Liquid metal

About: Liquid metal is a research topic. Over the lifetime, 6947 publications have been published within this topic receiving 77785 citations. The topic is also known as: liquid alloy & liquid metal alloy.

Retrograde Melting and Internal Liquid Gettering in Silicon

Abstract: COMMUNICATION Retrograde Melting and Internal Liquid Gettering in Silicon By Steve Hudelson , Bonna K. Newman , Sarah Bernardis , David P. Fenning , Mariana I. Bertoni , Matthew A. Marcus , Sirine C. Fakra , Barry Lai , and Tonio Buonassisi * Control of metal impurities has proven essential for developing modern semiconductor-based materials and devices. The prop- erties of high-performance integrated circuit, photovoltaic, and thermoelectric devices are tailored by the intentional introduc- tion of dopant species, as well as the removal and passivation of detrimental impurities. [ 1 , 2 ] In addition, the speed and uni- formity of several common semiconductor growth methods, including bulk crystal and vapor-liquid-solid (VLS) growth, are regulated by impurity-semiconductor interactions. [ 3 , 4 ] Precise control over impurity chemical states and spatial distributions requires a deep fundamental understanding of the thermodynamics and kinetics regulating impurity phase and transport. Impurity engineering in semiconductors typi- cally involves thermal annealing, as impurity solubility and diffusivity increase exponentially with temperature. However, because of the lack of suitable analytical tools for studying sub-micron-scale distributions of fast-diffusing impurities at elevated temperatures, the vast majority of experimental inves- tigations so far have been conducted at room temperature. As a result, much remains to be explored concerning fundamental impurity-semiconductor reactions at realistic processing temperatures. It was recently proposed [ 5 ] that certain silicon-impurity sys- tems can undergo melting upon cooling, a phenomenon known as retrograde melting . The controlled creation of liquid metal- silicon droplets within or on the surface of a silicon matrix of arbitrary shape could provide novel opportunities to engineer semiconductor-based systems via solid-liquid and vapor-liquid segregation. The phenomenon of retrograde melting, whereby a liquid phase forms from a solid phase upon cooling , has been observed and studied in several organic and inorganic systems, including Fe-Zr [ 6 ] and Mg-Fe-Si-O. [ 7 ] One common pathway [ ∗ ] S. Hudelson, [+] Dr. B. K. Newman, S. Bernardis, D. P. Fenning, Dr. M. I. Bertoni, Prof. T. Buonassisi Massachusetts Institute of Technology Cambridge, Massachusetts, 02139 (USA) E-mail: buonassisi@mit.edu Dr. M. A. Marcus, S. C. Fakra Advanced Light Source Lawrence Berkeley National Laboratory Berkeley, California, 94720 (USA) Dr. B. Lai Advanced Photon Source Argonne National Laboratory Argonne, Illinois, 60439 (USA) [ + ] Present address: 1366 Technologies, Lexington, MA 02421, USA for this process to occur is via the catatectic reaction, occur- ring at an invariant point on a binary phase diagram involving transformation from Solid → Solid + Liquid. [ 8 ] Many binary sys- tems exhibit such an invariant point, [ 9 ] including Ag-In, Cu-Sn, Fe-Mn, and Fe-S, [ 10 ] but very few are semiconducting mate- rials. [ 11 ] Retrograde melting in most common silicon-impurity systems cannot occur by this pathway, as these systems do not possess a catatectic point. [ 11 ] A second pathway for retrograde melting has been observed in the ternary Sb-Bi-Te system, wherein decreasing solubility of Te in Sb 2 Te 3 with decreasing temperature can lead to supersat- uration of Te and formation of liquid droplets at temperatures above the eutectic temperature. [ 12 ] We propose that a similar pathway could also produce retrograde melting in binary semiconductor-impurity systems that exhibit retrograde solu- bility. Due to the high enthalpy of formation of point defects in certain semiconductors, the solid solubility of an impu- rity within the crystal structure increases with temperature, reaching a maximum well above the eutectic temperature. Many dissolved elements in silicon demonstrate this property, [ 13 ] including many of the 3d transition metals such as iron, copper, and nickel. [ 14 ] It is hypothesized that retrograde solubility can lead to retrograde melting, [ 5 ] if supersaturation occurs at a tem- perature above the eutectic temperature (as demonstrated in Figure 1 a ). To study temperature-dependent silicon-impurity reactions at the micro-scale, we carried out synchrotron-based hard X-ray microprobe experiments at high temperatures (up to 1500 ° C). We adapted an in situ microscope hot stage (Linkam TS1500) at beamlines 10.3.2 at the Advanced Light Source [ 15 ] and 2-ID-D at the Advanced Photon Source. [ 16 ] X-ray fluorescence microscopy ( μ -XRF) mapping was used to investigate the spatial distribu- tion of transition metal-rich particles as small as 50 nm [ 17 , 18 ] in silicon matrices. The chemical state of precipitated impurities detected by μ -XRF was determined by X-ray absorption micro- spectroscopy ( μ -XAS). [ 18 ] To verify that μ -XAS can distinguish between liquid and solid phases in metal-Si systems, we prepared a standard sample (see Experimental , sample 1) consisting of a thin layer ( ∼ 1 μ m) of e-beam evaporated Cu, Ni, and Fe sandwiched between a mc-Si wafer and a thin piece ( < 15 μ m) of monocrys- talline Czochralski Si (CZ-Si). The sample was then heated to 1045 ° C, well above the Cu-Si and Ni-Si eutectic temperatures, to ensure a liquid metal-silicon mixture. μ -XRF mapping of the standard at 1045 ° C revealed that the previously continuous film had dewetted, suggesting the presence of a high-temperature liquid state. After cooling the sample to room temperature, a visual inspection revealed that the Si cap layer was fused to the

Neutron diffraction analysis of liquid zinc. — II

Abstract: Recent experimental data obtained by neutron spectrometry of zinc in several laboratories have been analysed with the purpose of producing the ion-ion potential in this liquid metal. The Percus-Yevick theory that is known to produce reliable inter-atomic potential in simple liquid insulators such as argon, as well as the hyperchain theory were used. Both theories are found to be inadequate in the case of liquid metals, at least if the experimental data presently available are used. Furthermore it seems that in order to proceed efficiently along this line, the region of low wave-vector transfer should be explored with much greater accuracy than now available both in X-ray and neutron-diffraction experiments.

Microstructural Changes in Brazing Sheet due to Solid-Liquid Interaction

Abstract: Aluminium brazing sheet is the material of choice to produce automotive heat exchangers. Although in Dutch the official translation of aluminium brazing sheet is “aluminium hardsoldeerplaat” the English name is used in the industry. Aluminium brazing sheet is basically a sandwich material and consists of an aluminium core alloy, typically an AA3XXX alloy (containing Mn) or an AA6XXX alloy (containing Mg and Si) with a clad alloy of the AA4XXX (containing Si) series. The core alloy gives the final product the desired properties after brazing. The core alloys are designed in such a way that after the brazing cycle, the condition is reached where the core has its optimum properties. Properties like strength and corrosion resistance are the main engineering parameters. Typically the core alloy is single side or both side clad and the thickness of the clad alloy ranges between 5 and 20% of the total thickness. The AA4XXX alloy used for aluminium brazing sheet has a melting range between 570°C and 610°C while the melting range of a typical AA3XXX core alloy lies above 610°C. This difference in temperature between the two alloys is used to join complex shaped products in “one shot”. At the brazing temperature, typically around 600°C, the AA4XXX clad alloy is completely molten. Due to capillary forces and surface tension differences, the molten clad alloy will flow to connect adjacent pieces. The process by which joining is taking place is called the brazing process. During this brazing process liquid metal from the clad alloy is in close contact with the solid core alloy. At this stage an interaction between the two phases can take place. Several types of interaction between the two phases can take place but the interaction that is referred to as Liquid Film Migration is the topic of study in this thesis. Liquid Film Migration is, however, not the only name given in literature to what seems to be the same interaction. Liquid Film Migration is causing a significant change in microstructure and element distribution of the core alloy. These changes are detrimental to the corrosion resistance of the final product. Although the name Liquid Film Migration was given to the process responsible for the observed changes no conclusive evidence has been presented to confirm the existence of a liquid film in aluminium brazing sheet. The literature available on Liquid Film Migration in aluminium brazing sheet has given some information on the conditions favourable for Liquid Film Migration to occur. These conditions are residual strain in the core alloy present at the peak brazing temperature and a small grain size of the core alloy. This thesis focussed on the occurrence of LFM in brazing sheet and the possible mechanism and driving forces behind it. Two different core alloys with the same clad alloy were processed and studied for their susceptibility to Liquid Film Migration. Conditions were created that according to literature should result in different degrees of Liquid Film Migration. Brazing took place in a standard brazing furnace or in a salt bath. The salt bath would enable a kinetic study since time and temperature are well controlled. The study of the samples after brazing indeed showed a different response to the applied processing. The main observation was that the onset of recrystallization of the core alloy plays a major role in the occurrence of Liquid Film Migration. A detailed study of the liquid film showed the accumulation of constituents and dispersoids which were originally present in the core alloy. The measured diffusion profiles of silicon in front of the liquid film coincide with theoretical diffusion profiles as if they originated from a moving boundary. From the diffusion profiles of silicon, the kinetics of the moving liquid film was extracted resulting in an inverse square root dependence of the velocity with time. An estimation of the energies available in the system showed that the coherency strain energy could not be the driving force for Liquid Film Migration in aluminium brazing sheet. Most likely the energy is supplied by the reduction of the (sub)grain size. This is supported by the fact that strained samples that do not recrystallize during brazing are the ones showing the highest degree of Liquid Film Migration. Based on these findings, the residual strain present in the form of sub-grains or dislocations is considered to provide the energy for the movement of the liquid film. A qualitative assessment of the residual strain present in samples after brazing supported the hypothesis that indeed this residual strain is the energy source needed for the movement of the liquid film. The mechanism behind liquid film migration has been found to be similar to the recrystallization process caused by Strain Induced Boundary Migration. It was presented that lowering the surface energy between grains by a liquid film would allow the boundary to move at lower dislocation densities compared to a non liquid infiltrated grain boundary. Recrystallization would reduce the dislocation density taking away the driving force to spport liquid film migration. Liquid film migration and strain induced boundary migration are both in competition for the same energy. Strain induced boundary migration can take place during the whole brazing cycle while liquid film migration only can occur in the presence of a liquid. This means that the clad alloy has to be at least partly molten to allow wetting of the grain boundaries. When recrystallization by strain induced boundary migration takes place no liquid film migration will occur. A theoretical approach to determine the velocity of the liquid film was in reasonable agreement with the observations. The main obstacle to completely quantify liquid film migration is the uncertainty of the development of the film thickness in time. As demonstrated, recrystallization plays a critical role in the onset of liquid film migration. A through Process Model was used to determine if such model could be used to predict recrystallization based on thermal mechanical process input parameters and alloy chemistry. Unfortunately the model lacks sensitivity to predict the onset of recrystallization in this study. However the model contains all necessary modules to ensure that after fine tuning for this application, more predictive power can be expected.