Nanocrystalline Aluminum Research Articles

Inverse Hall-Petch Behavior in Nanocrystalline Aluminum Using Molecular Dynamics

This work investigates the mechanical behavior of nanocrystalline aluminum, with special focus on deformation mechanisms, using molecular dynamics simulations with an interatomic potential parameterized by the authors. To this end, four nanocrystalline samples with grain sizes ranging from 8,2 to 14,2 nm were constructed, each with a volume of 15 x 15 x 20 nm3. As expected, the data from the tensile tests at a strain rate of 1,0 x 109 s−1 showed an inverse Hall-Petch relationship. The work hardening behavior revealed no significant gain in mechanical strength. The dislocation analysis indicated that perfect dislocation density decreases during tensile testing, while the Shockley partials increase. Grain boundary-mediated plasticity was evidenced with atomic diffusion along grain boundaries, as well as by grain rotation. Thus, it is concluded that the conventional plastic deformation mechanisms of metals are not preponderant for nanocrystalline aluminum.

Thin Film Piezoelectric Nanogenerator Based on (100)-Oriented Nanocrystalline AlN Grown by Pulsed Laser Deposition at Room Temperature.

In wearable or implantable biomedical devices that typically rely on battery power for diagnostics or operation, the development of flexible piezoelectric nanogenerators (NGs) that enable mechanical-to-electrical energy harvesting is finding promising applications. Here, we present the construction of a flexible piezoelectric nanogenerator using a thin film of room temperature deposited nanocrystalline aluminium nitride (AlN). On a thin layer of aluminium (Al), the AlN thin film was grown using pulsed laser deposition (PLD). The room temperature grown AlN film was composed of crystalline columnar grains oriented in the (100)-direction, as revealed in images from transmission electron microscopy (TEM) and X-ray diffraction (XRD). Fundamental characterization of the AlN thin film by piezoresponse force microscopy (PFM) indicated that its electro-mechanical energy conversion metrics were comparable to those of c-axis oriented AlN and zinc oxide (ZnO) thin films. Additionally, the AlN-based flexible piezoelectric NG was encapsulated in polyimide to further strengthen its mechanical robustness and protect it from some corrosive chemicals.

Nano-additive manufacturing of multilevel strengthened aluminum matrix composites

Nanostructured materials are being actively developed, while it remains an open question how to rapidly scale them up to bulk engineering materials for broad industrial applications. This study propose an industrial approach to rapidly fabricate high-strength large-size nanostructured metal matrix composites and attempts to investigate and optimize the deposition process and strengthening mechanism. Here, advanced nanocrystalline aluminum matrix composites (nanoAMCs) were assembled for the first time by a novel nano-additive manufacturing method that was guided by numerical simulations (i.e. the in-flight particle model and the porefree deposition model). The present nanoAMC with a mean grain size <50 nm in matrix exhibited hardness eight times higher than the bulk aluminum and shows the highest hardness among all Al–Al2O3 composites reported to date in the literature, which are the outcome of controlling multiscale strengthening mechanisms from tailoring solution atoms, dislocations, grain boundaries, precipitates, and externally introduced reinforcing particles. The present high-throughput strategy and method can be extended to design and architect advanced coatings or bulk materials in a highly efficient (synthesizing a nanostructured bulk with dimensions of 50 × 20 × 4 mm3 in 9 min) and highly flexible (regulating the gradient microstructures in bulk) way, which is conducive to industrial production and application.

Improved light transmission in nanocrystalline aluminum nitride (AlN)—Enabling a lightweight, thermal shock resistant transparent ceramic

Optically transparent ceramics were traditionally only viable using optically isotropic materials and limited birefringent materials with small Δn such as alumina (Δn≈0.008). Thus heavily birefringent materials such as AlN (Δn≈0.045), polycrystalline ceramics are usually not considered for optical applications due to their high scattering loss, despite its exceptionally appealing thermal conductivity, mechanical stiffness/toughness and relatively low density—attributes that are ideal for light weight optical windows for harsh environments. To further improve the transparency over coarse grain AlN ceramics that have been reported, subwavelength grain size is desired. Here we present significant improvements in visible, NIR and mid-IR transparency in AlN ceramics by reducing the average grain size to 230 nm. A newly developed, non-carbon thermal reduction for producing clean powders at low temperatures is key to the success. The nanocrystalline AlN ceramics reach full density at a densification temperature as low as 1500 °C and showed an in-line transparency of over 55% at 2.5 μm and over 70% in the mid-IR which is the highest transparency in AlN ceramics reported. The successful demonstration of transparent nanocrystalline AlN ceramics makes it the ideal choice for optical windows in quickly fluctuating temperature environments where weight is a consideration such as air and space vehicles.

Thermoluminescence characteristics of different phase transitions from nanocrystalline alumina

Nanocrystalline boehmite material was synthesized using the hydrothermal method. Different annealing temperatures have been used to transform boehmite into different alumina phases to study the effect of different phase transitions on the thermoluminescence properties of alumina. XRD analysis was carried out to investigate the crystal structure of the different alumina phases. The thermoluminescence glow curves for different alumina phases showed different structures; however, the sensitivity was almost constant for all the phase transitions of alumina over the applied dose ranging from 0.55 to 330 Gy.

Influence of Grain Boundary Character on Dopants Segregation in Nanocrystalline Aluminum

Influence of Grain Boundary Character on Dopants Segregation in Nanocrystalline Aluminum

Mechanisms of Densification and Bonding in the Ultrasonic Consolidation of Aluminum Powder

The densification and bonding of aluminum powder subjected to ultrasonic vibration under uniaxial pressure were characterized and their mechanisms were investigated by transmission electron microscopy (TEM). Full densification was achieved in just 1 second at a nominal consolidation temperature of 573 K and in 4 seconds at 473 K. The particle shape-change required for densification occurs by rapid plastic deformation, associated with dynamic recovery and continuous dynamic recrystallization, and repetitive formation and transfer of supercooled liquid into the particle interstices where the liquid solidifies into layers of amorphous and nanocrystalline aluminum. The liquid transfer also provides oxide-free surfaces required for metallurgical bonding at the particle junctions. The occurrence of rapid plastic deformation and formation of supercooled liquid is explained by the effects of excess vacancies generated in the deforming powder particles.

Investigation of Grain Boundary Content on Crack Propagation Behavior of Nanocrystalline Al by Molecular Dynamics Simulation

The nanocrystalline metal material has been an investigation hotspot due to its excellent mechanical property. The high content of grain boundaries (GBs) in microstructure is the key factor affecting its fracture behavior during service. Therefore, it is essential to investigate the effect mechanism of nanoscale GB on crack propagation. In this study, four molecular dynamics (MD) models of nanocrystalline aluminum (Al) with different GB contents are established. The results show that the high‐content GBs in polycrystals can increase the toughness and absorb energy, reducing the risk of brittle crack propagation. The presence of GB can reduce the stress concentration of the crack, and the dislocation emission from the crack tip can be absorbed by the front GB. The microstructure with high‐content GBs can actuate more plastic deformation mechanisms such as multiple slips, GB slip, and migration. The region with more GBs can induce a more even deformation of the whole microstructure by means of dislocation emission and GB migration to the region with fewer GBs. The purpose of this study is to provide a rational mechanism reference for the failure of nanocrystalline metal material.

NO2 gas sensing properties of chemically grown Al doped ZnO nanorods

This report highlights nanocrystalline aluminium doped ZnO (Al doped ZnO: AZO) samples deposited on seeded glass substrates using cost-effective reflux method. The doping concentration of ‘Al’ ions varied from 1 to 3 vol percentage (vol%). The deposited AZO samples exhibit wurtzite hexagonal crystal structure and the surface morphological analysis study confirms the formation of nanorods for undoped ZnO and reduction in diameter with the formation of the fine tips for AZO samples. The photoluminescence (PL) study confirms oxygen vacancies and defects in the AZO samples. Moreover, the AZO samples exhibit excellent gas sensing performance towards NO2 gas at an operating temperature of 175 °C. The gas sensitivity of 5 ppm NO2 gas was 85% at 175 °C. This work is promising for the practical approach for the low detection limit at low operating temperature in the field of gas sensor technology.

Study of XRD, dielectric properties and DC electrical conductivity of Li-Zn-Al ferrite synthesized by sol–gel combustion method

A series of nanocrystalline aluminum (Al3+) doped Lithium-Zinc ferrites (LZA) having general chemical formula Li0.5(1- x )Zn x Fe2.5- y Al y -0.5 x O4 with x = 0.1 and y = 0.2 (S1C1), 0.4 (S1C2), 0.6 (S1C3) and 0.8 (S1C4) were synthesized by citrate sol-gel combustion method. The structural characterizations of the samples were carried out by X-Ray Diffraction (XRD) studies. Morphological investigations were performed using Scanning Electron and Transmission Electron Microscopies. XRD analysis confirmed the formation of single phase of the reported samples. All the samples crystallized in spinel cubic crystal structure with Fd-3m (227) space group. Crystallite size was estimated in the range of 19 nm to 21 nm using Scherrer formula. The variation of D.C. electrical conductivity as a function of temperature of as-prepared samples revealed the semiconducting nature. The dielectric constant (ε'), dielectric loss factor (ε") and the dielectric loss tangent (tan δ) were studied in the frequency range of 1 KHz to 1 MHz at room temperature by using LCR Meter. The values of dielectric constant, dielectric loss factor and dielectric loss tangent were found to decrease with addition of Al3+ content as well as with the increase in the frequency. The observed dielectric dispersion at lower frequency is attributed to Maxwell-Wagner type of interfacial polarization due to hopping of charge between Fe+2 and Fe+3. High value of resistivity (≈106 Ω-cm) and low value of dielectric constant and dielectric loss are the prime achievements of the present research work which make these investigated nanoferrites useful for the microwave devices.

High pressure shear induced microstructural evolution in nanocrystalline aluminum

The microstructural evolution of 4.5 nm grain-sized nanocrystalline aluminum under high pressure shear loading is investigated using molecular dynamics simulations. A compression of 15 GPa is applied to the system concurrently with a shear deformation rate of 3.21 × 109 s−1. Results show that the microstructural evolution is marked by a transient grain refinement regime followed by a steady grain growth process within 1 ns, coarsening the microstructure from 4.5 nm to 5.8 nm average grain size. The grain growth process is mediated by three independent deformation mechanisms: i) grain rotation; ii) grain boundary sliding; iii) and grain boundary migration. While the first two mechanisms are inherent grain boundary deformation mechanisms, the latter is driven by intense dislocation activity. Data indicates that during the deformation the dislocation density surges from 5 to 15 × 1011 cm−2, highlighting the important role of dislocation dynamics in the evolution of the microstructure. These atomistic insights shed light on the underlying complex incipient deformation processes, which are activated during severe plastic deformation of nanocrystalline materials.

Influence of coating thickness on the microstructure, composition and optical properties of aluminum nitride thin films grown on silicon substrates via low-temperature PEALD

Within the framework of the study, fine-grained nanocrystalline aluminum nitride films were obtained using the PEALD method at the temperature of 250 °С and the number of deposition cycles from 80 to 500 on single-crystal silicon substrates. Low-temperature synthesis processes were carried out in the self-limited growth regime of the PEALD method at the constant growth rate of ∼0.115 nm/cycle. The obtained samples were studied by ellipsometry, FTIR-spectroscopy, X-ray diffraction analysis, Auger spectroscopy, Scanning electron and Atomic force microscopy. It was found that the samples in the IR-absorption spectra and ω/2θ-scans had bands and reflections characteristic for wurtzite AlN with hexagonal structure. It was shown that an increase in the coating thickness led to an increase in the crystallinity and optical density of the film material, the crystallites size, and values of the root-mean-square (RMS) roughness. In this case, significant changes in the crystal lattice parameter and relaxation of internal mechanical stresses within the obtained values of the AlN layers thicknesses were not observed.

(Electrodeposition Division Research Award Address) Electrodeposition in the Era of Additive Manufacturing

In the era of additive manufacturing (AM), great interest in electrodeposition has evolved, either as a post-printing finishing stage (e.g., on cellular AM’ed structures) or as a standalone three-dimensional (3D) printing technique. Over the years, a variety of electrochemical 2D patterning and 3D printing techniques have been developed, e.g., meniscus-confined electrodeposition (MCED), localized electrochemical deposition (LECD), electrochemical fountain pen nanofabrication (ec-FPN), electrochemical dip-pen nanolithography (E-DPN), electrochemical printing (EcP), electrochemical fabrication (EFAB), and nanotransfer printing (nTP).The MCED process relies on an electrolyte-containing micropipette with a micron/submicron-size orifice. As the micropipette approaches the surface of a conductive substrate, a meniscus (liquid bridge) is established between the dispensing orifice and the substrate’s surface (Fig. 1b). Electrical current (or potential), applied between a small conductive wire inside the pipette and the conductive substrate via the meniscus, causes reduction of the reducible species on the surface confined by the meniscus. The dispensing micronozzle is then withdrawn from the substrate at a speed that should match the deposition rate, allowing continuous fabrication. The size and shape of the meniscus established between the nozzle and the growing feature are determined mainly by the shape and size of the nozzle, its moving speed, the thermodynamic properties of the electrolyte, surface wettability, and the relative humidity.We recently suggested a novel setup and procedure for 3D electrochemical deposition, combining MCED with atomic force microscope (AFM) closed-loop control (see Fig. 1a).1,2 This aforementioned combination allowed us to print freestanding vertical pillars (Fig. 1c) and overhang structures with an overhang angle as high as 80° without the need to use supports (Fig. 1d). A copper-based electrolyte was used for feasibility tests. Fully dense, uniform and exceptionally smooth features were printed (Fig. 1c), with diameters ranging from 1.5 μm down to 250 nm and a high aspect ratio (> 100). The dense deposited copper features had a polycrystalline microstructure, with submicron grain size (see Fig. 1e), and were mechanically rigid and of good electrical conductivity.Other functional metals and alloys that will be printed in the near future in our lab include pure aluminum and cobalt-phosphorous amorphous alloy. To this aim, a recipe for successful electrodeposition of these materials under potentially relevant conditions should first be developed. Here, I will summarize our ongoing work on: (1) electrodeposition of pure and dense nanocrystalline aluminum from room-temperature ionic liquids (RTILs) on copper and nickel substrates; and (2) electrodeposition of exceptionally well adhered CoP coatings on copper.I will also present our novel electrochemical processing of functional core/multi-shell ZnAl/Ni/NiP powders.3,4 This process can be used for processing of functional powders for laser-based additive manufacturing (LAM) with either reduced or increased laser absorbance, but also processing of phase change materials (PCMs) or zinc-based anodes in zinc-air batteries. Figure 1

Processing and Thermal Conductivity of Bulk Nanocrystalline Aluminum Nitride.

Producing bulk AlN with grain sizes in the nano regime and measuring its thermal conductivity is an important milestone in the development of materials for high energy optical applications. We present the synthesis and subsequent densification of nano-AlN powder to produce bulk nanocrystalline AlN. The nanopowder is synthesized by converting transition alumina (δ-Al2O3) with <40 nm grain size to AlN using a carbon free reduction/nitridation process. We consolidated the nano-AlN powder using current activated pressure assisted densification (CAPAD) and achieved a relative density of 98% at 1300 °C with average grain size, ~125 nm. By contrast, high quality commercially available AlN powder yields densities ~75% under the same CAPAD conditions. We used the 3-ω method to measure the thermal conductivity, κ of two nanocrystalline samples, 91% dense, = 110 nm and 99% dense, = 220 nm, respectively. The dense sample with 220 nm grains has a measured κ = 43 W/(m·K) at room temperature, which is relatively high for a nanocrystalline ceramic, but still low compared to single crystal and large grain sized polycrystalline AlN which can exceed 300 W/(m·K). The reduction in κ in both samples is understood as a combination of grain boundary scattering and porosity effects. We believe that these are finest reported in bulk dense AlN and is the first report of thermal conductivity for AlN with ≤220 nm grain size. The obtained κ values are higher than the vast majority of conventional optical materials, demonstrating the advantage of AlN for high-energy optical applications.

Grain boundary relaxation in doped nano-grained aluminum

Simulation studies are done to understand the role of dopants that segregate preferentially to grain boundaries on the stability of nanocrystalline aluminum. A dopant design framework based on thermodynamic principles, is used to identify the specific dopant type with the highest potential to segregate to grain boundaries in nanocrystalline aluminum. Various elements are evaluated as potential dopants and magnesium is identified to have the highest tendency to segregate to grain boundaries and release the excess free energyleading to the relaxation of the grain boundaries. A systematic combination of atomic structure analysis is then done to correlate grain boundary relaxation and the mechanical response of the magnesium-doped nanocrystalline aluminum at ambient temperature. The atomistic simulations reveal that the preferential partitioning of magnesium dopants to the grain boundaries reduces the excess volume within this region which stabilizes the nanostructure. At low contents, the magnesium dopants are observed initially partition to the grain boundaries, but once saturation of the grain boundaries is reached, excess dopants are accommodated in the crystalline interiors. It is found that the addition of the magnesium dopants even in the dilute limit, enhances the strength of the nanocrystalline aluminum. The formation of large, disordered GBs in doped nanocrystalline aluminum under tensile load allowed it to accommodate the deformation and prohibit crack growth.

Molecular dynamics simulation and theoretical modeling of free surface effect on nanocrack initiation induced by grain boundary sliding in nanocrystalline materials

Free surfaces have significant effects on plastic deformation and fracture of nanocrystalline materials (NMs), especially for nano- or micro- scale specimens. To reveal the influence mechanism of free surfaces on crack initiation and growth, molecular dynamics (MD) simulations are carried out. The results show nanocrack initiation at triple junctions near free surfaces and the intergranular fracture due to nanocrack growth and coalescence. Based on the mechanism captured by MD, a theoretical model is established to describe the process of grain boundary (GB) dislocation evolution and nanocrack initiation. The critical stress for nanocrack initiation and the critical magnitude of GB sliding step are obtained for nanocrystalline nickel, aluminum and copper. Based on the proposed theoretical model, the fracture behaviors are discussed for the three nanomaterials.

Description of grain boundary structure and topology in nanocrystalline aluminum using Voronoi analysis and order parameter

Grain boundaries (GB) have a significant impact on mechanical, physical properties and microstructure of polycrystalline materials. Their studies are necessary for designing materials of improved properties. We present molecular dynamics based studies of grain boundary in nanocrystalline aluminum. A Voronoi tessellation based method (Voronoi analysis, VA) and order parameter approach were used for describing structure and topological features in nanocrystalline aluminum. We are portraying new functionality and usefulness of VA combined with order parameter (OP) to picturing structure nature of grain boundary (GB) and its order given in form of configuration entropy. Voronoi analysis unveiled sub-nanoscale topological features contributing to GB which correspond to lattice distortions and liquid-like GB behavior. Grouping of Voronoi cell indices to a crystal lattice distortion group was made together with corresponding to them OP values. Two complementary methods used in this work provide new information and open new possibilities for studying structure and properties of nanomaterials.

Size Effects in Internal Friction of Nanocrystalline Aluminum Films.

Al thin film is extensively used in micro-electromechanical systems (MEMS) and electronic interconnections; however, most previous research has concentrated on their quasi-static properties and applied their designs on larger scales. The present study designed a paddle-like cantilever specimen with metal films deposited on the upper surface to investigate the quasi-static properties of Al thin film at room temperature under high vacuum conditions at microscopic scales. Energy loss was determined using a decay technique in the oscillation amplitude of a vibrating structure following resonant excitation. Grain size and film thickness size were strictly controlled considering the quasi-static properties of the films. This study found that the internal friction of ultra-thin and thin Al films was more dependent on the grain boundaries than film thickness.

Effect of growth twins on strength and microstructural evolution of nanocrystalline aluminum

Effect of growth twins on strength and microstructural evolution of nanocrystalline aluminum

Local damage in grain boundary stabilized nanocrystalline aluminum

The excess free volume in grain boundaries of pure nanocrystalline aluminum leads to the grain coarsening, which degrades their desirable mechanical properties. The preferential segregation of dopants to the grain boundaries has been proposed to enhance the stability of the nanostructure. To shed light on the stabilization mechanisms, we present results from virtual mechanical tests on nanocrystalline aluminum samples with similar grain sizes but different magnesium dopant contents. The segregation of magnesium decreases the excess free volume within the grain boundaries and prohibits the nucleation and growth of intergranular cracks.