Al Droplets Research Articles

Reactive wetting of metallic/ceramic (Al/α-Al2 O3 ) systems: a parallel molecular dynamics simulation study

The reactive wetting process of a flat solid alumina (α-Al2 O3) ceramic surface by metallic aluminum (Al) nanodroplets with different shapes (spherical, cylindrical, and layer) is studied using parallel molecular dynamics (MD) simulations based on a variable charge MD method, with focuses on heat transfer, mass transfer, and the structure of the reactive region at the Al/α-Al2 O3 interface. We find that the diffusion of oxygen (O) atoms from the substrate into the droplet leads to the formation of a continuous layer of reaction product at the interface. The diffusion length of oxygen atoms into the spherical Al droplet is found to be ~7.3 Å, and the number density of O atoms at the ~5 top layers of the substrate decreases substantially. As a result, the structural correlations near the reactive region differ considerably from those in the solid substrate. Heat generated by the exothermic reactions in the reactive region is transferred to both the substrate and the droplet. The heat transfer is found to be sensitive to droplet shape.

In‐situ observation of AlN formation from Ni‐Al solution using an electromagnetic levitation technique

AbstractAluminum nitride is a promising substrate material for AlGaN‐based UV‐LED. In order to develop a robust growth processing route for AlN single crystals, fundamental studies of solution growth experiments using Ni‐Al alloy melts as a new solution system were performed. Al can be stably kept in solution the Ni‐Al liquid even at high temperature; in addition, the driving force of the AlN formation reaction from solution can be controlled by solution composition and temperature. To investigate AlN crystal growth behavior we developed an in situ observation system using an electromagnetic levitation technique. AlN formation behavior, including nucleation and growth, was quantitatively analyzed by an image processing pipeline. The nucleation rate of AlN decreased with increasing growth temperature and decreasing aluminum composition. In addition, hexagonal c‐axis oriented AlN crystal successfully grew on the levitated Ni‐40 mol%Al droplet reacted at low driving force (1960 K), on the other hand, AlN crystal with dendritic morphology appeared on the sample with higher driving force (Ni‐50 mol%Al, 1960 K). Thus, the nucleation rate and crystal morphology were dominated by the driving force of the AlN formation reaction.

Controllable synthesis of Al2OC-AlN solid solution by two-step sintering in resin-bonded Al-Al2O3 composites

A method of synthesizing Al2OC-AlN solid solution with cost-effective materials at lower temperature to achieve industrial production was proposed in this paper. Aluminum, tabular alumina and binder phenolic resin were used to prepare the resin-bonded Al-Al2O3 composites. The sintered samples were obtained by one-step sintering and two-step sintering, respectively. Then, the basic physical properties were tested. XRD, SEM and EDS were employed to investigative the sintered samples. The results indicated the controllable synthesis of Al2OC-AlN solid solution was successfully achieved in resin-bonded Al-Al2O3 composites containing 12 wt% Al by two-step sintering at 580 °C for 8 h and then 1600 °C for 3 h under N2. After sintering at 580 °C, the Al@AlN structure with AlN coating the surface of Al was formed. Active AlN and dispersed Al droplets were introduced after the disappearance of Al@AlN structure, thereby solving difficulties such as the introduction of active AlN and the agglomeration of liquid Al during direct sintering. The active AlN transient phase and liquid Al promoted the formation of Al2OC-AlN solid solution. Meanwhile, the binding agent resin provided active carbon resources for the formation of Al2OC-AlNss. The formation mechanism of Al2OC-AlN solid solution during two-step sintering was investigated.

Interfacial wetting mechanisms of Al liquid on cathode carbon blocks of aluminum reduction cell for developing wettable cathode materials

Molecular dynamics simulation had been performed to investigate the wetting properties of Al droplets on amorphous carbon surfaces and graphite surfaces. The effects of temperature, graphitization of substrate and roughness were considered. Our results show that the contact angles can be improved effectively with the increased temperature. Furthermore, the Al droplet on graphite surface has a better wettability than that on amorphous carbon surface. Similarly, the contact angle will also reduce with the increased degree of graphitization and the wetting state will change from the transition wetting state to the Cassie state when the roughness increases. In addition to the contact angles on the rough surface, the remaining contact angles are less than 90°, which is different from the previous reports. The free energy and thermodynamic properties analysis were applied to character the solid-liquid interface properties and explain the wettability. Another interesting finding is that the study reveals the reason of the poor wettability between Al liquid and cathode carbon blocks of aluminum electrolytic cell. These findings improve our understandings of the wetting behaviors of Al droplets on cathode carbon block surfaces at the atomistic level, which is profitable to develop the wettable cathode materials for aluminum electrolysis.

Effects of Ammonium Perchlorate Particle Size on the Aluminum Agglomeration in Primary Combustion of the Ammonium Perchlorate/Aluminum Binary Mixture with a High Aluminum Content

Effects of ammonium perchlorate (AP) particle size on the aluminum (Al) agglomeration in primary combustion of AP/Al binary mixture with a high Al content were investigated experimentally using a CO₂ laser ignition apparatus. The combustion of AP/Al binary mixture and Al agglomeration were observed by a high-speed camera. In addition, the particle size distributions and chemical compositions of the condensed combustion products were analyzed by a laser particle size analyzer, X-ray diffraction, and other experimental techniques. Results show that the increase in AP particle size can decrease the size of Al agglomeration, the combustion efficiency of Al powder, the brightness of oxide cap, and the distance from the flame front to the Al droplet but can increase the flame radius. The Al particle burns in evaporation mode, and the rotation phenomenon occurs in the Al droplet. The rotational velocity of the Al droplet decreases with increasing AP particle size. The Al agglomeration can be formed by two ways in the flame zone. For larger Al agglomerations, the spherical molten Al particles first aggregate, then some small aggregates detach from the surface of the big aggregates, and last, the remaining irregular aggregates become regular bigger spherical agglomerations. For small Al agglomerations, the agglomerates directly separate small agglomerations.

Droplet etching with indium – Intermixing and lattice mismatch

In contrast to Indium droplets the usage of Ga or Al droplets for the local droplet etching (LDE) technique is well established. It has been shown previously that In-droplets or In-alloys can be used for LDE with focus on small hole-depths at low etching temperatures. With our recent experiments we reveal that In-LDE is also well suited for the creation of very deep nanoholes (exceeding 100 nm) and very low densities in the range of 106 cm−2 for large average distances of several micrometers. The unexpected high thermal stability of the etching process itself as well as the formed wall-like structures clearly points to an intermixing between etching and substrate material during the LDE-process. This can be related to the lack of As during LDE. Additional experiments with capping layers also reveal high surface-roughness probably caused by a lattice mismatch, which can be also explained by intermixing. A second LDE step with Al can be applied to avoid roughening effects.

Coalescence and wetting mechanism of Al droplets on different types of carbon for developing wettable cathodes: a molecular dynamics simulation.

So far, there have been few studies on the microscopic wetting behavior of aluminum liquid on cathode surfaces, which is critical for developing wettable cathode materials. In the present study, an investigation on the coalescence and wetting mechanism of Al droplets on different carbonaceous substrates has been performed via molecular dynamics (MD) simulation for developing wettable cathodes. The growth rate of liquid bridge, the mean squared displacement, the balanced contact angle, and the time of full coalescence were calculated to describe the coalescence and wetting of the Al droplets. The results illustrate the sequence of full coalescence time for the Al droplets: DG < HCNT < VCNT ≈ AC and the corresponding balanced contact angles were 47.98°, 53.32°, 55.02°, and 63.12°, respectively. Furthermore, the presence of defects on DG will increase the time of coalescence and the contact angle but the directions of defects have little influence. The free energy analysis indicates that the defects reduce the solid-liquid interaction and the work done for removing the Al droplet from the substrates so that the wettability is weaker than that for perfect graphene, which also explains the balanced wettability of Al droplets on the other substrates. In addition, the surface roughness increases the contact angle of Al liquid on AC (from 62° to 113°-120°) and hence, the wettability is changed from good to poor. In general, our results can improve the understanding of the wetting of AC and graphene by Al liquid at the atomic level, which can provide direction and theoretical guidance for further research on wettable cathodes.

Faceting of local droplet-etched nanoholes in AlGaAs

Nanoholes, drilled in the (001) surface of AlGaAs by local Al droplet etching, are shown to consist of faceted inner walls. The most prominent facets of the inverted pyramidlike nano-sized etch pits are the {111}A and {$1\overline{1}1$}B surfaces, which differ in their atomic surface terminations. In the [110] direction, the {111} facets change to {112} and/or {113}, which are both stepped surfaces with (111)A terraces. Etching-temperature-dependent data indicate that this facet transition seems kinetically hindered up to etch temperatures above 660 \ifmmode^\circ\else\textdegree\fi{}C, at which point the walls along $[1\overline{1}0]$ have already evolved completely towards {$1\overline{1}1$}B facets. The redeposited ring material outside the nanohole develops facets with indices of (115) and higher, thereby forming relatively flat structures. The facets and their indices are unraveled by a combination of atomic force microscopy, scanning electron microscopy, and x-ray diffraction, which is performed on an ensemble as well as on a single hole using nanodiffraction. These results imply that this nanoconfined etching process can be largely understood in a vapor-liquid-solid scheme, which includes the bulk thermodynamics in the Al-Ga-As system, the surface energies of low index facets, and their etch rates and surface terminations.

Heterogeneous nucleation of pure Al on MgO single crystal substrate accompanied by a MgAl2O4 buffer layer

This paper investigates the correlations between interfacial reaction, crystallographic orientation relationship on the interface and the required undercooling for nucleation on different crystallographic planes of MgO. Thermal analysis and high resolution transmission electron microscopy were used to study the nucleation behavior of liquid, high-purity Al droplet on single crystal MgO substrates using a DSC with an integrated image capture system and a sessile drop apparatus. The results showed that the original substrate MgO would be completely replaced by the reaction product MgAl2O4 at the interface owing to the chemical reaction between liquid Al and the MgO substrates. In addition, the same crystal structure with the original MgO substrate is achieved in the new MgAl2O4 layer. The orientation relationship between MgAl2O4 and Al is consistent with the theoretical prediction according to the Bramfitt's lattice misfit theory and Edge-to-Edge model. Consequently, the generated MgAl2O4 significantly influences the detected undercooling.

Elimination of droplet rebound off soluble substrate in metal droplet deposition

This letter presents a method and its mechanism of using Ag coating to inhibit rebound of Al droplets over soluble substrates. Experiments show that Ag coating can eliminate droplet rebound, which makes it possible to print complex parts with high-quality inner surface. Further investigation reveals that Ag coating suppresses the rebound of Al droplets through strong chemical interaction between liquid Al and Ag layer, which results in fast Ag dissolution in liquid Al and formation of a dendritic network of Al-Ag intermetallic compounds (IMCs) on their interface. XRD and EDS observation indicates that the IMCs caused by Ag atom diffusion and precipitation are mainly composed of Ag2Al intermetallics and α-Al solid solutions. It is found that the thickness of the IMCs layer increases as the substrate temperature is raised. The temperature increase of the substrate will slow down the droplet cooling process, which allows more thorough chemical reaction.

Wetting, reactivity, and phase formation at interfaces between Ni–Al melts and TiB2 ultrahigh‐temperature ceramic

AbstractThe wetting, reactivity, and phase formation at the liquid Ni–Al/TiB2 ceramic interfaces have been investigated at the temperatures close to the Ni–Al liquidus line. The wetting kinetics has been studied by the sessile drop technique utilizing liquid drop dispension and high‐speed high‐resolution video imaging. It is established that the wetting behavior changes from a nonreactive for the Al‐rich melts to a dissolution‐reactive for the Ni‐rich melts. For the Ni concentration ≥40 at.%, TiB2 precipitates are found in the solidified Ni–Al droplets after the high‐temperature interaction of the melts with TiB2 substrates. Besides, new (Al,Ti)Ni3 and (Al,Ti)2Ni21B6 phases are formed due to dissolution of TiB2 ceramic in Ni‐rich melts and subsequent solidification.

Molecular dynamics study of high temperature wetting kinetics for Al/NiAl and Al/Ni3Al systems: Effects of grain boundaries

The high-temperature wetting of metal droplets on alloy metal substrates is of practical interest owing to its wide range of applications in brazing, welding and soldering. The surface and bulk of the alloy metals contain very many grain boundaries, whose effects on high-temperature wetting and the associated mechanisms have not been satisfactorily studied. In this work, two spreading scenarios are considered using molecular dynamics simulations to investigate the effects of grain boundaries on the wetting kinetics of the spreading of Al droplets on substrates with (polycrystalline NiAl) or without (monocrystalline NiAl) dissolutive reactions and at various rates of solution (monocrystalline and polycrystalline Ni3Al). Dissolutive reactions occur only when an Al droplet spreads over a polycrystalline NiAl substrate, and they significantly accelerate the wetting kinetics. Irregular atomic alignment and associated with grain boundaries, atomic migration or dissolution reactions occur more easily on grain boundaries than on ideal crystal substrates. Dissolutive reactions occur when Al droplets spread on either monocrystalline or polycrystalline Ni3Al substrates; however, grain boundaries influence the dissolution rates in various spreading stages. The dissolution rate of the Al(l)/polycrystalline Ni3Al(s) system with grain boundaries exceeds that of the Al(l)/monocrystalline Ni3Al(s) system in the initial stage of spreading, but is less than that of the Al(l)/monocrystalline Ni3Al(s) system in the final stage of spreading. Correspondingly, the spreading of the Al(l)/polycrystalline NiAl(s) system is faster than that of Al(l)/monocrystalline NiAl(s) system in the first stage but slower in the last stage. This result confirms that dissolutive wetting is dominated by dissolutive rate rather than by the total dissolutive quantity, and a high dissolution rate accelerates wetting.

Ni-Al-Ti Ohmic Contacts on 1 x 1020 cm-3 Al+ Ion Implanted 4H-SiC

The Al-Ti based alloys give ohmic contacts on p-type 4H-SiC with reasonably low specific contact resistance (≤ 10-4­ Ωcm2). Unfortunately during alloying the deposited metals go through a liquid phase [1] and contact areas lose their form. Moreover, during cooling the Al in excess segregates in Al droplets on the top of the contact surface. All that produces ohmic contacts that have not an homogeneous interface with the semiconductor and limits the minimum distance between adjacent contact areas. In previous studies it has been shown that a thin layer of metal with high melting temperature like Ni can be used as a cap on the top of the deposited Al-Ti layer for reducing or avoiding the enlargement of the melted Al-Ti film during alloying, depending on the combination of the Ni, Al and Ti thin film thicknesses [2,3]. In the same studies it was shown that it exists an optimum Ni : Al : Ti thicknesses ratios to preserve the contact form and with optically homogenous and shiny contact surfaces with specific contact resistance values in the low 10-4 Ωcm2 decade at room temperature for p-type resistivity in the range 0.1-1 Ωcm [3]. This study will focus on the electrical performence of these optimal contacts above room temperature. 4° off-axis <0001> n-type 4H-SiC homo-epitaxial specimens with a 200 nm p-type layer at the top surface have been used for this study. The p-type resistivity of these layers varied in the range 10-2 – 101Ωcm. Ni(50nm)/Al(40nm)/Ti(20nm) were evaporated on the surface of these samples and circular TLM devices (TLM-C) were obtained by the lift-off technology. The intra pad-distances of these devices was in the range 4 – 49 μm, and the internal-pad-radius in the range 51 – 508 μm. After lift-off the specimens were alloyed at 1000°C for 2 min in vacuum. The preservation of the pads form factor and of a mirror like contact surfaces after alloying [3] were verified by optical inspection. The ohmic behavior of the alloyed contact was tested by current-voltage (I-V) measurements in the temperature range 300-560 K. The specific ohmic resistance rc and transfer length LTwere obtained from the I-V resistance values of TLM devices versus intra-pads distance for constant internal-pad radius [4]. As a cross check, the so obtained values were verified to be good for fitting the TLM resistance data obtained from the I-V measurements of devices versus variable internal-pad radius and constant intra-pads distance [4]. The main result of this study is that, for temperatures in the range 300-560 K, the specific contact resistance of the studied contacts is in the range 2 – 4 × 10 -4 Ωcm2 over three decades of p-type material resistivity, i.e. 10-2 – 101 Ωcm, that corresponds to a range of 103- 6×104 Ωsq sheet resistance values, taking into account the thickness of the p-type layer. Reasons for such a perfomance will be discussed. [1] Susumu Tsukimoto, Kazuhiro Ito, Zhongchang Wang, Mitsuhiro Saito,Yuichi Ikuhara, Masanori Murakami, Materials Transactions 50(5), 1071-1075 (2009). [2] P. Fedeli, M. Puzzanghera, F. Moscatelli, R. A. Minamisawa, G. Alfieri, U. Grossner, R. Nipoti, Mater. Sci. Forum 897, 391-394 (2017). [3] R. Nipoti, M. Puzzanghera, M.C. Canino, P. Fedeli, G. Sozzi, submitted to ICSCRM2017, Washington, DC, Sept. 17-22, 2017 [4] D. K. Schroder, Semiconductor Material and Device Characterization, 3rd Edition, 2006, Wiley-IEEE Press.

Molecular dynamics simulations on dissolutive wetting of Al–Ni alloy droplets on NiAl substrate

Abstract Reactive and non-reactive wettings of pure Al and Al–Ni alloy droplets on an NiAl(100) substrate were numerically investigated. The embedded atoms method potentials were applied to account for interatomic interactions for the Al–Ni systems. The results show that since the Al–Ni alloy droplets has lower equilibrium contact angle, their spreading on the substrate will be slightly faster than the pure Al droplet. However, owing to atom interdiffusion effects on the relaxed substrate, enhanced dissolution of Ni atoms from the substrate to the droplet would remarkably accelerate the spreading using rigid substrate as control. Thus, the pure Al droplet yields the fastest spreading due to its corresponding farthest-from-equilibrium concentration, which is followed by 90% Al-10% Ni droplet and 80% Al-20% Ni droplet. The enhanced spreading kinetics is attributed to the decreased bulk viscous dissipation and local dissipation in the vicinity of the contact line caused by the dissolution.

Employing Al buffer layer with Al droplets-distributed surface to obtain high-quality and stress-free GaN epitaxial films on Si substrates

Al buffer layers with Al droplets-distributed surface have been employed to grow high-quality and stress-free GaN epitaxial films on Si substrates. The Al droplets are proved to efficiently improve the quality of as-grown GaN. On the one hand, they can act as nucleation seeds to facilitate the epitaxial growth, improving the crystalline quality and surface morphology of as-grown GaN epitaxial films. On the other hand, they also can compensate the huge compressive stress produced by Al buffer layer during the cooling process, achieving stress-free film. The density and volume of Al droplets greatly impact the properties of as-grown GaN epitaxial films. The GaN epitaxial film grown on the Al buffer layer with many small Al droplets uniformly distributed on it shows the best crystalline quality with the full-width at half maximum (FWHM) of GaN(0002) and GaN(10–12) as 0.5° and 0.7°, respectively, and flat surface with the smallest surface root-mean-square roughness of 3.8 nm. In addition, it also exhibits relatively better photoelectric properties with an FWHM of near band gap emission peak of 18 nm, carrier concentration of 2.0 × 1017 cm−3, and mobility of 137.1 cm2/Vs. This work has revealed the advantages of Al buffer layer and the important effects of buffer layer surface on achieving high-quality GaN by PLD, which is of significance for various applications of GaN-based devices.

The interface of heterogeneous nucleation on single crystal substrates

Under controlled nucleation process was achieved by solidifying a high purity Al droplet on a single crystal Al2O3 substrate in a high vacuum chamber. The following X-Ray Diffraction (XRD) analysis and measured undercooling prove that the nucleation was triggered by the substrate. Various lattice mismatches between new crystal and substrate (C/S) were obtained through this approach. Combining XRD patterns and high resolution transmission electron microscope analysis we found that the morphology of interface was affected by lattice misfit. An epitaxial layer was found at C/S interface with larger lattice misfit, as in Al(100)//Al2O3(0001) system. Further experiments on introduced alloying element, Sb, into liquid Al shows a suppressed epitaxial layer of Al. Chemical reaction between liquid and substrate also contributes to the formation of the interface. The nucleation of Al on the MgO substrates was actually nucleated on MgAl2O4, chemical reaction product of Al and MgO, rather than MgO.

Effects of phosphoric acid on the surface morphology and reflectance of AlN grown by MBE under Al-rich conditions

Aluminium nitride (AlN) films grown by molecular beam epitaxy (MBE) were exposed to phosphoric acid (H3PO4) heated to 70°C for 10min. The H3PO4 treatment removed excess aluminium (Al) from the surface of AlN grown under Al-rich conditions. A side effect of the H3PO4 process was the formation of hexagonal etch pits up to 180nm in size. Reflectance measurements were performed before and after the removal of excess Al from the film surface. Excess surface Al in the form of droplets was found to strongly influence the reflectance of the AlN films. Consequently the Al surface droplets (AlSD) introduced an error into film thickness values derived from Lorentz oscillator model fitting of the specular reflectance. The film thickness measurement error due to excess surface Al was quantified as a function of the AlSD surface coverage percentage. The effect of AlSD on reflectance-based film thickness measurements must be taken into account when using AlN grown in the Al droplet regime to characterize the nitrogen (N) growth rate.

(Invited) Electrochemical Detection of Collisions of Single Soft Nanoparticles-Emulsions and Viruses

Research on nanoparticle (NP) collisions with an ultramicroelectrode (UME) has been extended to “soft” NPs, like emulsion droplets and viruses. The electrochemical response and the frequency of collisions allow one to extract information about the NPs and even study electrode reactions in fL and aL droplet reactors. It also allows working with solvents, like toluene, that are not frequently employed for electrochemical reactions and to obtain information about ion transfer across water/oil interfaces. These studies also allow one to detect rather subtle changes in diffusion coefficient of viruses.

Effects of Surface Oxide on the Nitridation Behavior of Aluminum Particles

A detailed transmission electron microscopy study coupled with electron energy loss spectroscopy was conducted on AlN formed by the direct nitridation of Al particles under nitrogen atmosphere. The nitridation mechanism comprised two steps: the formation of AlN shell on Al particles and the growth of AlN with a lath type in Al droplets. Here, we found that the surface oxide layer of the Al particles acted as a channel layer, which supplied nitrogen in the atomic state to liquid Al, after being transformed into a thin AlON layer during the initial nitridation. In the Al particles, the inward growth of AlN with a shell structure occurred at the sub layer of the AlON layer. On the other hand, the extracted liquid Al droplets formed after the cracking of the AlN shell rested on the Al particles surrounded by the AlON layer. The nitridation of the droplets began at the interface between the Al particle and droplet and not at the free surface and grew outward from the droplet. Herein, based on the observation of the AlON layer formation, we propose a new mechanism for the nitridation of Al particles.

Urchin-like AlOOH nanostructures on Al microspheres grown via in-situ oxide template

Abstract Three dimensional (3D) metal oxide hydroxide nanostructures are highly desirable for numerous applications. Here, we present a simple, scalable method to grow urchin-like AlOOH nanostructures on the surface of Al microspheres. Al nanoparticles with native oxide film were melted in molten salts, forming an immiscible materials system. During ultrasonic processing of the mixture, emulsion of micro Al droplets occurs and Al 2 O 3 nanoparticles form from the native oxide films and self-assemble to form in-situ nanostructured templates on the surfaces of Al microspheres. When the solidified mixture of Al spheres and salt is exposed to water, urchin-like nanostructures spontaneously form on the Al microspheres due to a heterogeneous nucleation and growth of γ-AlOOH on the Al 2 O 3 templates. This work could be readily extended to other metals, providing a novel and scalable method for the fabrication of urchin-like metal oxide hydroxide nanostructures on metal microspheres.