Solubility Of Si Research Articles

Simultaneous Improvement of Thermal Conductivity and Strength for Commercial A356 Alloy Using Strontium Modification Process

The Aluminum–Silicon (Al–Si) die-casting alloys, such as the commercial A356 alloy, are expected to be used in heat-sink and the device with high thermal conductivity due to their high production efficiency of casting process. These fields also required them to possess sound mechanical properties. To meet these demands, the Strontium (Sr) was often utilized to modify the silicon phase. According to our current work, the secondary dendrite arm spacing of the α-Al grains prominently was decreased when 0.05%–0.25% strontium was added. The decrease of the secondary dendrite arm spacing (SDAS) lead to the enhancement of the mechanical strength as well as the improvement of the thermal and electrical conductivity. When the Sr dosage was 0.15% in the commercial A356 alloy, the strength and the thermal conductivity of the A356 alloy simultaneously reached the maximum value. The improvement of the electrical and thermal conductivity might by contributed by the formation of a good conductor, Al2Si2Sr phase, on the Si surface. Further investigations suggested that the improvement of thermal conductivity was mainly due to the modification effect of Sr on the eutectic Si phase, which enlarged the specific area between the α-Al/eutectic Si interface. The WDS analysis indicated that the solubility of Si decreased in aluminum matrix by increasing in the Sr dosage. However, the excessive dosage of strontium would result in the coarsening of the modified silicon phase, deteriorating the strength and the thermal and electrical conductivity of the modified A356 alloys. In this work, the commercial A356 alloy was modified by adding small amounts of strontium (Sr). This kind of aluminum alloy with sound fluidity could be formed by using casting process, and it was potential to be applied in the structural parts. After the Sr modification, the plate-like silicon (Si) phase was transformed to fibrous shape in the raw commercial A356. Meanwhile, it was found that the content of silicon in the Al solid solution was reduced.The XRD and SEM analysis suggested that the Al2Si2Sr phase was formed on the modified silicon phase, which was favorable to the conduction of free electron.

Unusual Iron Nitride Formation Upon Nitriding Fe-Si Alloy

The influence of Si, substitutionally dissolved in ferritic Fe-2 at. pct Si and Fe-4.5 at. pct Si alloys, on the nucleation and growth of γ′ iron nitride upon controlled gaseous nitriding was investigated. The nitrided specimens were characterized by XRD, light microscopy, TEM, EELS, EPMA and EBSD. The combination of difficult and thus delayed precipitation of (1) silicon nitride, because of a large misfit with the ferrite matrix, and (2) γ′ iron nitride, because of the negligible solubility of Si, was shown to lead to a series of unusual, nonequilibrium phenomena: high nitrogen supersaturation, development of Si-containing metastable γ′ phase of peculiar morphology, precipitation of amorphous silicon nitride in γ′ and development of ε nitride at thermodynamic conditions unlikely to allow its formation.

Preparation of Ni–Si–B nano-crystalline film and mechanism analysis using first-principle calculations

In this work, Ni–Si–B nano-crystalline film was prepared successfully based on supersaturated solid solution method using magnetron sputtering technology. The phase structure of Ni–Si–B nano-crystalline film is composed of Ni (111) phase completely, and the average diameter of Ni grain is 4.77 nm. The high-multiple morphology of the film shows the uniform paste-like structure, which is constituted by a little of pure nickel (Ni) and much of nickel-based solid solution (NiSS). The solid solution of elements Si and B in host Ni can enhance the nano-crystalline degree of the film effectively, and the solid solubility of Si is much larger than that of B, which are c (Si) = 14.6 at.% and c(B) = 0.00124 at.%, respectively. The solid solubility of substitutional solute element Si and interstitial solute element B are both influenced by the bonding contribution and structural contribution simultaneously. For substitutional solute element, the structural contribution and bonding contribution are negative for improving the solid solubility, which are harmful to the formation of nano-crystalline. While for interstitial solute element, although the influence of structural contribution is negative for improving the solid solubility, the bonding contribution plays the positive role, which is conductive to the formation of nano-crystalline.

Microstructural Characterization and Mechanical Properties of Rapidly Solidified Al-Si Systems by Chill Block Melt-Spinning Technique.

AlـSi alloys with compositions (0, 0.1, 0.5, 0.9 and 1.3 wt.% Si) were manufactured by chill block melt spinning method. The resulting ribbons samples have been characterized by xـray diffraction (XRD) and scanning electron microscope (SEM). Detailed analysis of (XRD) shows that presence of f.c.c Al solid solution and Si particles embedded within the aluminum grains. Microstructural examination resulted that microstructure of the melt spun ribbons are more fine and uniformly distributed. Rapid solidification technology led to increase the solubility of Si in αـAl as confirmed by XRD. Micro hardness measurements were also carried out by Vickers microـhardness tester at applied load 25gm forced and different dwell time. It is concluded that the Vickers hardness values are sensitive to applied load and indentation time. It is also found that the highest values of Hv is sensitive to presence of Si as columnar shape with fine grained of Al by high cooling rate.

Microcharacterization and mechanical performance of an Al-50Si alloy prepared using the sub-rapid solidification technique

In this reported work, the hypereutectic Al-50Si alloys with a wide mushy zone of 465 °C were successfully fabricated using twin roll casting technique. The experimental results showed that the overall hardness of the roll-cast alloy increased, as a result of the decrease in the size of Si crystal and the increase in the relative solubility of Si in the α-Al. However, the EBSD results showed that the primary Si crystals in the twin-roll cast alloy grew according to twin plane re-entrant edge (TPRE) mechanism.

Effects of CO2–water interaction with coal on mineral content and pore characteristics

Abstract There are a large number of abandoned coalmines in China, and most of them are located around major coal-fired power stations, which are the largest emission sources of carbon dioxide (CO2). Considering the injection of CO2 into abandoned coalmines, which are usually in the flooded condition, it is necessary to investigate the effect of CO2–water–coal interaction on minerals and pore structures at different pressures, temperatures and times. It reveals that the CO2–water–coal interaction can significantly improve the solubility of Ca, S, Mg, K, Si, Al, Fe and Na. By comparing the mineral content and pore structure before and after CO2–water–coal interaction, quartz and kaolinite were found to be the main secondary minerals, which increased in all samples. The structures of micropores and mesopores in the range of 1.5–8 nm were changed obviously. Specific surface areas and pore volumes first increased and then decreased with pressure and time, while both increased with temperature. By using the Frenkel–Halsey–Hill model, the fractal dimensions of all samples were analyzed based on Ds1 and Ds2, which reflected the complexities of the pore surface and pore volume, respectively. The results show that fractal dimensions had very weak positive correlations with the carbon content. Ds1 had a positive correlation with the quartz and kaolinite contents, while Ds2 had a negative correlation with the quartz and kaolinite contents.

The Isothermal Section of the Al-Si-Ti Ternary System at 550 °C

The isothermal section of the Al-Si-Ti ternary system at 550 °C has been determined using scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. Results show that there are 12 three-phase equilibrium regions in the isothermal section of the system. The maximum solubility of Si in Al3Ti is 16.4%, and the solubility of Al and Ti in Si is small. There exists only one ternary compound in the isothermal section with a composition of Al7.1-13.9Ti33.5-34.2Si51.9-59.1.

Determination of heat treatment parameters by experiments and CALPHAD for precipitate hardening of Cr-Alloys with Si, Ge and Mo

Binary Cr–Si alloys as well as ternary Cr–Si-Ge and Cr–Si–Mo-systems on the Cr rich side of the phase diagram are examined at 1050 °C and 1350 °C regarding the maximum solubility in the bcc solid solution matrix and the A15-phase volume fraction. SEM, EDS and image analysis measurements showed, that the addition of 2 at.% Ge has the same effect on the A15-phase formation as Si but leads to a coarser microstructure. The addition of 2 at.% Mo leads to a decreased A15-phase volume fraction but a small-sized precipitate distribution. The maximum solubility of Si in the binary Cr–Si system is still not clear and has to be verified in further examinations. Thermo-Calc calculations showed, that a reduction of Si content and the addition of Ge and Mo is beneficial for an increase in the process window for solution heat treatment and a decrease of the solvus temperature.

Microstructure, microhardness and oxidation behavior of Mo-Si-B alloys in the Moss+Mo2B+Mo5SiB2 three phase region

A series of Mo-Si-B alloys in the Moss+T2 (Mo5SiB2)+Mo2B three phase region were designed to examine the effect of the lower Si solubility limit in the Moss phase on the microstructure, hardness and oxidation behavior. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Vickers hardness tests and Electron Probe Microanalysis (EPMA) were carried out for the evaluation. The XRD results show an unusual lattice constant expansion for the Moss that is consistent with B as an interstitial solute. SEM images establish the microstructure evolution during the annealing. The analysis of the indentations produced by the Vickers hardness tests demonstrate that the samples in the Moss + Mo2B + T2 three phase region have about a 50% higher fracture toughness than comparable samples in Moss + Mo3Si + T2 three phase region due to the lower Si solubility in the Moss phase. The thermogravimetric analysis (TGA) results show that the oxidation resistance is directly related to T2 phase fraction in the microstructure. Alloys with a Moss + Mo2B + T2 microstructure exhibit a similar oxidation behavior to those with a Moss + Mo3Si + T2 microstructure when the T2 phase fraction in the two alloys is similar. Mo-Si-B alloys in the Moss + Mo2B + T2 three phase region could achieve better mechanical properties than alloys in the Moss + Mo3Si + T2 three phase region and are attractive candidates for high temperature applications.

Processing and characterization of MoAl1-xSixB solid solutions

The MAB phase MoAlB has attractive properties both at room and high temperatures. To further improve its properties, a possible way is to form solid solutions by partially substituting one of constituting elements in MoAlB. This work reports on the synthesis of MoAl1-xSixB solid solutions by powder metallurgy. Influences of sintering temperature, dwelling time, and composition of starting mixture on the solubility limit of Si in MoAlB have been investigated. The maximum solubility of Si was 0.91 at.%, corresponding to x = 0.03. Increase of Si content in the starting mixtures caused the formation of other competing phases. Phase composition and microstructure were characterized by x-ray diffraction method and scanning electron microscopy. A dense MoAl0·97Si0·03B solid solution has been achieved by hot-pressing a MoB/1.3Al/0.1Si mixture at 1200 °C with 25 MPa for 1 h, and its mechanical properties have been measured. An increase in strength and hardness is achieved by solid-solution strengthening. The flexural strength and the hardness of the solid solution are higher than those of MoAlB, viz. 385 MPa vs 307 MPa, and 14.89 GPa vs 8.43 GPa.

Monitoring of High Selectivity Chemistry for Si3N4 Etch

Concentrated phosphoric acid at nearly boiling temperature (165°C) has been used for many years in Selective Silicon Nitride etch process. Traditionally, inorganic silicate is added to improve the selectivity of Si3N4 over SiO2. Recent semiconductor processes such as 3D Memory and FinFET logic require an enhancement in Si3N4:SiO2 selectivity in excess of 100. The low solubility of Si in H3PO4 limits this Si-addition approach for process that requires high Si3N4:SiO2 selectivity. In recent years, H3PO4 etchant containing organosilicate has been developed to improve the solubility of Si, enabling both high selectivity and desirable etch rate. Another important function of organosilicate is to prevent etched SiO2 from re-deposition to the wafer surface. This high selectivity process using organosilicate presents new metrology challenges. A reagent electrochemical method was developed to monitor Si in hot phosphoric process tanks. The same method with improved reagent was also successfully applied for monitoring organosilicate. The accuracy of this is evaluated by off-line ICP-MS method, which is shown in Figure 1. Samples were obtained from an actual process tank. The results from this improved fluoride method match those from ICP-MS. Good stability of organosilicate results by the same method is shown in Figure 2. This test was performed on samples with a known prepared concentration for a period of one week. All measured results have an accuracy of less than 2% (solid lines on Figure 2). A recent study with the combination of the two methods above demonstrates the capability of measuring two different Si forms. In this study, two separate measurements using two different reagents were done first. Two equations were derived from signals/results obtained from these two tests, to calculate both organo- and inorganic Si. Figure 3 shows these calculated organo- and inorganic Si results in comparison with expected values. In addition, ICP provides an alternative metrology option to measure both silicon and carbon elements. The organo-Si portion can be derived from carbon concentration assuming no other source of carbon in the matrix. However, organic solvents are commonly used in preparation of H3PO4 etchant containing organosilicate, and evaporation at high process temperature would break the correlation between organosilicate and total carbon. Figure 4 compares the ICP results before and after the heat treatment, and a dramatic decrease in the total carbon concentrations is observed, while the same trend was not seen in total Si concentrations. Slight increase in Si concentrations is due to the loss of solvent during heating. Therefore, ICP measurement of organosilicate targeting carbon element needs to be scrutinized case by case to avoid possible solvent interference. Reference: [1] S.J. Baffat, M.S. Lucey, M.R. Yalamanchilli Hot Phosphoric Acid APC for Silicon Nitride Etch. Semiconductor International, 8/1/2002 [2] Hong et al. Compositions for etching and methods of forming a semiconductor device using the same US Patent Application 20130092872 [3] Cho et al. Etching composition and method for fabricating semiconductor device using the same US Patent Application 20130157427 [4] Shalyt et al. Analysis of silicon concentration in phosphoric acid etchant solutions US Patent 8,008,087 [5] Shalyt et al. Analysis of silicon concentration in phosphoric acid etchant solutions United States Patent Application 20160018358 [6] Bai et al. Metrology for High Selective Silicon Nitride Etch Solid State Phenomena 255:81-85 Figure 1

The Zinc-Rich Corner of the Fe-Si-Sn-Zn Quaternary System at 450 °C

The 450 °C isothermal sections of the Fe-Si-Sn-Zn quaternary system with Zn composition fixed at 70 and 93 at.% were determined experimentally using optical microscopy, scanning electronic microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffractometry (XRD). Five four-phase regions were identified in the 70 at.% Zn section, whereas no four-phase region was found in the 93 at.% Zn section. The liquid phase was found to be in equilibrium with almost all phases in the two sections, including the FeSn, FeSi, FeSi2, Γ1, δ, ζ, and α-Fe phases. The solubility of Sn in the ζ, FeSi, and FeSi2 phases was rather limited; however, the maximum solubility of Si in the FeSn phase was 0.5 wt.%. No quaternary compound was found in the study.

Episyenites\u2014Characteristics, Genetic Constraints, and Mineral Potential

Episyenites are sub-solidus, quartz-depleted alkali-feldspar-rich rocks. They form veins and lenticular bodies in granitoid rocks and migmatites in a late- to post-orogenic or anorogenic setting. Leaching of quartz is usually a response to a flux of weakly saline hydrothermal solution in circulation cells above cooling intrusions, where sufficient fluid-rock ratios and thermal gradients are achieved. Fluid Si-undersaturation is achieved by rapid cooling within the field of retrograde Si solubility or by temperature and pressure increase outside retrograde conditions. Some quartz may also be consumed in metasomatic reactions and in response to pressure fluctuation in sealed episyenite bodies. The small size and overall rarity of episyenites imply that conditions for episyenite formation are not commonly encountered in the crust. In addition to quartz depletion, episyenites record complex histories of metasomatic alteration and hydrothermal mineral growth. Nearly all episyenites have undergone Na-metasomatism, which may have led to the formation of nearly monomineralic albitite, and which is occasionally followed by late K-metasomatism, phyllic alteration, and argillization. Depending on the effectiveness of later compaction, recrystallization and vug-filling episyenites may preserve the macroscopic porosity formed by quartz dissolution and brittle deformation. Vuggy episyenites can act as significant sinks for metals carried by crustal fluids and host many significant U, Sn, and Au deposits worldwide. Rare earth-critical syenitic fenites around alkaline intrusions share mineralogical and genetic traits with episyenites.

Performance of local fly ash geopolymers under different types of acids

A new class of ceramic materials, known as geopolymers, possess comparable or even higher mechanical and chemical properties than ordinary Portland cement-based materials. Because geopolymers can be used in a wide variety of areas, from civil engineering applications to decorative objects manufacturing, they can get in contact with different types of acidic corrosive substances. The degradation degree depends mostly on the acidic solution concentration and the exposure time. Due to the interaction between the geopolymer compounds, Al, Si and Fe ions solubility and the corrosive environment substance, there may follow severe consequences on the material mechanical properties. These consequences are highly related to the geopolymers three-dimensional structure degeopolymerization. The performance of geopolymers under severe environmental condition depends on the raw material, mineralogical composition and activation method. The aim of this paper is to present the effects of nitric acid, sulfuric acid and hydrochloric acid exposure on the local fly ash geopolymers compressive strength and microstructure.

Influence of the calcination temperature on the properties of a mica mineral as a suitability study for the use as SCM

Calcined clays as SCM have the potential of compensating the future decrease of slag and fly ash. The present work dealt with a mica mineral, a common component of raw clays, and its change of properties during calcination. Raw and at four temperature levels calcined mica were investigated on the following properties: Water absorption capacity, specific surface area, particle size distribution, lattice parameters by XRD, heat flow and Al-/Si solubility. The results showed a low ion solubility throughout all samples. Thus, the contribution to the chemical pozzolanic reaction due to calcination was low. However, mica's physical and hygroscopic properties influenced the cement hydration. Contrary to common knowledge, which predicts a decrease in water absorption capacity with a decrease in BET specific surface area, calcined mica behaved in the opposite direction. To conclude, for mica containing raw clays the calcination temperature should be lower in regard to mica's physical-hygroscopic properties, but higher in regard to pozzolanic reactivity of other phyllosilicates. Therefore, a balanced calcination temperature should be applied. These results can be used to assess their suitability as SCM in mortar and concrete.

Influences of Cu Content on the Microstructure and Strengthening Mechanisms of Al-Mg-Si-xCu Alloys

The effects of the Cu content on the microstructure and strengthening mechanisms of the Al-Mg-Si-xCu alloys were systematically investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and mechanical tensile tests. The results show that, the strengthening mechanisms change with the Cu content. For as-quenched alloys, solution strengthening (σSS) is predominant when the Cu content ≥2.5 wt.%, and of equivalent importance as grain size strengthening (σH-P) when the Cu content ≤1.0 wt.%. With respect to peak-aged alloys, precipitation strengthening (σppt) is predominant when the Cu content ≥2.5 wt.%, but σSS becomes predominant when the Cu content is 4.5 wt.%. As the Cu content increases from 0.5 to 4.5 wt.%, the main type of precipitates in alloy tends to change from a β″ phase to Q′ phase, and then to a θ′ phase. Among the three types of precipitates, θ′-precipitate causes the largest increase in yield strength (σ0.2) and the largest decrease rate in elongation. β″-precipitate leads to the smallest increase in σ0.2 and the smallest decrease rate in elongation. The increase of Cu content reduces Si solubility in the Al matrix and thus decreases the nucleation rate of β″ phase during subsequent aging.

Influence from Size and Morphology of Mn5Si3 on Wear Resistance of Cu-Zn-Al-Mn-Si Alloys

In this study, the microstructure, hardness, and wear resistance of two kinds of copper alloy are demonstrated to be affected by both composition, annealing temperature, and phases present with a sliding block-on-ring test, where the ring is 316 stainless steel. With a higher content of Zn, the solubility of Mn and Si would decrease, resulting in increased volume fraction of the hard ω-Mn5Si3 phase (with a high nanoindentation hardness of 12.9 GPa). Moreover, the higher contents of Mn and Si lead to the formation of large primary polyhedron Pω-Mn5Si3 cylinders during solidification. With the help of two-stage forging, the primary polyhedron Pω-Mn5Si3 grow along [0001], enabling the large primary Mn5Si3 cylinders all parallel to each other. This special microstructure could significantly improve the wear resistance in comparison with the samples with a microstructure containing only small and randomly oriented precipitates. Such parallel primary polyhedron Pω-Mn5Si3 cylinders can act as reinforcements to grab the matrix, effectively improving the wear resistance.

Silicon Availability from Chemically Diverse Fertilizers and Secondary Raw Materials.

Crops may require Si fertilization to sustain yields. Potential Si fertilizers include industrial byproducts (e.g., steel slags), mined minerals (CaSiO3), fused Ca-Mg-phosphates, biochar, ash, diatomaceous earth, and municipal sewage sludge. To date, no extraction method was shown to accurately predict plant availability of Si from such chemically diverse Si fertilizers. We tested a wide range of products in greenhouse experiments and related the plant Si content to Si extracted by several common Si fertilizer tests: 5-day extraction in Na2CO3-NH4NO3, 0.5 mol L-1 HCl, and Resin extraction. In addition, we tested a novel sink extraction approach for Si(OH)40 that utilizes a dialysis membrane filled with ferrihydrite ("Iron Bag"). Wheat straw biochars and ash exhibited equivalent or marginally higher Si solubility and availability compared to wheat straw. Thermo-chemically treated municipal sewage sludge, as well as diatomaceous earth, did not release substantial amounts of Si. The Resin and the Iron Bag extraction methods gave the best results to predict plant availability of Si. These methods better reproduce the conditions of fertilizer dissolution in soil and around the root by (1) buffering the pH close to neutral and (2) extracting the dissolved Si(OH)40 with ferrihydrite (IronBag method) for maximum quantitative extraction.

Microstructure, mechanical properties and strengthening mechanisms of in-situ prepared (Ti5Si3 + TiC0.67)/TC4 composites

Hybrid (Ti5Si3 + TiCx) (x = 0.67) particles reinforced TC4 matrix composites were in-situ prepared by hot-press sintering, basing on the reactions between Ti3SiC2 and Ti. To research the effects of Ti3SiC2 content on the microstructures and mechanical properties, 1–10 wt% Ti3SiC2 were selected. When Ti3SiC2 content was less than 5 wt%, Si element dissolved in the matrix, and TiCx distributed in the matrix homogeneously. While the content exceeded 5 wt%, Si was precipitating as Ti5Si3 attaching to TiCx particles, which resulted in the aggregations of reinforcements. The micro-hardness of matrix was improved with the solid solubility of Si increasing in matrix, and the compressive strength was improved with Ti3SiC2 content increasing. The hardness was mainly affected by solid solution strengthening. The compressive strength was affected by the solid solution strengthening, fine-grain strengthening and the improvement effect of reinforcements. It was determined that solid solution strengthening played the major role by theoretical calculation.

Constraining silicon isotope exchange kinetics and fractionation between aqueous and amorphous Si at room temperature

Abstract Silicon (Si) isotopes are useful tracers for the modern and ancient Si cycle, but their interpretation is limited by inadequate understanding of Si isotope exchange kinetics and fractionation factors at low temperature. This study investigated Si isotope exchange and fractionation between aqueous and amorphous Si at circumneutral pH and room temperature through a series of 29Si-spiked isotope-exchange experiments. Four different amorphous Si solids with varied surface areas were reacted with aqueous Si solutions of high ionic strength similar to seawater, or low ionic strength typical of freshwater, under conditions close to chemical equilibrium with respect to amorphous Si solubility. In contrast to the common perception of negligible Si isotope exchange at low temperature, ∼50–85% isotope exchange was achieved between aqueous and amorphous Si within ∼60 days. Larger solid surface areas and higher aqueous ionic strength generally promoted Si isotope exchange. Drying/aging of Si gel, however, impedes Si isotope exchange between amorphous and aqueous Si relative to freshly prepared Si gels. Excluding the experiments that used the aged Si gel, temporal trajectories of Si isotope evolution of the two phases from all other experiments showed significant curvature in three-isotope space (29Si/28Si and 30Si/28Si). These results can be best explained by a model that comprises two Si isotope exchange processes with different exchange rates and fractionation factors during the interactions between aqueous and amorphous Si towards isotope equilibrium. The faster exchange is associated with surface sites, and slower exchange occurs between exterior and interior Si atoms of the solid. Exchange with surface sites tends to partition heavy Si isotopes in the aqueous phase relative to the solid surface, whereas exchange between surface and interior sites in the solid tends to enrich heavy Si isotopes in the interior. Two experiments that achieved >80% isotope exchange provided the best estimates of equilibrium Si isotope fractionation factors between bulk amorphous Si solid and aqueous monomeric silicic acid H4SiO4 (Δ30Siamorphous–aqueous) at 23 °C: +0.52‰ (±0.15‰, 1sd) at seawater ionic strength, and −0.98‰ (±0.12‰) at freshwater ionic strength. The observed “salt effect” on Si isotope exchange kinetics and fractionation factor is interpreted to reflect an influence of cations on Si speciation of solid surfaces. This work highlights the value of three-isotope method in studying both reaction kinetics and isotope fractionation mechanisms. The observed Si isotope exchange between amorphous and aqueous Si at low temperature implies that Si isotope re-equilibration, a previously neglected process, may be important in controlling Si isotope compositions of natural samples.