Polycrystalline Alumina Research Articles

Stress engineering of polycrystalline aluminum nitride thin films for strain sensing with resonant piezoelectric microbridges

For an optimized performance of micro electromechanical systems (MEMS) double-clamped bridge-type resonators for mechanical strain sensing, a modified sputter process was developed which exploits the influence of varying sputter pressure during deposition on the intrinsic stress component of functional thin films. In detail, four different, polycrystalline aluminum nitride layers, synthesized with different sputter parameter sets were characterized related to their microstructure with techniques, such as X-ray diffraction, scanning electron microscopy and atomic force microscopy, respectively. Furthermore, the intrinsic thin film stress and the longitudinal piezoelectric coefficient were evaluated. The best performing layers were integrated in the fabrication process of two MEMS resonant strain sensor devices to study the stress-related impact on the resonant device performance. The initial static buckling of the sensor devices was studied by white light interferometric measurements, whereas the frequency response as a function of externally applied as well as intrinsic strain was analyzed by laser Doppler vibrometry. The behavior of the sensor devices was compared to theoretical predications and the influence of intrinsic thin film stress on the resonance frequency as a function of strain was studied. With a precise tailoring of the intrinsic film stress, a responsivity of ∼17000 is measured, representing an improvement by a factor of ∼5 compared to state-of-the-art resonant strain sensors.

Simulation of deformation and fracture in polycrystalline aluminum alloy under dynamic loading

Simulation of deformation and fracture in polycrystalline aluminum alloy under dynamic loading

A study of macroscopic plastic strain localization during creep of polycrystalline aluminum

This paper studies the deformation behavior of commercially pure aluminum 1050A during creep tests at room temperature. The plastic flow parameters at the stage of steady-state creep at different stress levels are determined. The kinetics of localized plasticity zones in the creep mode is studied using the digital image correlation method. The stress dependence of the spatial period of plastic strain localization is established.

High pressure phase transition and strength estimate in polycrystalline alumina during laser-driven shock compression

Alumina (Al2O3) is an important ceramic material notable for its compressive strength and hardness. It represents one of the major oxide components of the Earth’s mantle. Static compression experiments have reported evidence for phase transformations from the trigonal α-corundum phase to the orthorhombic Rh2O3(II)-type structure at ∼90 GPa, and then to the post-perovskite structure at ∼130 GPa, but these phases have yet to be directly observed under shock compression. In this work, we describe laser-driven shock compression experiments on polycrystalline alumina conducted at the Matter in Extreme Conditions endstation of the Linac Coherent Light Source. Ultrafast x-ray pulses (50 fs, 1012 photons/pulse) were used to probe the atomic-level response at different times during shock propagation and subsequent pressure release. At 107 ± 8 GPa on the Hugoniot, we observe diffraction peaks that match the orthorhombic Rh2O3(II) phase with a density of 5.16 ± 0.03 g cm−3. Upon unloading, the material transforms back to the α-corundum structure. Upon release to ambient pressure, densities are lower than predicted assuming isentropic release, indicating additional lattice expansion due to plastic work heating. Using temperature values calculated from density measurements, we provide an estimate of alumina’s strength on release from shock compression.

Growth of GaN Hexagonal Arrays on Partially Crystallized Sapphire Nanomembranes for Micro-Light-Emitting Diodes

100-nm-thick hexagonal fully and partially crystallized sapphire nanomembrane arrays were demonstrated as growth templates for hexagonal micro-sized GaN arrays for micro-light-emitting diodes (LEDs). By simply controlling the thermal treatment time for crystallization of the amorphous membrane, the partially crystallized nanomembrane which has both single- and polycrystalline alumina on the top surface was fabricated. The unique crystalline structures contributed to unique growth behavior which contains selective and lateral overgrowth of GaN, compared to that of GaN on fully crystallized sapphire nanomembranes, leading to superior structural and optical properties of GaN. Threading dislocation densities were reduced by 75% and 1.29 times higher integrated photoluminescence intensity was achieved compared to GaN on the fully crystallized sapphire nanomembranes. Stress in GaN was also fully relaxed due to the ultrathin membrane structure as a compliant substrate. The superior properties of GaN as well as the truncated inverted pyramid structure are expected to improve the performance of micro-LED devices. Hexagonal partially crystallized sapphire nanomembrane technology also provided advances in the fundamental study of epitaxial growth of GaN-based materials.

Mechanical characterization of aesthetic orthodontic brackets by the dynamic indentation method.

The objective of this study is to investigate the mechanical properties, such as the dynamic hardness and indentation elastic modulus, of commercially available aesthetic orthodontic brackets, such as ceramic and plastic brackets, by the dynamic micro-indentation method. Five ceramic brackets, which were made of alumina (both monocrystalline and polycrystalline forms) or zirconia, and two plastic brackets, which were made of glass fiber-reinforced polycarbonate or polyamide, were tested. There were significant differences in the mechanical properties of the monocrystalline and polycrystalline alumina brackets. The mechanical properties of the glass fiber-reinforced plastic bracket were significantly superior to these of the non-glass-fiber-reinforced plastic bracket. The differences in the crystal structures of the ceramic brackets surface affected the dynamic hardness and indentation elastic modulus. Furthermore, the short glass fibers contained in the plastic bracket might contribute to the improvement of the mechanical properties.

Study on Polishing of Polycrystalline AlN Using Sol–Gel Polishing Tool

Polycrystalline aluminum nitride has extensive application prospects in substrate materials, insulating layer materials, and packaging materials. Polycrystalline aluminum nitride is sintered from powder. The traditional mechanical polishing method is easy to cause aluminum nitride grains to fall off to form pits. In this paper, to improve the polishing effect, based on the hydration of polycrystalline aluminum nitride, a sol-gel (SG) polishing tool was proposed to polish polycrystalline aluminum nitride. The polishing process was divided into rough polishing and fine polishing. The diamond particle size of the SG tool used for rough polishing was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$40\mathbf {\mathrm {\mu }}\text{m}$ </tex-math></inline-formula> . The diamond particle size of the SG tool used for fine polishing was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\mathbf {\mathrm {\mu }}\text{m}$ </tex-math></inline-formula> . Moreover, the polycrystalline AlN was processed with or without deionized water. The results showed that in the rough polishing stage, the material removal rate (MRR) of the polishing method without deionized water was four times higher than that of the polishing method with deionized water. However, in the fine polishing stage, the MRR of the polishing method with deionized water was slightly higher than that of the polishing method without deionized water. The results showed that mechanical removal is the primary material removal method in the rough polishing stage. In the fine polishing stage, hydration can improve the polishing effect of polycrystalline aluminum nitride.

Correlating Corrosion to Surface Grain Orientations of Polycrystalline Aluminum Alloy by Scanning Electrochemical Cell Microscopy.

The study of grain-dependent corrosion behaviors of practical polycrystalline metals remains challenging due to the difficulty in eliminating the influences of other microstructural features, such as intermetallic particles and grain boundaries. In this work, we performed thousands of microscopic potentiodynamic polarization measurements on a polycrystalline aluminum alloy AA7075-T73 using the spatially resolved oil-immersed scanning electrochemical cell microscopy measurement. Data were extracted only from grain interior areas excluding intermetallic particles and grain boundaries. Based on the multiple potentiodynamic polarization measurements, the differences between grains can be revealed. Cathodic currents exhibited a strong grain orientation dependence with a decreasing order of {101} > {001} > {111}, agreeing with the prediction from the order of atomic planar density. By contrast, the dependence of anodic currents on grain orientation was weak, and pitting was independent of grain orientation, which could be due to the limited mass transport of ions within the surface oxide film. This work highlights the capability of oil-immersed scanning electrochemical cell microscopy in resolving small electrochemical differences, which will greatly promote the study of grain-dependent behaviors of practical polycrystalline samples.

Finite element simulation and experimental investigation of crystallographic orientation-dependent residual stress in diamond cutting of polycrystalline aluminum

The residual stress of crystalline materials induced by deformation at grain level strongly correlates with crystal orientation. In the present work, the dependence of residual stress on crystallographic orientation in diamond cutting of polycrystalline aluminum is investigated by crystal plasticity finite element modeling and simulations. Furthermore, corresponding ultra-precision diamond cutting experiments, which have the same parameter setup with the finite element model, are also carried out. The crystallographic orientations of the specimens are depicted by electron back-scattered diffraction characterization, and the surface residual stress is measured by X-ray diffraction. Both experiment and finite element simulation results demonstrate the anisotropic characteristics of the residual stress within machined polycrystalline aluminum, which exhibits the significant correlation with crystallographic orientation. Furthermore, finite element simulations of bi-crystal cutting are also performed, which indicate that the grain boundary also has a strong influence on the generation of residual stress.

Graph-component approach to defect identification in large atomistic simulations

The graph-theoretical concept of connected components is employed to extract the evolution of defect configurations in a polycrystalline aluminum structure containing ∼8.3 million atoms. This graph-component approach is applied to reveal details of defect formation, transport, and transformation in the polycrystalline Al under large shear deformation. Building upon standard nearest neighbor analysis, graph theory and associated tools are used to reduce the multi-million-atom system into discrete component subgraphs that represent distinct structural defects. This method allows the automated identification, characterization, and tracking of defective regions within large volumes of data representing atomic-scale processes. Such analysis elucidates relationships between external stimuli, such as strain, and defect distributions, which have a large influence on material properties. The Graph Analytics for Large Atomistic Simulations (GALAS) codebase that implements this analysis, together with user guidance, is openly available at https://github.com/pnnl/galas.

Finite Element Analysis of AA 6016-T4 Sheet Metal Forming Operations Using a New Polycrystalline Model

A major challenge in forming polycrystalline aluminum alloy sheets is their strong plastic anisotropy, namely the flow stresses and plastic strains depend on the loading direction. This plastic anisotropy is due to the anisotropy of the constituent crystals and the preferred orientations that they assume in the polycrystalline material i.e. crystallographic texture. Recently, in [1] we developed a single-crystal yield criterion that involves the correct number of anisotropy coefficients such as to satisfy the intrinsic symmetries of the constituent crystals and the condition of yielding insensitivity to hydrostatic pressure. This single-crystal criterion is defined for any stress state. It is shown that a polycrystalline model that uses this single-crystal criterion in conjunction with appropriate homogenization procedures leads to an improved prediction of the plastic anisotropy in macroscopic properties under uniaxial tension and shear loadings for polycrystalline aluminum alloy 6016-T4. Moreover, results of FE simulations of cup forming operations demonstrate the predictive capabilities of this polycrystalline model.

Piezospectroscopic analysis of mechanical stresses in Si3N4 and AlN irradiated with high-energy bismuth ions

Depth-resolved Raman piezospectroscopy was used to study residual mechanical stress profiles in polycrystalline silicon and aluminum nitrides irradiated with 710 MeV bismuth ions to fluences of 1×1012, 2×1012, and 1×1013 cm-2. It was found that stress fields of opposite signs are formed in the irradiated Si3N4 layer, separated by a buffer zone located at a depth coinciding with the thickness of the sample layer, amorphized at high ion fluences due to multiple overlapping of track regions. At great depths, tensile stresses witch magnitude reaches their maximum value in the region of the end of the ion range are detected. In contrast to Si3N4, radiation-stimulated changes in mechanical stresses in AlN were within the measurement error throughout the entire thickness of the irradiated layer, except of the near-surface region. The observed effect is associated with the different structural sensitivity of silicon and aluminum nitrides to high-density ionization - the formation of amorphous latent tracks in Si3N4 and their absence in AlN.

The dependence of the surface roughness of aluminum nitride films on the processing methods when magnetron sputtering is used

The paper investigates the dependence of the surface roughness of aluminum nitride films on the modes of magnetron deposition. The paper shows the difference between the surfaces of polycrystalline aluminum nitride films on ceramized glass and silicon substrates grown under the same conditions. Atomic force microscopy was used to compare the surface roughness parameters of aluminum nitride obtained at different substrate temperatures and the power dissipation in the target. The optimal conditions for forming aluminum nitride films through magnetron sputtering have been determined.

Large-scale atomistic studies of sliding friction in polycrystalline aluminum interfaces

We discuss large-scale non-equilibrium molecular dynamics (NEMD) simulations of ductile metal sliding comprising up to 1.8 × 109 atoms over time scales of 100 ns. The results of these simulations have identified a variety of physical mechanisms that are important in determining the steady-state frictional force for a wide range of velocities at compressed metal–metal interfaces. These include grain growth and refinement, the evolution of large plastic strains and strain rates, material mixing, and melting. These phenomena can be included in a strain, strain rate, and grain size model that gives good agreement with the NEMD simulations and can be applied to macroscopic continua.

Reducing Friction in Orthodontic Brackets: A Matter of Material or Type of Ligation Selection? In-Vitro Comparative Study.

(1) Background: Orthodontic appliances have changed and improved with the increasing demand for orthodontic treatment of the general population. Patients desire for shorter orthodontic treatments and for the wearing of more aesthetic devices has led to the technological development of orthodontic brackets; these were manufactured from aesthetic materials (ceramics, composite polymers) and presented different designs regarding the way archwires are ligated to the bracket. The aim of this study was to determine whether there were any differences between the static frictional forces generated by stainless steel (metallic) and polycrystalline alumina (ceramics) conventional and self-ligating brackets. (2) Methods: Static friction assessment was carried out in vitro with a universal testing machine, HV-500N-S (Schmidt Control Instruments, Hans Schmidt & Co. GmbH), intended for measuring compression and traction forces. (3) Results: The study revealed significant differences in static frictional forces at the bracket-archwire interface between the tested brackets. Stainless steel brackets produced lower static friction forces than polycrystalline alumina and self-ligating brackets generally produced lower static frictional forces than conventional brackets. The reduction of frictional forces was noticeable in the first stages of treatment, when thin, flexible orthodontic archwires (0.016” NiTi) are used. Engaged with large rectangular stainless steel archwires, (0.019 × 0.025” SS), the frictional forces produced by conventional and self-ligating metal brackets were similar, no significant differences being observed between the two types of metallic design. However, in the case of tested ceramic brackets, the results showed that the self-ligating type allows a reduction in frictional forces even in advanced stages of treatment compared to conventionally ligation. (4) Conclusions: From the perspective of an orthodontic system with low frictional forces, metal brackets are preferable to aesthetic ones, and self-ligating ceramic brackets are preferable to conventional ceramic brackets.

Luminescent Dy3+ and Dy3+/Cr3+ doped transparent Al2O3 ceramics: Microstructure and optical properties

Highly transparent Dy3+ and Dy3+/Cr3+ polycrystalline alumina ceramics were prepared with the real in-line transmittance (RIT) up to 55% (at λ = 632 nm), one of the highest values reported for luminescent rare-earth elements doped alumina. The RIT of doped alumina decreased more sharply with increasing mean grain size than predicted by the models for pure alumina. Co-doping with Dy3+ and Cr3+ resulted in a moderate increase of the RIT, which was independent on grain size. EDX analysis revealed that dysprosium segregated at grain boundaries and chromium was dispersed throughout the alumina matrix. A thermodynamic model for diffusion of multiple species in systems with multiple sorts of traps was proposed. The photoluminescence excitation and emission spectra of Dy3+-doped alumina showed peaks characteristic for Dy3+-doped materials. In co-doped Dy3+/Cr3+ alumina, Dy3+ emitted in the blue and yellow regions and Cr3+ in the red region, creating a suitable combination for obtaining white light.

Probing Hundreds of Individual Quantum Defects in Polycrystalline and Amorphous Alumina

Quantum two-level systems (TLSs) are present in the materials of qubits and are considered defects because they limit qubit coherence. For superconducting qubits, the quintessential Josephson junction barrier is made of amorphous alumina, which hosts TLSs. However, TLSs are not understood generally -- either structurally or in atomic composition. In this study, we greatly extend the quantitative data available on TLSs by reporting on the physical dipole moment in two alumina types: polycrystalline $\mathrm{\mathrm{\gamma-Al}_{2}\mathrm{O}_{3}}$ and amorphous $\mathrm{a-Al}\mathrm{O_{x}}$. To obtain the dipole moments $p_z$, rather from the less-structural coupling parameter g, we tune individual TLSs with an external electric field to extract the $p_z$ of the TLSs in a cavity QED system. We find a clear difference in the dipole moment distribution from the film types, indicating a difference in TLS structures. A large sample of approximately 400 individual TLSs are analyzed from the polycrystalline film type. Their dipoles along the growth direction $p_z$ have a mean value of 2.6$\pm$0.3 Debye (D) and standard deviation $\sigma$ = 1.6$\pm$0.2 D . The material distribution fits well to a single Gaussian function. Approximately 200 individual TLSs are analyzed from amorphous films. Both the mean $p_z$ =4.6$\pm$0.5 D and $\sigma$ =2.5$\pm$0.3 D are larger. Amorphous alumina also has some very large $p_z$, > 8.6 D, in contrast to polycrystalline which has none of this moment. These large moments agree only with oxygen-based TLS models. Based on data and the candidate models (delocalized O and hydrogen-based TLSs), we find polycrystalline alumina has smaller ratio of O-based to H-based TLS than amorphous alumina.

Effects of temperature and grain size on diffusivity of aluminium: electromigration experiment and molecular dynamic simulation

Understanding the atomic diffusion features in metallic material is significant to explain the diffusion-controlled physical processes. In this paper, using electromigration experiments and molecular dynamic (MD) simulations, we investigate the effects of grain size and temperature on the self-diffusion of polycrystalline aluminium (Al). The mass transport due to electromigration are accelerated by increasing temperature and decreasing grain size. Magnitudes of effective diffusivity (D eff) and grain boundary diffusivity (D GBs) are experimentally determined, in which the D eff changes as a function of grain size and temperature, but D GBs is independent of the grain size, only affected by the temperature. Moreover, MD simulations of atomic diffusion in polycrystalline Al demonstrate those observations from experiments. Based on MD results, the Arrhenius equation of D GBs and empirical formula of the thickness of grain boundaries at various temperatures are obtained. In total, D eff and D GBs obtained in the present study agree with literature results, and a comprehensive result of diffusivities related to the grain size is presented.

Twins and grain boundaries-dominated the reverse Bauschinger effect and tension-compression asymmetry

The failure modes and mechanisms of single and polycrystalline aluminum under different stress states are very different. Therefore, molecular dynamics methods were used to study the tension-compression asymmetry and Bauschinger effect of single and polycrystalline aluminum under different loads. We found both single and polycrystalline aluminum exhibit work hardening under tensile and compressive loads. The difference in this hardening phenomenon is manifested in that the characteristics of single and polycrystalline aluminum are brittle fracture and ductile fracture, respectively. Furthermore, we found that the tension-compression asymmetry of polycrystalline aluminum is more obvious than that of single crystal aluminum, and it manifested as a reverse tension-compression asymmetry. The Bauschinger effect is caused by the defects introduced by preloading, and the asymmetry of the Bauschinger effect is caused by different microscopic defects. Dislocations introduced by the pre-stretching of single crystal aluminum degrade its performance under reverse loading. Polycrystalline aluminum has more twinned regions under compressive load, which hinder the movement of dislocations and grain boundaries during reverse stretching, showing the reverse Bauschinger effect. This study has important theoretical guiding significance for a comprehensive and in-depth understanding of the mechanical properties and microstructural evolution mechanism of single and polycrystalline aluminum.

Homogenisation of micromechanical modelling results for the evaluation of macroscopic material properties of brittle ceramics

This paper presents an approach to evaluate the macroscopic properties of ceramic materials based on the homogenisation of the results of micromechanical simulations performed on Representative Volume Elements (RVEs) of polycrystalline microstructures. The RVEs are defined using a bespoke algorithm to generate numerical model geometries statistically representative of the microstructure of polycrystalline ceramics. The material properties of single crystals and grain boundaries, obtained from direct experimental measurements at the relevant scales, are used for the generation of RVEs, which are then subjected to unit load cases. The results of the numerical simulations, performed using the explicit Finite Element Method (FEM), are used to calculate the macroscopic material parameters governing elastic deformation, and brittle failure due to initiation and propagation of cracks at the grain scale. The automated approach adopted to generate and analyse the response of RVEs under a specified set of loading conditions permits a statistically relevant number of simulations to be performed on different combinations of microstructural morphologies, distributions of crystallographic orientations, and defects, thus demonstrating a homogenisation and upscaling methodology that captures several important aspects of the stochastic variability typically exhibited by ceramic materials. The numerical simulations of elastic deformation and brittle failure of polycrystalline alumina (Al2O3) are compared against both experimental measurements and analytical calculations, showing good agreement for both elastic deformation - expressed in terms of homogenised elastic moduli - and failure strength - observed both as changes in the crack pattern at the microscale and when measured as the value of the stress at the onset of failure.