brittle mode of failure. On the other hand, the bimodal nanocomposite showed mixed mode of fracture, ductile in microcrystalline Al regions and brittle in nanocrystalline Al regions.

Comparative study of irradiation effects on nanosized and microsized Gd2Zr2O7 ceramics

The heavy-ion irradiation effects of nanocrystalline (78 nm) and microcrystalline (2 μm) Gd2Zr2O7 ceramics were comparatively studied by irradiation using 5 MeV Xe20+ with fluence of 4 × 1015 ions/cm2. The grazing incidence X-ray diffraction results demonstrate that the amorphization rate after irradiation decreases from 63.2% to 34.1% as the grain size increases from nanometer to micrometer. Combined with Raman analysis, the disorder of crystal structure after irradiation is mainly caused by the disorder of cations. The microstructure analysis confirms that the amorphization of nanocrystalline ceramic is more serious than microcrystalline counterpart after irradiation. In other words, the anti-irradiation performance of Gd2Zr2O7 microcrystalline ceramic should be better than the nanocrystalline counterpart.

Hierarchical nanotwins in single-crystal-like nickel with high strength and corrosion resistance produced via a hybrid technique.

High-density growth nanotwins enable high-strength and good ductility in metallic materials. However, twinning propensity is greatly reduced in metals with high stacking fault energy. Here we adopted a hybrid technique coupled with template-directed heteroepitaxial growth method to fabricate single-crystal-like, nanotwinned (nt) Ni. The nt Ni primarily contains hierarchical twin structures that consist of coherent and incoherent twin boundary segments with few conventional grain boundaries. In situ compression studies show the nt Ni has a high flow strength of ∼2 GPa and good deformability. Moreover, the nt Ni has superb corrosion behavior due to the unique twin structure in comparison to coarse grained and nanocrystalline counterparts. The hybrid technique opens the door for the fabrication of a wide variety of single-crystal-like nt metals with unique mechanical and chemical properties.

Micro-sized polycrystalline cubic boron nitride with properties comparable to nanocrystalline counterparts

High-performance polycrystalline cubic boron nitride (PcBN) was sintered without binders at 1500 °C in a pressure range from 11 to 15 GPa using commercial micrometre cubic boron nitride (cBN) with a diameter of approximately 2–4 μm. The results demonstrated that the sample sintered at 12 GPa and 1500 °C had the best mechanical properties and thermal stability. Its average Vickers hardness, fracture toughness, and thermal stability was 63 GPa, 15 MPa m1/2, and 1315 °C, respectively. The considerable improvement in the mechanical properties was mostly attributed to the high compactness, close bonding between grains, and the sample's internal defect structures. The relatively small specific surface area of the micron grains provides an advantage due to its high thermal stability. The amorphous regions observed in the sample's local areas may provide a new strengthening mechanism under high pressure and temperature.

Micrometer-sized titanium carbide with properties comparable to those of nanocrystalline counterparts

The influence of sintering pressure on the mechanical properties of bulk titanium carbide (TiC) fabricated through work hardening at high pressure and high temperature is investigated systematically. A series of pure polycrystalline TiC samples are prepared by sintering micrometer-sized TiC powders at a pressure of 9.0–14.0 GPa and a temperature of 1500 °C. These samples are then characterized by various techniques for determining their residual stress, grain size, density, microstructural defects, hardness, and fracture toughness. The results demonstrate that the Vickers hardness HV and the fracture toughness KIC depend strongly on the sintering pressure. It is found that the mechanical properties of the sintered samples improve with increasing sintering pressure. The relative density increases with increasing sintering pressure, reaching near full density at 14.0 GPa. The hardness and fracture toughness of the sample sintered at 1500 °C at 14.0 GPa pressure are 31.2 GPa and 4.2 MPa m1/2, respectively. The high-pressure and high-temperature environment causes severe plastic deformation of the grains, as well as a high density of dislocations, resulting in a dislocation pileup. The latter, together with the production of defects such as sub-boundaries and stacking faults, provides strengthening and stabilizing effects and improves the material hardness.

Magic high-pressure strengthening in tungsten carbide system

High-pressure strengthening is reported to be an effective approach to strengthen materials in addition to traditional nanocrystalline strengthening. Numerous studies have proven that high pressure is an important factor for improving the mechanical properties of materials. Here, we sintered high-purity sub-micron tungsten carbide (sm-WC) ceramics without any additives under a high pressure of 10 GPa and temperatures ranging from 1000 °C to 1500 °C. Numerous microdefects (stacking faults and twins) were introduced inside the grains, and the Vickers hardness of the well-sintered WC bulks reached a high value of 33 GPa, which could be compared with that of single crystal and nanocrystalline counterparts. Thus, the high-pressure strengthening can be applied to the WC system and improves the mechanical properties. The strengthening mechanism is that the high pressure can make the grains take place bulk yield, promote a severe plastic deformation, and introduce numerous microdefects. These defects act as obstacles against dislocations glide during deformation, and thus improve the mechanical properties.

3D Printing of Nanotwinned Metals

Microstructure of a metal determines its properties. A major effort in the field of additive manufacturing (AM) of metals is controlling and predicting the microstructure. Nanotwinned (nt)-metals exhibit superior mechanical and electrical properties compared to their coarse-grained and nano-grained counterparts. nt-metals have a unique microstructure with grains that contain a high density of layered nanoscale twins divided by coherent twin boundaries (TBs). These metals often show higher strength and ductility compared to their nanocrystalline counterpart, since TBs can effectively block dislocation motion. These metals show more resistance to electromigration, which is a common problem for metals at the nano/ microscale. nt-metals in film and bulk forms are obtained using physical and chemical processes including pulsed electrodeposition (PED), plastic deformation, recrystallization, phase transformation, and sputter deposition. However, currently, there is no process for 3D printing (AM) of nt-metals. Given their unique properties, it is desirable to establish an AM process for nt-metals. We developed a new process for 3D printing of nt-metals, termed localized PED (L-PED). We demonstrate microscale 3D printing of nt-Cu with high density of coherent TBs using this process. This cost-effective process, which is performed at ambient environment, enables direct printing of 3D microscale structures of pure metals with nanoscale twins. The 3D printed nt-Cu is fully dense, with low to none impurities, and low microstructural defects, and without obvious interface between printed layers, which overall result in good mechanical and electrical properties, without any postprocessing steps.

Plasmonic and Photonic Effects on Hydrogen Evolution over Chemically Modified Titania Inverse Opals

AbstractThe photonic effects of chemically modified titania inverse opals can be exploited for improving photocatalytic hydrogen production. This work reports the in situ preparation of gold‐platinum nanoparticle (NP)‐loaded and nitrogen‐fluorine co‐doped titania inverse opal photocatalysts with different photonic band gaps (PBGs). In situ loading of gold and platinum NPs was carried out during the preparation of titania inverse opals by treating the titania precursor with gold chloride and platinum chloride. Au−Pt/N−F TiO2 IO‐460 samples with the photonic effects matching the surface plasmon resonance region of Au NPs displayed a maximum activity with a hydrogen production rate of 134 mmol h−1 g−1. The hydrogen yield (97 mmol h−1 g−1) obtained for Au−Pt/N−F TiO2 IO‐215, with the photonic effects close to the electronic bandgap of titania was also high when compared to that of the nanocrystalline counterpart, Au−Pt/N−F nc‐TiO2 (49 mmol h−1 g−1). Excellent solar hydrogen evolution profile was observed for in situ gold‐platinum NP‐loaded and nitrogen‐fluorine co‐doped titania inverse opal photocatalysts when compared to recent reports.

Localized Pulsed Electrodeposition Process for Three-Dimensional Printing of Nanotwinned Metallic Nanostructures.

Nanotwinned-metals (nt-metals) offer superior mechanical (high ductility and strength) and electrical (low electromigration) properties compared to their nanocrystalline (nc) counterparts. These properties are advantageous in particular for applications in nanoscale devices. However, fabrication of nt-metals has been limited to films (two-dimensional) or template-based (one-dimensional) geometries, using various chemical and physical processes. In this Letter, we demonstrate the ambient environment localized pulsed electrodeposition process for direct printing of three-dimensional (3D) freestanding nanotwinned-Copper (nt-Cu) nanostructures. 3D nt-Cu structures were additively manufactured using pulsed electrodeposition at the tip of an electrolyte-containing nozzle. Focused ion beam (FIB) and transmission electron microscopy (TEM) analysis revealed that the printed metal was fully dense, and was mostly devoid of impurities and microstructural defects. FIB and TEM images also revealed nanocrystalline-nanotwinned-microstructure (nc-nt-microstructure), and confirmed the formation of coherent twin boundaries in the 3D-printed Cu. Mechanical properties of the 3D-printed nc-nt-Cu were characterized by direct printing (FIB-less) of micropillars for in situ SEM microcompression experiments. The 3D-printed nc-nt-Cu exhibited a flow stress of over 960 MPa, among the highest ever reported, which is remarkable for a 3D-printed material. The microstructure and mechanical properties of the nc-nt-Cu were compared to those of nc-Cu printed using the same process under direct current (DC) voltage.

Room temperature ferromagnetic‐like response in “bulk” Y‐doped CeO2

Strong ferromagnetic‐like response is observed at room temperature in “bulk”‐ and nano‐crystalline Y‐doped CeO2. The saturation magnetization for “bulk” (∼600 nm) crystallites of CeO2 doped with 25 at.% Y is more than an order of magnitude higher compared to its nanocrystalline (∼10 nm) counterpart. High frequency electron spin resonance and 89Y nuclear magnetic resonance measurements indicate clustering of electronic defects in the “bulk” crystallites. The remarkable size dependence of the magnetic behavior likely originates from a collective magnetic response of defect‐lined nanodomain interfaces in the “bulk” crystallites, consistent with the giant orbital paramagnetism model.

Corrosion resistance of Al and Al–Mn thin films

Magnetron-sputtered aluminum (Al) and aluminum–manganese (Al-Mn) films with structures ranging from nanocrystalline to amorphous were obtained by tuning the Mn% up to 20.5at.%. Corrosion behavior of the films was investigated in 0.6M and 0.01M NaCl aqueous solutions by potentiodynamic polarization and electrochemical impedance spectroscopy. Pitting corrosion was found to be strongly affected by alloy composition. The amorphous Al–20.5at.% Mn exhibited the best pitting resistance during short term exposure. However, over longer immersion in 0.01M NaCl up to 108h, nanocrystalline Al–5.2at.% Mn showed the highest corrosion resistance. The dual-phase Al-11.5at.% Mn alloy was found to have higher nominal corrosion rate compared to its nanocrystalline or amorphous counterparts.

Interplay between grain boundary segregation and electrical resistivity in dilute nanocrystalline Cu alloys

The relationships between microstructure, controlled by alloying elements prone to grain boundary segregation, and electrical resistivity in sputtered nanocrystalline Cu were investigated. We find a non-monotonic dependence of the mean grain size on solute concentration for both Cu-Nb and Cu-Fe dilute alloys, with a concentration regime where the grain size increases over that of pure Cu before refining with further alloying. The electrical resistivity follows the same trend, suggesting a non-equilibrium processing route that remarkably gives rise to dilute nanocrystalline Cu alloys with lower resistivity, thermal stability, and enhanced mechanical properties relative to their pure nanocrystalline counterpart.

Exceptional high fatigue strength in Cu-15at.%Al alloy with moderate grain size.

It is commonly proposed that the fatigue strength can be enhanced by increasing the tensile strength, but this conclusion needs to be reconsidered according to our study. Here a recrystallized α-Cu-15at.%Al alloy with moderate grain size of 0.62 μm was fabricated by cold rolling and annealing, and this alloy achieved exceptional high fatigue strength of 280 MPa at 107 cycles. This value is much higher than the fatigue strength of 200 MPa for the nano-crystalline counterpart (0.04 μm in grain size) despite its higher tensile strength. The remarkable improvement of fatigue strength should be mainly attributed to the microstructure optimization, which helps achieve the reduction of initial damage and the dispersion of accumulated damage. A new strategy of “damage reduction” was then proposed for fatigue strength improvement, to supplement the former strengthening principle. The methods and strategies summarized in this work offer a general pathway for further improvement of fatigue strength, in order to ensure the long-term safety of structural materials.

Molecular dynamics study of creep mechanisms in nanotwinned metals

Nanotwinned structures have shown great promise as optimal motifs for evading the strength–ductility trade-off. In this paper, we present a study of high temperature creep in polycrystalline nanotwinned face-centered cubic metals using molecular dynamics. The simulations reveal that the nanotwinned metals exhibit greater creep resistance with decreasing twin boundary spacing over a large range of applied stresses. The findings also indicate that the presence of twin boundaries entails higher stress for the onset of power-law creep compared to the nanocrystalline counterparts. Nanotwinned metals with very high density of twin boundaries exhibit a new creep deformation mechanism at high stresses governed by twin boundary migration. This is in contrast to nanocrystalline and nanotwinned metals with larger twin spacing, which exhibit a more conventional transition from grain boundary diffusion and sliding to dislocation nucleation.

Copper thin films by ion beam assisted deposition: Strong texture, superior thermal stability and enhanced hardness

Nanocrystalline metals generally exhibit exceptionally high strength. However, their susceptibility to grain growth restricts their applications in high temperature environments. The current study presents that nanocrystalline Cu thin films produced by ion beam assisted deposition (IBAD) are able to sustain their as-deposited microstructure and high hardness upon annealing at high temperatures. IBAD-Cu films exhibit a strong (111) fiber texture, which is caused by the ion beam induced effects of substrate cleaning, preferential damage and preferential sputtering. The microstructure of the IBAD-Cu films is stable at temperatures up to 800°C (80% of the melting point of Cu). The hardness of the as-deposited IBAD-Cu films can reach a maximum value of 3.85GPa. Even after annealing, their hardness is still much higher than that of the normally deposited (without ion beam) films as well as their bulk nanocrystalline counterparts before heat treatment. The excellent thermal stability of microstructure is attributed to the formation of nanometer-sized voids and their pinning effect on grain boundary migration. The kinetics of void formation, the contribution of twin boundaries and ion beam induced defects to the hardness are analyzed and discussed. The findings in this study demonstrate that IBAD is an effective method for the stabilization of microstructure and mechanical properties of nanocrystalline metal thin films.

Modifying Oxide Single Crystal Surfaces to Study Photoinduced Electron Transfer

Single crystal oxide semiconductor electrodes provide an ideal experimental system for the elucidation of the models and parameters that control light absorption, charge separation, and transport at semiconductor/liquid interfaces. Such fundamental investigations can prove very valuable in the understanding of dye sensitized nanocrystalline solar cells. The single crystal experimental system is much less complex than its nanocrystalline counterpart and the study of well-defined crystallographic surfaces allows for more definitive conclusions to be made concerning the relation between sensitizer attachment and surface structure and the sensitizer’s performance. I will report recent work on the sensitization of low index faces of both anatase and rutile forms of TiO2 and ZnO single crystals with dyes, quantum dots and polymers. We employ atomic force microscopy (AFM) to verify that atomically flat terraced oxide surfaces are prepared and to elucidate the structure of the sensitizers adsorbed on these surfaces. Photocurrent spectroscopy and attenuated reflection spectroscopy are used to correlate the photocurrent response with the optical absorption of the sensitizing species and the relative electron injection efficiencies of for instance dye monomers and aggregates. We have also recently begun to grow homoepitaxial layers on both anatase and rutile using atomic layer deposition (ALD). These layers can have superior electronic properties compared to the substrate crystal and allows us to control the doping density in the near surface region. The primary focus of this talk will be on CdSe and PbS quantum dots as sensitizers. PbS quantum dots adsorbed on anatase (001) surfaces irradiated with light of >2.5 times their band gap have been shown to produce more than one injected electron per incident photon (1), a process known as carrier multiplication or multiple exciton generation (MEG). Harnessing MEG may result in more efficient solar conversion devices.1. Justin B. Sambur, Thomas Novet and B. A. Parkinson, “Multiple Exciton Collection in a Sensitized Photovoltaic System”, Science, 330, 63-66, (2010)