Si Sr Research Articles

On the formation of Al2Si2Sr phase and its behavior in Al–Si–Sr alloys

Sr element is commonly used to modify eutectic Si in Al–Si alloys, while Al2Si2Sr phase is quite easy to precipitate when Sr content is high to tens of ppm level. In this study, the growth models of Al2Si2Sr crystal as well as its behavior in Al–xSi–2 Sr (x=5, 8, 12, 18, 25 and 40, wt%) alloys were investigated. Polyhedral, triangular pyramid and dendritic morphologies of Al2Si2Sr crystals were detected, and their growth models were discussed. Based on the crystallographic parameters, the disregistry between (0001)Al2Si2Sr and (111)Si is calculated to be 9.7%. Transmission electron microscopy analysis confirms that the Al2Si2Sr phase is not likely to be a good nucleation site for Si. Differential scanning calorimetry analysis indicates that Al2Si2Sr phase is firstly precipitated in Al–5, 8, 12, 18Si–2 Sr alloys, while its precipitating temperature is close to primary Si in Al–25Si–2 Sr alloy and it falls behind to primary Si in Al–40Si–2 Sr alloy. Interestingly, amounts of Al2Si2Sr dendrites were engulfed by primary Si in Al–40Si–2 Sr alloy. Based on the widely–known impurity–induced twinning mechanism in Sr modified Al–Si alloys, the Al2Si2Sr dendrites were deduced to be in–situ evolved from Sr–rich clusters, which are absorbed on {111}Si planes. This work may provide a new perspective for understanding the modification mechanisms of Sr element on Si crystal in Al–Si–Sr alloys.

In situ SEM characterization of tensile behavior of nano-fibrous Al–Si and Al–Si–Sr eutectics

In situ SEM characterization of tensile behavior of nano-fibrous Al–Si and Al–Si–Sr eutectics

Pressure dependence of mechanical and thermodynamic properties of Al2Si4Sr3: A first-principles study

The influence of pressure (0–50 GPa) on mechanical properties and thermodynamic behaviors of Al2Si4Sr3 compound has been investigated by performing first-principles calculations. It is found that the formation energy of Al2Si4Sr3 compound is −25.64 kJ/mol at 0 GPa. The results show that Al2Si4Sr3 is dynamically and thermodynamically stable under pressure. Moreover, the calculated elastic constants showed that Al2Si4Sr3 possesses the maximum bulk modulus of 187.35 GPa and the minimum microhardness of 1.588 GPa at 40 GPa. In addition, it is shown that the Debye temperature and minimum thermal conductivity are increasing to 651.63 K and 0.086 Wm−1K−1 at 45 GPa, respectively. The analysis of electronic structures revealed that the Si–Sr and Al–Sr in (010) plane can form the ionic and covalent bonds, respectively, whereas Al–Sr ionic bonds turn to be anti-bonding states under pressure.

Alternating Current Electrophoretic Deposition of Chitosan-Gelatin-Bioactive Glass on Mg-Si-Sr Alloy for Corrosion Protection.

Magnesium alloys have gained significant attention as degradable implant materials, but the fast and localized corrosion behavior leading to hydrogen gas evolution and alkaline poisoning limits their clinical application. In this research, the possibility of controlling the fast degradation rate of an experimental Mg-Si-Sr alloy by applying hybrid biopolymer chitosan (CS)-gelatin (G)-bioactive glass (BG) coatings was investigated. Electrophoretic deposition using alternating current fields (AC-EPD) was employed for surface coating and the influence of suspension parameters (biopolymer type and concentration, BG particle size), and key AC-EPD parameters (voltage amplitude, frequency, and time) on the coating quality were investigated. Stable suspensions of positively charged biopolymer/BG particles deposited on the Mg alloy coupled as a cathode during the high-amplitude peak. Furthermore, coating homogeneity improved with increasing peak-to-peak-voltage and the hybrid nature of the coatings was confirmed by scanning electron microscopy and Fourier transform infrared spectroscopy. Corrosion studies revealed a significantly decreased corrosion rate down to 0.08 mm/year for the Mg-Si-Sr alloy incorporating CS-G-BG b AC-EPD coating.

Thermodynamic descriptions of ternary Al–Si–Sr system supported by key experiments

Abstracts All the experimental phase equilibria and thermodynamic properties in the Si–Sr and Al–Si–Sr systems were critically reviewed. Based on the reviewed experimental data, the thermodynamic descriptions of the Si–Sr binary system were updated by using the CALculation of PHAse Diagram (CALPHAD) technique, resulting in better agreement with the experimental data, especially for the standard enthalpies of formation, than those in the previous assessment. After that, the thermodynamic assessment of the Al–Si–Sr ternary system was carried out using the CALPHAD approach. Comprehensive comparisons indicate a good agreement between the calculated phase equilibria/thermodynamic properties and the experimental/first-principles data in Al–Si–Sr ternary system. Moreover, the as-cast microstructures of 4 Sr-additional Al–Si alloys were experimentally investigated by using the scanning electron microscope and electron probe microanalysis techniques. The solidification sequence and volume fractions of precipitated phases in the 4 alloys predicated from Scheil-Gulliver solidification simulations agree reasonably with the presently measured data, which further validates the reliability of the present thermodynamic descriptions.

Experimental Study of Refining and Modification of Steel with Si–Ca, Si–Sr, and Si–Ba Alloys

The results of thermodynamic calculations and experimental studies of the deoxidation, desulfurization, and modification of low-carbon steel with aluminum and Si–Ca, Si–Sr, and Si–Ba alloys are presented. The metal treatment with silicocalcium leads to the formation of corrosive non-metallic inclusions (CANI). The replacement of calcium-containing alloys with Ba and Sr alloys helps to clean steel from CANI, the formation of smaller complex oxysulfide non-metallic inclusions and a change in the grain size of metal. The calculated data are consistent with the experimental results.

Geochemical study of Cretaceous magmatic rocks in Chuzhou region, low Yangtze River metallogenic belt: implications for petrogenesis and Cu–Au mineralization

ABSTRACTThe low Yangtze River metallogenic belt (LYRMB) is one of the most important poly-metal deposit belts in China. The Chuxian, Machang and Shangyaopu intrusions in the LYRMB are intermediate rock series, mainly composed of monzonite and quartz monzonite. In this study, bulk rock major and trace elements, zircon U–Pb dating and Hf isotope were analysed. Five ages have been obtained as (1) Chuxian, 121.8 ± 1.9 and 124.0 ± 1.4 Ma, respectively, (2) Machang intrusion, 123.1 ± 2.0 Ma and (3) Shangyaopu, 126.6 ± 1.8 and 123.4 ± 1.9 Ma, indicating that the regional igneous activity was in Early Cretaceous, being consistent with the massive Yanshanian magmatic events in eastern China. These three intrusions are identified as a high-Mg adakite, most of them showing geochemical features of high Si, high Na and low Sr, which can be interpreted as partial melting of subducted oceanic crust. High Mg# characteristics indicate the magmas reacted with the mantle. The negative zircon εHf values of these adakites suggest that the magmas have assimilation of old crustal material, e.g. Archaean continental crust, the basement of the south Tancheng–Lujiang (Tan–Lu) fault. Biotite Ti temperature result (about 700°C) shows that intrusive magma has a relatively low temperature. Petrogenesis and regional Cu–Au mineralization mechanism may be explained by Pacific plate subduction during about 125–180 million years subducted to southwest towards the LYRMB. Magmas formed by partial melting of subducted oceanic crust have systemically high Cu–Au contents, which are conducive to corresponding mineralization.

Expanding the Repertoire of Molecular Linkages to Silicon: Si-S, Si-Se, and Si-Te Bonds.

Silicon is the foundation of the electronics industry and is now the basis for a myriad of new hybrid electronics applications, including sensing, silicon nanoparticle-based imaging and light emission, photonics, and applications in solar fuels, among others. From interfacing of biological materials to molecular electronics, the nature of the chemical bond plays important roles in electrical transport and can have profound effects on the electronics of the underlying silicon itself, affecting its work function, among other things. This work describes the chemistry to produce ≡Si-E bonds (E = S, Se, and Te) through very fast microwave heating (10-15 s) and direct thermal heating (hot plate, 2 min) through the reaction of hydrogen-terminated silicon surfaces with dialkyl or diaryl dichalcogenides. The chemistry produces surface-bound ≡Si-SR, ≡Si-SeR, and ≡Si-TeR groups. Although the interfacing of molecules through ≡Si-SR and ≡Si-SeR bonds is known, to the best of our knowledge, the heavier chalcogenide variant, ≡Si-TeR, has not been described previously. The identity of the surface groups was determined by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and depth profiling with time-of-flight-secondary ionization mass spectrometry (ToF-SIMS). Possible mechanisms are outlined, and the most likely, based upon parallels with well-established molecular literature, involve surface silyl radicals or dangling bonds that react with either the alkyl or aryl dichalcogenide directly, REER, or its homolysis product, the alkyl or aryl chalcogenyl radical, RE· (where E = S, Se, and Te).

Identification and description of intermetallic compounds in Mg–Si–Sr cast and heat-treated alloys

The ternary Mg–Si–Sr system is experimentally investigated in this work. Twelve compositions located in the Mg rich corner of the ternary system are processed. The microstructure and phase content of the different alloys are analysed in the as-cast condition and after additional heat treatments.Apart from the Mg matrix, various intermetallic phases are observed. The binary compounds Mg2Si and Mg17Sr2 are identified, as well as, two ternary phases MgSiSr and MgSi2Sr.The morphology and distribution of the intermetallic phases is discussed with particular interest on the ternary intermetallics MgSiSr and MgSi2Sr for which only scarce information is available in literature. The phase stability and the stability of the intermetallic phases in these alloys are discussed.The Mg-rich side of the Mg–Si–Sr phase diagram is constructed based on descriptions of the binary phase diagrams Mg–Si, Mg–Ca and Ca–Si from literature and assuming complete solubility (i.e. a line compound) between the ternary phase MgSiSr and the binary phase Sr2Si. It is also assumed that MgSi2Sr is a stoichiometric compound. These assumptions were made based on the chemical similarities between Ca and Sr and between Si and Sn, and the similarity in crystal structures between the two studied ternary compounds of the Mg–Si–Sr diagram and the ternary compounds present in the Ca–Mg–Si and Mg–Sn–Sr system. A good agreement is found between the constructed phase diagram and the experimental results.

Impact of P and Sr on solidification sequence and morphology of hypoeutectic Al–Si alloys: Combined thermodynamic computation and phase-field simulation

Even small amounts of Phosphorus and Strontium strongly affect the microstructure of hypoeutectic Al–Si alloys. P is an unavoidable trace element in commercial Al-alloys which causes formation of AlP particles as potent nucleation sites for eutectic (Si). Sr, in contrast, is purposely added to modify the morphology of eutectic (Si) towards fine coral-like fibers. It is hypothesized that Sr does not only alter the growth kinetics of (Si), but additionally prevents detrimental (Si) nucleation due to neutralization of AlP particles by Al2Si2Sr formation. This presumes that both AlP and Al2Si2Sr precipitate prior to (Si). Using a newly assessed thermodynamic database for the Al–Si–Sr–P system, critical P and Sr thresholds for pre-silicon formation of AlP and Al2Si2Sr were evaluated and mapped under equilibrium and Scheil conditions. The competitive precipitation of AlP, Al2Si2Sr and (Si) and its impact on the evolution of the eutectic morphology was further studied by 3D phase-field simulations. Effective anisotropy functions for the (Si) interface mobility considered Sr-induced internal twinning. Depending on whether subcritical or supercritical P and Sr contents were selected, either a fine lamellar structure, a coarse flaky structure, or the targeted fine fibrous eutectic structure was reproduced.

Borane-Catalyzed Si–H Activation Routes to Polysilanes Containing Thiolato Side Chains

Dehydrocoupling and hydrosilation reactions of the Si–H bonds in poly(phenylsilane) catalyzed by B(C6F5)3 allow the preparation of new polymers containing both Si–H and Si–SR side chains. This postpolymerization modification takes place without any observable competing Si–Si bond cleavage, unlike other Lewis acid, transition-metal, or radical mediated routes. The −SR-functionalized polymers have been characterized by GPC, IR, UV–vis, elemental analysis, and 1H, 13C, and 29Si NMR.

First-principles investigation of the Al–Si–Sr ternary system: Ground state determination and mechanical properties

Twelve different ternary phases of the Al–Si–Sr system were characterized using first-principles calculations based on Density Functional Theory. Two different exchange correlation functionals were considered to extract its influence on the global results. In addition to the four phases (AlSiSr, Al 2Si 2Sr, Al 2Si 2Sr 3 and Al 16Si 30Sr 8) reported for this system, we explored eight other ternary phases (Al 2Si 3Sr 3, Al 2Si 4Sr 3, Al 2Si 7Sr 5, Al 3Si 7Sr 10, Al 6Si 3Sr 20, Al 6Si 9Sr 10, Al 8Si 3Sr 14 and AlSi 6Sr 4) that has been reported in chemically analogous systems. The mechanical stability of the phases was determined through the analysis of the elastic constants, and the energetic stability was established using the calculated formation enthalpy. The thermodynamic and vibrational properties of the AlSiSr, Al 2Si 2Sr 3 and Al 2Si 4Sr 3 phases were studied considering the harmonic, quasi-harmonic as well as simple anharmonic corrections. All phases were found to be stable, except for the AlSi 6Sr 4 and Al 6Si 9Sr 10. An isothermal section of the phase diagram was calculated at 0 K with the Thermocalc software, where it was found that the Al 2Si 4Sr 3 and Al 6Si 3Sr 20 phases can belong to the ternary ground state. The surface energy sections show that the other phases are metastable with a ΔG 0 K ≈ 2 kJ/mol−atom, all stables and metastable phases show a metallic behavior and high compressibility.

A thermodynamic reassessment of the Si–Sr system

The Si–Sr binary system has been thermodynamically reassessed in the present work based on the critically reviewed experimental data available in the literature, especially newly published experimental phase diagram data in the Si-rich side. The liquid phase has been modeled with both the substitutional solution model and the associate model, and two sets of self-consistent thermodynamic parameters that describe the system are thus obtained. The shortcomings of the previous assessment are removed, and a better agreement with experimental data is achieved compared with the previous assessment.

Thermodynamic modeling of the Al–Si–Sr–O quaternary system

A thermodynamic description for the Al–Si–Sr–O quaternary system was developed in this study. Unlike previous publications which mostly focused on one particular pseudo-binary or pseudo-ternary section, the thermodynamic description developed in this study can be applied to the entire compositional range. For the six constituent binary systems, Al–O and Si–O were re-evaluated, while thermodynamic descriptions for the other four binaries were adopted from the published assessments. Thermodynamic descriptions for the Al–Si–O and Al–Sr–O ternaries were then obtained on the basis of the reevaluated Al–O and Si–O binaries. In addition, the Si–Sr–O ternary system was, for the first time, assessed in this study based on the literature experimental data. In the present study, associate model was used to describe the liquid phase and a four-sublattice compound energy model was used to describe the mullite (3Al 2O 3·2SiO 2) phase. The developed thermodynamic description was validated by the experimental data available in the literature for this quaternary system.

The Si–Sr and Si–Ba phase diagrams over the Si-rich composition range

For the first time, Si–Sr and Si–Ba phase diagrams have been investigated over the Si-rich composition range. The solid solubilities of Sr and Ba impurity getters in silicon were measured in the temperature range from 1173 to 1573 K by analytical methods, including microhardness and instrumental neutron activation analysis. The liquidus curves for the primary crystallization of silicon were determined by differential thermal and activation analyses. The alloys were prepared from pure components using induction heating with subsequent quenching. The samples were subjected to isochronous homogenization annealing at different temperatures for 100 h. The elements Sr and Ba exhibit retrograde solubility in silicon. The maximum equilibrium solubility of Sr in Si is 1.15 at.% at 1573 K, while the Ba solubility limit is 0.98 at the same temperature. The thermodynamic description of the experimental liquidus curves for the primary crystallization of Si indicates that the Si–Sr and Si–Ba liquid alloys do not inherently have a regular nature. We expect that the clathrate structure can exist in the Si-rich melts of the Si–Sr and Si–Ba systems.

Thermodynamic modeling of the Si–Sr system

Thermodynamic modeling of the Si–Sr system was carried out using the CALPHAD method. In this system, three of the intermetallic compounds, Sr 2Si,Sr 5Si 3,SrSi, were considered to be stoichiometric and SrSi 2 was modeled as a non-stoichiometric phase. For the liquid phase, two different models were used. In one case, Sr and Si were assumed to constitute a random solution, with its excess Gibbs energy being described through the Redlich–Kister polynomial. In the second case, the liquid phase was described through the associate model, considering the species Si, Sr and SrSi. The terminal solid solutions, ( β Sr ), ( α Sr ) and (Si), are treated with negligible solubilities. A set of self-consistent thermodynamic parameters is obtained by considering experimental phase equilibria, measurements of the mixing enthalpy in the liquid phase, as well as measured and calculated–via first-principles methods based on density functional theory–formation enthalpies of the intermetallic compounds.

Sr effect on the microstructure and tensile properties of A357 aluminum alloy and Al 2O 3/SiC-A357 cast composites

The effect of strontium as a modifier on the microstructures and tensile properties of two castable particulate metal matrix composites has been studied. The particulate metal matrix composites had similar matrix alloy (A357) but different reinforcing fine particles (silicon carbide and alumina). Results showed that the addition of 0.03% strontium makes a modest improvement to the yield strength, ultimate tensile strength and elongation percentage values, and the scatter of these properties, but makes a significant improvement to minimum strength and elongation results. Microstructural examinations by scanning electron microscope and energy dispersive spectroscopy analysis of metal matrix composites showed segregation of strontium on both the silicon carbide and alumina particles. Further results showed that the addition of higher strontium levels contributes to the over-modification of the eutectic silicon and promotes the formation of an Al–Si–Sr intermetallic compound on the particle/matrix interface.

Structure and chemical properties of mononuclear and dinuclear silylrhodium complexes. Activation of the Si–C bond and formation of Si–Cl and Si–SR bonds promoted by Rh complexes

Recent studies of the structures and chemical properties of organosilylrhodium complexes are surveyed. Oxidative addition of secondary and tertiary silanes to chlororhodium(I) complexes with PMe 3 and with P( i-Pr) 3 ligands leads to the formation of chloro(hydrido)silylrhodium(III) complexes. The resulting Rh(III) complexes are often in equilibrium with Rh(I) complexes via a reversible reductive elimination of the organosilane and its reoxidative addition in solution. Irreversible coupling of chloro and silyl ligands of the complexes with an octahedral or a square-pyramidal Rh(III) center occurs at room temperature when the two ligands are at cis coordination sites. Complexes with chloro and organosilyl ligands at trans positions are thermally stable and release chlorosilanes upon heating above 80°C. Hydridorhodium(I) complexes formed in the above reaction mixture readily undergo oxidative addition of organosilane to give dihydrido(silyl)rhodium(III) complexes. Dinuclear silylrhodium(III) complexes, L(H)(Ar 3Si)Rh(μ-H)(μ-Cl)Rh(SiAr 3)(H)L (Ar=Ph, C 6H 4F; L=P( i-Pr) 3), prepared from the reaction of triarylsilane with RhCIl 2, cause thermally induced Si–C bond cleavage of a triarylsilyl ligand to afford L(H)Rh(μ-Cl)(μ-SiAr 3)(μ-SiAr 2)Rh(H)L, accompanied by the evolution of arene. The produced complexes contain a bridging triarylsilyl ligand coordinated symmetrically to the two Rh centers. RhH(SAr)(SiHAr′ 2)(PMe) 3-type complexes with thiolato and diarylsilyl ligands at trans positions undergo clean thermal rearrangement to give complexes with the Si(SAr)Ar′ 2 ligand. The Rh complex that promoted the Si–S bond forming reaction is extended to the synthesis of a new disilarhodacyclopentane and to the RhCl(PPh 3) 3-catalyzed polycondensation of diarylsilane with dithiols to form new Si-containing polymers.

Engulfment of Al 2O 3 particles during solidification of aluminum matrix composites

Through the analysis of forces acting on the particles at the solid/liquid interface based on the interfacial energies, we propose a new theoretical particle transfer model. This model can predict the behavior of the particles at the interface. The main concept of this model is proved by the mutual wetting behavior among the solid, liquid and particle phases. If the contact angle at a solid/liquid interface and a particle is <90°, the particle can be engulfed into the solid, and if the contact angle is >90°, the particle would be pushed. This contact angle is measured by unidirectional solidification (UDS) experiments. But if solidification experiments were carried out for particle dispersed composites in a conventional UDS, the particles completely descended before freezing, therefore, a zone-UDS was used in the experiment. In Al–Sr, Al–Ca and Al–Ce matrix composites, Al 2O 3 particles are pushed by the solid because the contact angles are >90°. On the other hand, in Al–12.6wt.%Si–Sr–Al 2O 3 composites alloyed with Ca, the Al 2O 3 particles are engulfed into the solid because the contact angle is <90°.

Effects of silicon, magnesium and strontium content on the qualities of Al-Si-Mg alloys

According to the specification of AA standard, the magnesium content of 356.1 alloy ranges from 0.25 to 0.45%. In producing Al-Si-Mg alloy the strontium content for the modified 356 (Al-7%Mg) alloy might range from 0.005 to 0.02%. Therefore, 356 alloys might be produced with different percentages of Mg and/or Sr. The effects of changes in Si content (from 6.6% to 10.8% in gravity casting and 6.5% to 10.3% in squeeze casting), Mg content (from 0.36% to 0.48% in gravity casting and 0.3% to 0.44% in squeeze casting), and Sr content (from 0.0007% to 0.0158% in gravity casting and 0.0015% to 0.04% in squeeze casting) on the microstructures, density, mechanical properties and strength of different Al-Si-Mg alloys were fully investigated and discussed. Different melts were poured in the ASTM B108 permanent mould and dies in a vertical squeeze machine to produce bar castings. These bar castings were then machined and the mechanical properties tested. Experimental results showed that if a high strength and a high elongation were desired in a squeeze casting after T6 treatment, an increase in Si and Sr content to 9.9% and 0.019% (Mg at 0.3%) would lead to 280 MPa strength, 12% elongation and Weibull modulus of 34 in reliability of strength. If both strength and reliability were desired in a squeeze casting after T6 treatment, an increase in Si content to 10.3% (Sr at 0.0015% and Mg at 0.3%) would result in 294 MPa strength, 8.7% elongation and a great Weibull modulus of 67. In gravity casting, after T6 treatment, increasing Si content to 10.4–10.55% and Sr content in the range of 0.0007–0.0208% and 0.35% Mg could develop 282–284 MPa strength at about 6.6% elongation with Weibull modulus of around 31. In Al-7Si-Mg alloy, increasing Mg content from 0.26% to 0.48% decreased the tested elongations of both as-cast and heat treated squeeze castings and gravity castings.