The applied materials characterization methods in the present research are shortly described as follows.
3.2.1 Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS is a type of mass spectrometry for elemental analysis and capable for detection of most of the elements in periodic table with high precision and sensitivity at the level of parts per billion. Samples are introduced in liquid and further decomposed to its constituent elements and transformed into ions by the ionization source named inductively coupled plasma, which generates ultra-high temperature of approximately 8000 °C. The generated ions are then introduced to the mass spectrometer for measurement with extremely high sensitivity and up to 70 elements can be determined simultaneously in a single sample analysis.
In the presented work, high-resolution ICP-MS (Agilent 8800) was employed to measure the impurities content to ensure a low detection limit. The workflow of ICP-MS sample preparation mainly consists of the following three steps: Firstly, weighing sample with target amount (30 to 60 mg) using high-precision balance and clean PFA bottles. Secondly, digestion of weighted samples through specific acids combination (1.5 mL HNO3 (69%) + 0.5 mL HF (40%)). Thirdly, further dilution of the digested solution using ultrapure water to around 200 mL and then filled in specific clean tube for ICP-MS analysis.
In order to obtain accurate and reliable results with impurity content at ppmw levels, both the sample preparation and measurement follow restrict and standardized rules for all samples in the lab. Every Si sample was measured at least two parallels for average and the NIST 57b Si powder sample was used as the reference material. In the digestion procedure, electronic grade acid mixture of HF and HNO3 were employed and carefully added by droplets into clean PFA bottles. The PFA bottles are only used for Si sample digestion and cleaned by ultra-pure deionized water at least three times before and after use. The digestion of Si samples lasts at least 6 hours to make sure all Si particles fully dissolved, and no remaining solids or precipitations could be observed by naked eyes. The digested solution was further carefully diluted by ultrapure deionized water for ICP-MS measurement.
71 3.2.2 Scanning Electron Microscopy (SEM)
SEM is commonly used in materials science and engineering for the micro-scale topography observation and often coupled with energy dispersive x-ray spectroscopy (EDS) to detect chemical composition on the sample surface. As the name suggests, the SEM produces images by scanning the sample surface with a focused electron beam in a vacuum chamber as schematically illustrated in Figure 3-5. The electrons interplay with sample surface and results in the excitation of different signals for detection such as secondary electrons, backscattered electrons, and characteristic X-rays. Characteristics X-ray for EDS analysis can be collected from points or areas of the sample (depending on the size of the phases) and the technique was used to identify approximate chemical compositions of the co-existing samples in this study.
Figure 3- 5. Sketch of the working principles of SEM and EDS.[165]
In this work, the microstructure of obtained Si alloys was investigated by field-emission scanning electron microscopy (LVFSEM, Zeiss Supra 55VP) with installed energy-dispersive X-ray spectrometers (EDX). Before the observation, samples were mounted in EpoFix cold-setting resin and further ground and polished using Struers automatic grinder and polisher (RotoPol-31). Carbon coating was also applied to improve the electrical conductivity of the samples surfaces (to prevent electron charging over the surface in SEM) using SEM Turbo Coater (AGB7230).
3.2.3 Electron Probe Micro-Analyzer (EPMA)
EPMA is a microbeam instrument used for non-destructive elemental analysis of micron-sized volumes on sample surface. It fundamentally shares the same principle as SEM but with advanced capability for quantitative chemical analysis by wavelength-dispersive spectroscopy (WDS).
In this work, JXA-8500F was used for the high-resolution elemental mapping and the precipitate composition was measured through the WDS. The acceleration voltage was set as 15 kV with a probe current 9.9 × 10−9 A. The advantage of WDS in EPMA compared to EDS is the more accurate chemical composition measurement and also the possibility to analyze more fine phases, i.e. about one micron and larger.
72
3.2.4 Secondary Ion Mass Spectrometry (SIMS)
SIMS is a surface destructive technique to analyze the composition of solid materials by sputtering primary ions over the sample surface. The ejected secondary ions are further collected and measured by mass spectrometer which provides in-depth distribution analysis of trace elements with a detection limit down to ppb level.
In this work, 2D elemental distributions were measured by SIMS using CAMECA IMS 7f microanalyzer. The measurements were performed in the imaging mode using 15 keV Cs+ ions as the primary beam for Si-Mg sample and 10 keV O2+ for Si-Sn sample. The primary beam rastered over an area of 500 × 500 μm2. A typical scanning result is presented in Figure 3-6.
Figure 3- 6. Typical example of SIMS elemental mapping results shown by a studied Si-Ca-Mg alloy (sample CM1-Casted in paper 4) with presented signals of Ca+Si, Si-Ca-Mg+Si, and P.
3.2.5 Electron Backscatter Diffraction (EBSD)
EBSD is a microstructural-crystallographic characterization technique used to perform quantitative microstructural analysis, grain size and orientation analysis. In this work, the crystallographic properties were also measured using NORDIF system. The EBSD scan was conducted in a Quanta 650 scanning electron microscope (SEM, ThermoFisher Scientific Inc.) operated at an accelerating voltage of 20 kV, an aperture of 100 μm and a spot size of4.0. The working distance was about 20 mm for the 70° pre-tilt specimen, and the dynamic focus in the SEM was adopted to improve the focal distance over a relatively large probe area of about 1.2
× 1.2 mm2. A typical EBSD mapping results can be seen in Figure 3-7.
P Mg+Si
Ca+Si
73 Figure 3- 7. Typical EBSD crystallographic mapping results shown by studied Si-Ca-Mg
alloys (a) sample CM1 in paper 4, (b) sample CM1-Casted in paper 4, where the colored regions indicate Si phase and the white regions are precipitates. The color-coded according to
the legend in the bottom-right corner. The lines presented indicate grain boundaries of Si.
3.2.6 Particle size distribution test
The particle size distribution of studied Si materials was measured by laser scattering particle size distribution analyzer Horiba Partica LA-960. The measured particle size ranges between 10 nm to 5 mm. Si particles were dispersed in deionized water and their sizes measured by the laser scattering technique. The refractive index value of pure Si is adopted for analyzing Si-rich alloy samples as an approximation since the real value should be reasonably close to the adopted value. The particle size measured by laser scattering is usually larger than the sieving particle size, as it is the diameter of a sphere with the particle’s equivalent volume and the sieving particle size is the minimum diameter passing through the sieve aperture.
3.3 Computational methods