Al-Ge Alloys Research Articles

Thermal Properties of Cu–Al–Ge Alloy PCM for Improving Thermal Energy Storage of Current CSP

Metallic phase change material was proposed and examined at melting/eutectic temperatures less than 600 °C for latent heat storage in concentrated solar power. The PCM alloy can potentially charge/discharge thermal energy as a sensible/latent heat at the temperature range of 414-527 °C. The working temperatures of PCM alloy are in operation temperatures of current molten-salt based CSP plant. In this study, thermal response testing of PCM alloy and sensible/latent storage unit that PCM alloy was loaded in a ceramic honeycomb were prepared in a laboratory scale. The thermal charging/discharging performance of PCM alloy and sensible/latent storage unit were experimentally examined and compared. The storage unit provided thermal release/absorption behaviour at close to temperatures of the thermodynamic equilibrium state.

Microstructure evolution of dendrite and eutectic in undercooled Al70Ge30 alloy

Al-Ge eutectic alloy, as a desirable bonding material, has been widely used in microelectromechanical systems (MEMS) packaging. The bonding property of Al-Ge eutectic alloy is closely related to their microstructures. However, the microstructure evolution and underlying mechanisms in undercooled eutectic Al-Ge alloy remain unclear. In this work, Al70Ge30 alloy was prepared and subjected to undercooling using a cycled superheating purification technology. The maximum undercooling can reach 90 K (0.13TL). The solidified Al70Ge30 alloy exhibits concomitant dendrites and eutectic microstructures. The dendrites undergo rapid refinement at low undercooling and subsequently maintain nearly constant size as undercooling increases. Intriguingly, undercooling has a significant influence on eutectic microstructures. The eutectic microstructure transformed from feather structures to fishbone structures as undercooling increases then transformed into a line-like structure in eutectic cells. The evolution mechanisms of dendrites and eutectic structures are further elucidated based on several classical thermodynamic models. This study can provide theoretical and experimental guidance for the microstructure evolution of undercooled eutectic alloys with similar eutectic structures.

In-situ X-ray diffraction analysis of SiGe liquid phase growth on Si using Al–Ge paste

We performed in-situ two-dimensional X-ray diffraction measurements during a silicon-germanium (SiGe) layer formation process to analyze the liquid-phase epitaxial growth of a SiGe layer on silicon (Si) substrate by a simple method of aluminum-germanium (Al–Ge) paste screen-printing and annealing. The aluminum (Al) and germanium (Ge) from the Al–Ge paste were found to be in the liquid phase starting around 540 °C for the Al–Ge paste containing Al and Ge powders, while it starts around 430 °C for the one containing Al–Ge alloy powder. During the cooling process from the peak temperature of 900 °C down to 430 °C, a SiGe layer was epitaxially grown in the same orientation as the Si substrate. Around 430 °C, the remaining material in the liquid phase formed during the temperature increase solidified and the reaction was terminated. The XRD results suggest that most of the Al in the liquid phase exists as liquid phase from the peak temperature down to 430 °C. The Al liquid phase mediates the melting of Si and Ge, and the Si substrate functions both as a Si source, and as a seed crystal for the growth of the SiGe layer. Both samples show that the Ge concentration is sharply increased in the SiGe layer to around 13 at% at the interface and retains a uniform concentration distribution up to the Al paste matrix/SiGe interface. The change in crystallinity during the liquid phase growth indicates that the properties of the SiGe layer formed by this method can be controlled by adjusting the composition of the Al–Ge paste and the annealing temperature profile.

Mathematical modeling of dendrite growth in an Al–Ge alloy with convective flow

A theory of stable dendrite growth in an undercooled binary melt is developed for the case of intense convection. Conductive heat and mass transfer boundary conditions are replaced by convective conditions, where the flux of heat (or solute) is proportional to the temperature or concentration difference between the surface of the dendrite and far from it. The marginal mode of perturbation wavelengths is calculated using the linear morphological stability analysis. Combining this analysis with the solvability theory, we have derived a selection criterion that represents the first condition to define a combination of dendrite tip velocity and tip diameter. The second condition—the undercooling balance—is derived for intense convection. The theory under consideration determines the dendrite tip velocity and tip diameter for low undercooling. This convective theory is combined with the classical theory of dendritic growth (conductive boundary conditions), which is valid for moderate and high undercooling. Thus, the entire range of melt undercooling is covered. Our results are in good agreement with experiments on Al–Ge crystallization.

Structure of liquid Al[sbnd]Sn alloys

The short-range order of liquid AlSn alloys (0, 3, 8, 13, 20, 35, 50, 75, 100 at. % Sn) at 973 and 1273 K has been studied by X-ray diffraction and Reverse Monte Carlo simulation. The analysis of the experimental and simulation data points out the presence of atomic clusters with a local structure similar to liquid Sn even at low tin content (8 at. %). An impact of these clusters on the formation of short-range order of AlSn melts becomes dominant at 35 at. % Sn, resulting in the similarity between experimental structure factors and pair correlation functions of binary melts and liquid tin. On the other hand, the short-range order of liquid binary alloy with high Al content (97 at. %) is structurally homogeneous and based on the local atomic structure of liquid aluminium. The comparison between the short-range order of the liquid AlSn, AlGe, and AlSi alloys has been discussed.

Dislocation anelasticity in Al–Si/Ge alloys obtained by directed solidification

Young’s modulus defect, internal friction (logarithmic decrement), and flow stress at a constant anelastic strain measured at frequency about100 kHz at variable silicon or germanium content near the eutectic points in Al–Si and Al–Ge alloys are considered. Different behavior of anelastic characteristics induced by component contents are observed in these alloys. It is supposed that the important role in the anelasticity is determined by the influence of Guinier-Preston zones on dislocation vibration motion in solid solution of silicon or germanium in aluminum.

A comparative in situ X-radiography and DNN model study of solidification characteristics of an equiaxed dendritic Al-Ge alloy sample

In situ X-radiography imaging during the isothermal solidification of a 200 µm thin Al-24 at.% Ge sample was performed to determine dendritic growth rates, liquid concentrations and projected solid fractions. The experimental data allowed to follow dendrite tip growth velocities with high accuracy all the way from the initial transient growth related decrease of velocity via the expected increase of velocity during free dendrite growth to its final decrease during neighbor interacted growth. Concentration profiles in front of individual dendrite tips were measured to analyze the increase in far-field solute concentration due to solute accumulation. By time-resolved imaging of almost the entire 12 mm diameter sample, the global melt concentration could be derived. This enabled to estimate the initial nucleation undercooling of this un-refined alloy, which was so far unresolved. The experimental results are compared to a three-dimensional dendrite needle network model that simulates the experimental conditions, such as hexagonal dendrite structures and confined sample geometry. Although the simulation results are very sensitive to variations of the parameters undercooling and dendrite selection constant, one model parameter set is found that reproduces all experimentally measured solidification characteristics. The thorough experiment-model comparison indicates that the dendritic growth rates in this experimental configuration can be lower by a factor of 7 depending on undercooling than in the non-confined case.

Theoretical studies on alloying of germanene supported on Al (111) substrate**Project supported by the National Natural Science Foundation of China (Grant No. 11674129).

Using density functional theory, we study the alloying of the buckled hexagonal germanene superlattice supported on Al (111)-(3 × 3), the sheet composed of triangular, rhombic, and pentagonal motifs on Al (111)-(3 × 3), and the buckled geometry on Al (111)-()(19°), which are denoted, respectively, by BHS, TRP, and SRT7, to facilitate the discussion in this paper. They could be alloyed in the low doping concentration range. The stable configurations BHS, TRP, and SRT7 of the pure and alloyed germanenes supported on both Al (111) and its Al2Ge surface alloy, except the SRT7 pure germanene on Al2Ge, could re-produce the experimental scanning tunneling microscopy images. The relatively stable Al–Ge alloy species are the Al3Ge5 BHS-2T, Al3Ge5 TRP-2T, and Al3Ge3 SRT7-1T on Al (111) while they are the Al4Ge4 BHS-1T, Al3Ge5 TRP-2T, and Al27Ge27 SRT7-(3 × 3)-9T on Al2Ge (the n in the nT means that there are n Ge atoms per unit which sit at the top sites and protrude upward). In addition, the Al3Ge5 BHS-2T and Al4Ge4 BHS-1T are the most stable alloy sheets on Al (111) and Al2Ge, respectively. Comparing with the experimental studies, there exists no structural transition among these alloyed configurations, which suggests that the experimental conditions play a crucial role in selectively growing the pure or the alloyed germanene sheets, which may also help grow the one-atomic thick honeycomb structure on idea Al (111).

Account of the diversity of tunneling spectra at the germanene/Al(1 1 1) interface

Despite the wealth of tunneling spectroscopic studies performed on silicene and germanene, the observation of a well-defined Dirac cone in these materials remains elusive. Here, we study germanene grown on Al(1 1 1) at submonolayer coverages with low temperature scanning tunneling spectroscopy. We show that the tunnelling spectra of the Al(1 1 1) surface and the germanene nanosheets are identical. They exhibit a clear metallic behaviour at the beginning of the experiments, that highlights the strong electronic coupling between the adlayer and the substrate. Over the course of the experiments, the spectra deviate from this initial behaviour, although consecutive spectra measured on the Al(1 1 1) surface and germanene nanosheets are still similar. This spectral diversity is explained by modifications of the tip apex, that arise from the erratic manipulation of the germanium adlayer. The origin of the characteristic features such as a wide band gap, coherence-like peaks or zero-bias anomalies are tentatively discussed in light of the physical properties of Ge and AlGe alloy clusters, that are likely to adsorb at the tip apex.

Single and multi domain buckled germanene phases on Al(111) surface

The simultaneous formation of single domain (3×3) and multi domain (√7×√7)R(±19.1°) germanene phases on Al(111) surface in the sub-monolayer range was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) based simulations. Experimental results revealed that both germanene phases nucleate and grow independently from each other and regardless of Al substrate temperature within significantly expanded range Ts = 27–200 °C. Our results unambiguously showed that STM images with hexagonal contrast yield correct-resolved structure for both germanene phases, while honeycomb contrast is a result of an artificial tip-induced STM resolution. First-principles calculations suggested atomic models with strongly buckled germanene (2×2)/Al(111)(3×3) and (√3×√3)R30°/Al(111)(√7×√7)R(±19.1°) with one of eight and one of six Ge atoms protruding upward respectively, that consistently describe the experimentally observed STM images both for single and multi domain surface phases. According to the DFT based simulations both germanene (2×2) and (√3×√3)R30° superstructures have a stretched lattice strain with respect to the ideal free-standing germanene by 6.2% and 13.9%, respectively. Hence, numerous small domains separated by domain boundaries in the (√3×√3)R307Al(111)(√7×√7)R(±19.1°) germanene phase tend to reduce the surface energy and prevent the formation of extended single domains, in contrast to the (2×2)/Al(111)(3×3) phase. However, our experimental results showed that the nucleation and growth of germanene on Al(111) surface yield strong modifications of Al surface even at room temperature (RT), which may be contributed to the formation of Al-Ge alloy due to Ge surface solid-states reactivity that was ignored in recent studies. It is already evident from our present findings that the role of Al atoms in the formation of (3×3) and (√7×√7)R(±19.1°) germanene phases is worthy to be carefully studied in the future, which could be an important knowledge for large-quantity fabrication of germanene on aluminum.

Structure of liquid Al-Ge-Ni melts in a wide composition range

X-ray diffraction study of the Al-Ge-Ni melts along different sections of the ternary phase diagram has been discussed. The measurements were taken at 1000 °C or at temperatures on 50° above liquidus for compositions with high melting point. It has been shown that AlNi interactions play the most important role in the formation of the local atomic structure in the Al-Ge-Ni melts. The most significant decreasing of the nearest interatomic distance (R1) takes places with addition of Ni to binary AlGe alloys. There is a competition between Al and Ge atoms in the formation of local atomic structure around Ni atoms. For ternary melts with high Ge content the influence of GeGe pairs increases resulting in the formation of the clusters with structure similar to the liquid germanium. The ribbons of the Al79Ge15Ni6, Al61Ge29Ni10 and Al61Ge15Ni24 alloys have been obtained by rapid solidification from the liquid state. The phase analysis of the as-quenched and annealed ribbons has been discussed.

Evaluation of laser Induced Breakdown Spectroscopy for analysis of annealed Aluminum Germanium alloy at different temperatures

Laser Induced Breakdown Spectroscopy (LIBS) technique has been applied to study the annealing effect of Al-Ge alloy at 150, 175, 240, 280, 325 °C. LIBS spectra of all samples prepared at these annealing temperatures were recorded. The signal intensity of the Al and Ge spectral lines increases with increase in annealing temperature. This increase in the signal intensity is due to structural modification of Al-Ge sheets surface with annealing temperatures. This variation in intensity verifies the assessment of annealed materials which can be performed with LIBS technique.

Self-diffusion in liquid Al-Ge investigated with quasi-elastic neutron scattering

Quasi-elastic neutron scattering measurements are carried out to investigate the self-diffusion of Ge in liquid Al-Ge alloys in a wide temperature range at and below the melting point of the respective pure elements. In all investigated Al-Ge alloys, the self-diffusivity of Ge follows an Arrhenius behavior over the entire investigated temperature range and shows a standard deviation of only 4%, which is significantly more precise when compared to classical measurements in long capillaries. At the melting point of pure Ge, the self-diffusivity of Ge in all investigated alloys is higher by some 22%–28% than that of pure Ge and shows a diffusion maximum at 41 at. % Ge. This can be understood as an increase in chemical interactions between Al and Ge in this composition region and is in line with previous experimental findings regarding fast dendrite growth during solidification.

Aluminum-Germanium Eutectic Bonding for MEMS: Behaviour and Solidification of Liquid Al-Ge on Different Substrates

Micro Electro-Mechanicals Systems (MEMS) include moving elements in some cases and they require a controlled atmosphere to operate. Hence they are protected by a cap assembled to the sensor by wafer-level packaging. For specific MEMS packaging, the process should provide strong and hermetic sealing walls of reduced size, less than 100 μm which also ensure electrical conductivity. If a high-temperature-resistant sealing (>200 °C) is also required, Al-Ge eutectic bonding is suitable. In eutectic bonding, the constituents of the alloy are deposited on at least one of the two wafers which are brought into contact. The melting and then the solidification of the solder close the interface to form an assembly. The Al-29.5at%Ge eutectic alloy, with a melting temperature of 424 °C [1], has been shown allowing strong and void-free bonds [2-4]. Nevertheless, during bonding, the eutectic liquid tends to squeeze out of the seal ring which can lead to void formation at the sealing interface and areas with unwanted metal. Moreover, influences of the underlayer on the bonding mechanisms have not been investigated whereas it is known to have an impact on Al/Ge bilayers [5]. In this paper we study the formation and behaviour of liquid Al-Ge alloy, its solidification, the resulting microstructures and the appearance of voids. The main investigated parameters are: - the underneath layer which is related to the wettability of liquid Al-Ge eutectic - the annealing temperature and cooling rate and their influence on solidified microstructure - the seal rings geometry The 200 mm silicon substrates used in this study are either thermally oxidized or covered by a 50nm-thick titanium nitride layer to prevent metal diffusion into silicon substrate. Blanket and patterned Al/Ge deposited wafers with 0.59 μm Ge/1 μm Al are used as single wafers and in bonded structures. The seal rings are 5 mm square with widths varying from 50 to 200 μm. Thermal treatment and bonding are performed under either vacuum (10-3 mbar) or inert gas (N2) at atmospheric pressure. Experimental temperature is varied from 400 °C to 500 °C with cooling rates varying from 1 °C/min to 20 °C/min. In order to evaluate the influence of the underneath layer on the behaviour of liquid Al-Ge eutectic alloy during bonding, the wetting of different substrates is investigated by dispensed drop method, under a vacuum of 5x10-7mbar. High purity aluminum and germanium (99.999 %) are used to prepare the alloy in an alumina crucible ending by a capillary to deposit a droplet on the studied surface. Observations are made in situ by camera and analyzed using Drop Shape Analysis software (Fig 1a). In addition, the melting and solidification process of many 1x1 cm2samples from blanket Al/Ge deposited wafers are studied by Differential Scanning Calorimetry (DSC) by using different cooling rates. The microstructures are then analyzed by optical microscopy, Secondary Electron Microscopy SEM and Energy Dispersive X-ray spectrometry (EDX). In some cases large dendrites are observed, which could be critical for a bonded structure (Fig 1b). Finally blanket and patterned Al/Ge deposited wafers are bonded to silicon wafers either thermally oxidized or covered by a titanium nitride layer. The influence of underneath layer, geometry (for patterned wafers) and cooling rates on squeeze-out and voiding phenomenon are characterized using Scanning Acoustic Microscopy SAM and FIB SEM (Fig 2). Image analysis is used to compare voiding densities between bonded samples (Fig 3). The results appear to be dependent on the process conditions. Underneath layer and cooling rate seem of great importance to control the squeeze-out and solidified microstructures in Al-Ge eutectic sealing and to determine the quality of the final assembly. Thanks to these characterizations and especially to the wettability measurements, mechanisms will be proposed to explain the squeeze-out result. REFERENCES [1] Rodney P. Elliott et al., Bulletin of Alloy Phase Diagrams, vol 1, n° 1, pp 65-68, 1980 [2] B.Vu et al., J. Vac. Sci. Technol.B, vol. 14, n° 4, pp. 2588-2594, 1996. [3] F.Crnogorac et al., J. Vac. Sci. Technol.B, vol. 30, n° 6, 06FK01, 2012. [4] S.Sood et al., Advancing Microelectronics, vol 41, n° 5, pp 30-37, 2014. [5] K. Nakazawa et al., Jpn. J. Appl. Phys., vol 53, n°4, 04EH01, 2014. Figure 1

Aluminum-Germanium Eutectic Bonding for MEMS: Behaviour and Solidification of Liquid Al-Ge on Different Substrates

In this paper we study the formation and behaviour of liquid Al-Ge alloy, its solidification, the resulting microstructures and the appearance of voids in bonded structure for MEMS packaging. The underneath layer influences the wettability of liquid Al-Ge as well as its solidification which can lead to undesirable Al dendrites. Each underneath layer imposes a critical value for the cooling rate above which dendrites are likely to appear. Blanket and patterned Al/Ge deposited wafers are bonded together with process parameters optimized thanks to previous results. The interfaces present a dendritic-free eutectic structure. Micro-voids are observed at the interfaces between Al and Ge phases. The Kirkendall effect and thermal stresses are two possible reasons for voiding phenomenon.

In-situ solute measurements with a laboratory polychromatic microfocus X-ray source during equiaxed solidification of an Al-Ge alloy

An important tool to study dendritic growth in metallic alloys is the in-situ polychromatic X-radiography. Since solutal interaction is a dominant factor influencing dendrite growth, the determination of liquid concentrations is crucial to interpret the image data. Laboratory experiments with polychromatic X-ray sources require reference measurements of samples of known composition and thickness to convert intensities into concentrations because of the wavelength dependency of the absorption coefficient. Calibration measurements and fitted intensity functions are used to determine solutal concentrations in an Al-Ge sample without using in-situ reference samples.

Refining and nucleating behaviors of AlP on primary phase in Al–Ge and Al–Ge–Si alloys

The present study addresses refining behavior of AlP on primary phases in Al–Ge and Al–Ge–Si alloys. In binary hypereutectic Al–70Ge alloy, the addition of Al–P master alloy leads to a refinement performance for primary Ge. It is found that AlP can act as the nucleation sites for primary Ge particles, and between them the lattice mismatch is 3.85%. Besides, the introduction of AlP in Al–Ge alloy results in an increased undercooling for the precipitation of primary Ge. While for Al–60Ge–5Si alloy, Si and Ge form continuous solid solution, with Si distributing in the center of the primary particles and Ge the outer layer. The primary solid solutions can also be significantly refined by AlP. Moreover, due to the nucleation behavior of AlP particles, primary Si crystals doped with a certain amount of Ge are refined to about 30μm from more than 140μm in Al–20Ge–18Si alloy.

Structure, microhardness, and strength of a directionally crystallized Al-Ge alloy

The structure, microhardness, and strength of binary directionally crystallized aluminum alloys with 35, 43, 53, 57, and 64 wt % germanium have been investigated. It has been shown that the eutectic microhardness is constant in the composition region under study. The microstrength of primary crystals of the solid solution of germanium in aluminum with the dendrite structure increases with increasing germanium concentration. However, the difference in the microhardnesses of the eutectic and dendrites, which was determined for each of compositions on the same specimen, does not exceed the measurement error. It has been assumed that the change in the strength of the alloy having the composition in the hypoeutectic region is determined by the redistribution of the volume fractions of the eutectic (α-Al and eutectic germanium) and the domains of primary crystals of the solid solution. This dependence can be described by the mixture rule. Above the eutectic composition, the alloy decomposes in a brittle manner; its strength is likely dependent not only on the content of the components, but also on the form and orientation of primary germanium crystals.

New evidence for the formation and growth mechanism of the intermetallic phase formed at the Al/Fe interface

Abstract

Particle and liquid motion in semi-solid aluminium alloys: A quantitative in situ microradioscopy study

Semi-solid melts exhibit a very unpredictable rheology and filling dynamics, when injected into thin-walled components. Optimization of the process requires an insight into the casting process during injection. For this purpose we injected semi-solid an Al–Ge alloy into two different thin channel geometries while recording high resolution radiographs at fast frame rates (up to 1000 images per s). Comparison of a bottleneck channel, which has previously been used for slower experiments, with a right-angle turn geometry reveals a significant influence of the channel shape on the flow behaviour of the particle–liquid mixture. While the bottleneck is quickly sealed with densified solid, turbulences in the right-angle turn apparently permit solid particles and clusters to move conjointly with the liquid and thus achieve a more complete filling. Single particle trajectories and rapid break-up of solid skeletons in such a system have been observed for the first time in situ.