Decreasing Si Content Research Articles

Microstructure and corrosion performance of CoCrFeNiSix coatings deposited by magnetron sputtering at various negative bias voltages

In this work, CoCrFeNiSix coatings were fabricated by magnetron sputtering at the negative bias voltage ranging from 0 to 100 V. The effect of negative bias voltage on the composition, microstructure and corrosion performance was studied. The results show that all CoCrFeNiSix coatings consisted of amorphous and nanocrystalline FCC phases. However, the ratio of both amorphous/nanocrystalline phases and Si/(Co + Cr + Fe + Ni) atoms decreased with increasing negative bias voltage. Applying negative bias voltage also led to the improved surface quality. Accordingly, the corrosion rate of CoCrFeNiSix coatings in 0.5 mol/L H2SO4 solution exhibited a decreasing trend. Meanwhile, the CoCrFeNiSix coating deposited at 100 V had the lowest passive current density, highest coating resistance and lowest carrier density. This was because that Si competed with Cr to suppress the formation of Cr2O3 in the compact inner layer, and thus the decreasing Si content caused by increasing negative bias voltage led to the improved protection effect of the passive film.

Interface mediated crystallization of plate-like SAPO-41 crystals to promote catalytic hydroisomerization

Silicoaluminophosphates SAPO-41 is a superior unidimensional zeotype catalyst used in hydroisomerization. Generation of hierarchically structured SAPO-41 provides an effective way to promote its hydroisomerization selectivity, which is nonetheless difficult to achieve due to the formation of impurities. Herein, we report crystallization of plate-like crystals of pure phase SAPO-41 in a toluene-aqueous biphasic media in the presence of both Pr2NH and oleylamine. The obtained material has a larger adjustable range of silicon content (nSi/(nAl+nSi+nP) = 0.02−0.15), intermediate acidity and reduced diffusion path length along the micropore channel direction. Catalytic tests for n-heptane hydroisomerization on Pt loaded plate-like SAPO-41 crystals showed that the yield of the isomerized products could reach up to 68 %, with respect to 57 % for a control sample. The selectivities for isomers were found to increase with decreasing Si contents and enhanced diffusion properties, suggesting side β-scission reaction was disfavored at reduced acid site density and shortened diffusion path length.

Non-stoichiometry in Ca2Al2SiO7 enabling mixed-valent europium toward ratiometric temperature sensing

Eu2+/Eu3+ mixed-valence couple co-doped material holds great potential for ratiometric temperature sensing owing to its different electronic configurations and electron-lattice interaction. Here, the correlation of nonstoichiometry in chemical composition, phase structures and luminescence propertis of Ca2Al2Si1−xO7:Eu is discussed, and controlled Eu2+/Eu3+ valence and tunable emission appear with decreasing Si content. It is found that the 2Ca2+ + Si4+ ↔ Eu2+ + Eu3+ + Al3+ cosubstitution accounts for the structural stability and charge balance mechanism. Benefiting from the diverse thermal dependent emission behaviors of Eu2+ and Eu3+, Ca2Al2Si1−xO7:Eu thermometer exhibits excellent temperature sensing performances with the maximum absolute and relative sensitivity being 0.024 K−1 (at 303 K) and 2.46% K−1 (at 443 K) and good signal discriminability. We propose that the emission quenching of Eu2+ is ascribed to 5d electrons depopulation through Eu2+/Eu3+ intervalence charge transfer state, while the quenching of Eu3+ comes from multi phonon relaxation. Our work demonstrates the potential of Ca2Al2Si1−xO7:Eu for noncontact optical thermometry, and also highlights mixed valence europium containing com pounds toward temperature sensing.

Impact of strained silicon on the device performance of a bipolar charge plasma transistor

In this manuscript we analyze a unique approach to improve the performance of the bipolar charge plasma transistor (BCPT) by introducing a strained Si/SixGe1−x layer as the active device region. For charge plasma realization different metal work-function electrodes are used to induce n+ and p+ regions on undoped strained silicon-on-insulator (sSOI or SixGe1−x) to realize emitter, base, and collector regions of the BCPT. Here, by using a calibrated 2-D TCAD simulation the impact of a Si mole fraction x (in SixGe1−x) on device performance metrics is investigated. The analysis demonstrates the band gap lowering with decreasing Si content or effective strain on the Si layer, and its subsequent advantages. This work reports a significant improvement in current gain, cutoff frequency, and lower collector breakdown voltage (BVCEO) for the proposed structure over the conventional device. The effect of varying temperature on the strained Si layer and its implications on the device performance is also investigated. The analysis demonstrates a fair device-level understanding and exhibits the immense potential of the SixGe1−x material as the device layer. In addition to this, using extensive 2-D mixed-mode TCAD simulation, a considerable improvement in switching transient times are also observed compared to its conventional counterpart.

Microencapsulation of eutectic and hyper-eutectic Al-Si alloy as phase change materials for high-temperature thermal energy storage

Thermal energy storage using phase change materials (PCMs) has been world-widely accepted as an effective technology for energy saving. In this study, Micro-Encapsulated PCMs (MEPCMs) were developed from Al-Si alloys, in which four kinds of Al-Si microspheres with different Al-Si compositions: Al-12%Si, Al-17%Si, Al-20%Si, and Al-30%Si (mass%) were encapsulated by two facile steps for controlling heat storage property. First, boehmite film was formed over the Al-Si microspheres as a precursor shell during boiling in distilled water. Subsequently, the boehmite-coated particles were oxidized by pure oxygen at the high temperatures to ensure the formation of a stable Al2O3 shell. Three different temperatures, 1100 °C, 1150 °C, and 1200 °C, were chosen to study the effect of temperature on the product; the shell morphology, structure, and latent heat storage capacity. Interestingly, the results revealed an increase in MEPCM thermal storage capacity with decreasing Si content and lowering the temperature. The MEPCM melting point was almost identical to its eutectic temperature at ~577 °C, in contrast the larger supercooling was observed for samples with the higher Si content. The cyclic durability of MEPCM was also evaluated through repeated heating and cooling processes in air. The obtained results showed no significant change in both MEPCM structure and thermal storage capacity. It indicated a good repetition durability of MEPCMs oxidized at high temperatures. In conclusion, the Al-Si microencapsulated PCMs appealed great potential as MEPCMs for use in high-temperature thermal energy applications.

Reworked restite enclave: Petrographic and mineralogical constraints from the Tongshanling intrusion, Nanling Range, South China

Microgranular enclaves, which can provide important petrogenetic indications for the host granitoids, are commonly observed in the Middle-Late Jurassic Cu-Pb-Zn-bearing granodiorites in the Nanling Range of South China. The origin of these Cu-Pb-Zn-bearing granodiorites is still controversial, with two different hypotheses: crust-mantle mixing and partial melting of the mafic lower crust, primarily based on geochemistry and geochronology. Detailed petrographic and mineralogical studies have been carried out on the Tongshanling granodiorite and its microgranular enclaves to provide new textural and compositional constraints on the petrogenesis. The microgranular enclaves have dioritic compositions with a mineralogy dominated by plagioclase, amphibole, and biotite. Abundant residual materials, such as mafic mineral clots, inherited and metamorphic zircon, and Ca-rich core plagioclase, occur in the enclaves and are in favor of a restite origin. The amphibole-rich clots are considered as vestiges of residual pyroxene-rich precursors from the source. Three different types of amphibole, i.e., magmatic, metamorphic, and magma reworked metamorphic amphibole, have been recognized in the granodiorite and its enclaves. The magmatic amphibole occurs as enclosed and idiomorphic isolated crystals in the granodiorite with Al and Si contents of 1.34–2.12 apfu (atoms per formula unit) and 6.25–6.88 apfu, respectively, and a ΣREE content of 307–764 ppm. The metamorphic amphibole, with a granoblastic triple-junction texture, occurs as aggregated crystals dominantly in the enclaves and a few in the granodiorite. It has actinolitic compositions (Al: 0.31–0.81 apfu, Si: 7.33–7.72 apfu) and distinctly lower contents of incompatible elements (ΣREE: 99–146 ppm). The magma reworked amphibole has intermediate compositions (Al: 0.81–1.59 apfu, Si: 6.71–7.35 apfu, ΣREE: 317–549 ppm) between the magmatic and metamorphic amphibole. The zonal amphibole-rich clots exhibit increasing Al and decreasing Si contents from interior amphibole to exterior amphibole and also from core to rim in amphibole grains, and the outer parts of enclave magmatic zircon have higher ThO2 + UO2 contents and lower Zr/Hf ratios than the inner parts, showing the process of magma reworking of the restite enclaves. Based on the textural and compositional evidence, these microgranular enclaves are thought to be reworked restite enclaves. This is also supported by thermobarometric calculation. A model illustrating the formation process of reworked restite enclave has been proposed. Combined with previous experimental studies of partial melting, the Tongshanling granodiorite is deduced to be derived from dehydration melting of amphibolite in the mafic lower crust. The fertile amphibolitic source is beneficial to the formation of Cu-Pb-Zn-bearing granodiorites in the Nanling Range.

The oxidation behavior of the quinary FeCoNiCrSix high-entropy alloys

The oxidation of three quinary FeCoNiCrSix high-entropy alloys (x = 0.25, 0.5 and 1.0) was investigated at 700–900 °C in dry air and at 900 °C in various O2-containing atmospheres (FeCoNiCrSi0.5 only). The two lower Si-content alloys contained an FCC single-phase structure, while the equimolar alloy had a BCC plus FCC dual-phase structure. The oxidation kinetics of all the alloys followed the parabolic rate law, with their oxidation rate constants increased with increasing temperature and decreasing Si content. In addition, the oxidation rate constants of the FeCoNiCrSi0.5 alloy steadily increased with increasing oxygen partial pressure, indicative of a typical oxide scale exhibiting a p-type semiconductivity. The scales formed on all the alloys consisted mostly of Cr2O3 and minor amounts of SiO2 (α-quartz). The oxidation mechanism of the HEAs is due primarily to outward diffusion of cations.

Development of hot rolled coils in S355J2+N grade with Si content <0.03% on Zelezara Smederevo hot strip mill

Because of suitability for hot-dip zinc-coating, there is some concern about the use low silicon (Si) content in normalized or normalizing rolling S355J2+N structural steel with minimum specified yield strength of 355 MPa. However, decreasing Si content leads to a decrease of solid solution hardening, whereas decrease in strength should be compensated by alternative mechanism. One possible solution is the use of microalloying with niobium (Nb). Based on available results obtained on own and competitive material, we projected and produced S355J2+N hot-rolled coils (HRC). The results show that the produced material fully satisfies the requirements of the EN 10025-2/2004 quality standard. Furthermore, strength values in normalized conditions were lower than those in normalizing rolling condition.

Structure and composition of Fe–OM co-precipitates that form in soil-derived solutions

Iron oxides represent a substantial fraction of secondary minerals and particularly affect the reactive properties of natural systems in which they formed, e.g. in soils and sediments. Yet, it is still obscure how transient conditions in the solution will affect the properties of in situ precipitated Fe oxides. Transient compositions, i.e. compositions that change with time, arise due to predominant non-equilibrium states in natural systems, e.g. between liquid and solid phases in soils. In this study, we characterize Fe–OM co-precipitates that formed in pH-neutral exfiltrates from anoxic topsoils under transient conditions. We applied soil column outflow experiments, in which Fe2+ was discharged with the effluent from anoxic soil and subsequently oxidized in the effluent due to contact with air. Our study features three novel aspects being unconsidered so far: (i) the transient composition of soil-derived solutions, (ii) that pedogenic Fe oxides instead of Fe salts serve as major source for Fe2+ in soil solution and (iii) the presence of exclusively soil-derived organic and inorganic compounds during precipitation. The experiments were carried out with two topsoil materials that differed in composition, texture and land use. Derived from Mössbauer spectroscopy, broad distributions in quadrupole splittings (0–2mms−1) and magnetic hyperfine fields (35–53T) indicated the presence of low-crystalline ferrihydrite and even lower crystalline Fe phases in all Fe–OM co-precipitates. There was no unequivocal evidence for other Fe oxides, i.e. lepidocrocite and (nano)goethite. The Fe–OM co-precipitates contained inorganic (P, sulfate, silicate, Al, As) and organic compounds (proteins, polysaccharides), which were concurrently discharged from the soils. Their content in the Fe–OM co-precipitates was controlled by their respective concentration in the soil-derived solution. On a molar basis, OC and Fe were the main components in the Fe–OM co-precipitates (OC/Fe ratio=0.5–2). The elemental composition of the Fe–OM co-precipitates was in accordance with the sequential precipitation of Fe(III)phosphates/arsenates prior to the formation of ferrihydrite. This explains decreasing Si contents in the Fe–OM co-precipitates with increasing availability of P. With respect to constant mean quadrupole splittings and slightly decreasing mean magnetic hyperfine fields, increasing contents of OC, P and Al in the Fe–OM co-precipitates did not further increase the structural disorder of the Fe polyhedra, while the crystallite interactions slightly decreased. Scanning electron microscopy and dynamic light scattering revealed the coincidental presence of variably sized aggregates and a considerable amount of Fe–OM co-precipitates, which remained dispersed in solution for months. Thus, variably composed Fe–OM co-precipitates with highly diverse aggregate sizes and comparably constant poor crystallinity can be expected after the oxidation of Fe2+ in transient, soil-derived solutions.

Effect of metallurgical factors on the bulk magnetic properties of non-oriented electrical steels

Non-oriented electrical steel (NOES) is one of the most common material used in electrical motors. Core loss and permeability are the most important properties that the motor manufacturers look for. Both these properties are structure sensitive and depend on several metallurgical factors; such as chemistry, grain size, crystallographic texture, cleanliness and stress states in non-oriented electrical steels. It has been observed in this course of the study that the grain size and Si content of NOES are the primary controlling factors to core loss, especially at higher frequencies. On the contrary, crystallographic texture plays an important role at lower frequencies. At higher frequency, core loss increases with increasing grain size and decreasing Si content of the steels. Small difference in grain size (~50μm) at lower frequency range has little influence on the magnetic properties but has significant adverse effect as frequency reaches high enough.

Impact of the hydrogen content on the photoluminescence efficiency of amorphous silicon alloys

This paper analyzes the impact of hydrogen on the photoluminescence (PL) efficiency of the three wide gap silicon alloys: silicon carbide (a-SiCx), silicon nitride (a-SiNx): silicon oxide (a-SiOx). All three materials behave similarly. The progression of the PL efficiency over the Si content splits into two regions. With decreasing Si content, the PL efficiency increases until a maximum is reached. With a further decrease of the Si content, the PL efficiency declines again. A comprehensive analysis of the sample structure reveals that the PL efficiency depends on the degree of passivation of Si and Y atoms (Y = C, N, O) with hydrogen. For samples with a high Si content, an effective passivation of incorporated Y atoms gives rise to an increasing PL efficiency. The PL efficiency of samples with a low Si content is limited due to a rising amount of unpassivated Si defect states. We find that a minimum amount of 0.2 H atoms per Si atom is required to maintain effective luminescence.

Dislocation dynamics in SiGe alloys

The dislocation velocities and mechanical strength of bulk crystals of SixGe1−x alloys grown by the Czochralski method have been investigated by the etch pit technique and compressive deformation tests, respectively. Velocity of dislocations in the SiGe alloys of the composition range 0.004 < x < 0.08 decreases monotonically with an increase in Si content at temperature 450–700°C and under stress 3–24MPa. In contrast, velocity of dislocations in the composition range 0.92 < x < 1 first increases, then decreases and again increases with a decrease in Si content at temperature 750-850°C and under stress 3–30MPa. The velocity of dislocations was quantitatively evaluated as functions of stress and temperature. Stress-strain behaviour in the yield region of the SiGe alloys of composition 0 < x < 0.4 is similar to that of Ge at temperatures lower than about 600°C. However, the yield stress becomes temperature-insensitive at high temperatures and increases with increasing Si content. The stress-strain curves of the SiGe alloys of composition 0.95 < x < 1 are similar to those of pure Si at temperatures 800–1000°C and the yield stress increases with decreasing Si content down to x = 0.95. The yield stress of the SiGe alloys is dependent on the composition, being proportional to x(1−x), showing a maximum around x ≈ 0.5. Built-in stress fields related to local fluctuation of the alloy composition and the dynamic development of a solute atmosphere around the dislocations, may suppress the activities of dislocations and lead to the hardening of SiGe alloys.

The formation of phosphoran olivine and stanfieldite from the pyrometamorphic breakdown of apatite in slags from a prehistoric ritual immolation site (Goldbichl, Igls, Tyrol, Austria)

In this study we report P-rich olivine and the tric- calciumphosphate(TCP)stanfielditeinpartiallymoltenquartz- phyllites from the ritual immolation site at the Goldbichl, near Innsbruck in the Tyrol, Austria. During partial melting, foamy patches of dark glassy material formed at the surface of the rocks and also as layers within the rocks. The pyrometamorphic rocks contain mostly the mineral assemblage olivine + ortho- pyroxene + plagioclase + spinel + glass. During the investiga- tion of slag samples from this prehistoric ritual immolation site, extremely P-rich, apatite-bearing micro-domains were found. In these domains phosphoran olivine was found whose P con- tents are approaching the maximum P contents in olivine according to the experimental investigations of Boesenberg and Hewins (Geochim Cosmochim Acta 74:1923-1941, 2010). The textures within these domains indicate strongly disequilibrium conditions. The phosphoran olivines formed due to reactions involving apatite and the mineral assemblage of the quartzphyllites, and coexist with plagioclase and a tri- calcium phosphate phase (TCP) showing stanfieldite Ca4(Mg, Fe 2+ ,M n 2+ )5(PO4)6 composition. In terms of its chemical composition, olivine shows a wide range in composition with P ranging from 0.3 to 0.54 a.p.f.u, which corresponds to max- imal 23 wt.% P2O5. These are the highest P-contents in olivine reported from rocks so far. The incorporation of P correlates with decreasing Si contents according to the charge balancing scheme 2P 5þ þ M1;2 ¼ 2Si 4þ þ Mg;Fe ðÞ 2þ M1;2. Therefore P can only be incorporated in combination with a vacancy on the M1,2 position. Micro-Raman spectroscopy of phosphoran olivines indicates that these olivines can easily be identified with this method due to the strong signals of the SiO4 and PO4 vibrations. The external vibrations of the M1,2 sites at low wave-numbers are more complex than for P-free olivine. This might be due to the effect of P 5+ on the M1 2+ and M2 2+ positions and the formation of vacancies on these sites. Since micro- Raman investigations of the TCP phase yielded no conclusive match with a known Raman spectrum of a phosphate mineral so far, therefore it is most likely that the TCP phase is stanfieldite, whose Raman spectrum has not been obtained yet. Schematical Schreinemakers analysis in the system CaO-Al2O3-FeO-SiO2- P2O5-H2O shows that P-rich olivine (fayalite-sarcopside solid solution) can form from mineral reactions involving chlorite, apatite and quartz and show that the occurrence of P-rich Fe- olivines spans a large T-range but is restricted to domains with high aSiO2. The mineral assemblage in the P-rich micro- domains shows that the formation of phosphoran olivine is not only restricted to the interaction between bone material and rocks in slags from ritual immolation sites as suggested by Tropper et al. (Eur J Mineral 16:631-640, 2004 )f rom the immolation site in Oetz but can form locally due to the pyro- metamorphic breakdown of a P-rich accessory precursor phase such as apatite.

Evolution of a very deeply subducted metasediment from As Sifah, northeastern coast of Oman

Near the coastal village of As Sifah, NE Oman, eclogite-facies rocks occur in the Saih Hatat window. We investigated a metapelite from this area, which is composed of mm-sized garnet and greenish phengite and minor epidote, blue amphibole, paragonite, albite, quartz, rutile, opaque phases, barite, and carbonate. Garnet exhibits a chemical zonation with Gro17Alm66Pyr6Spe11, Gro22Alm72.5Pyr5Spe0.5, and Gro25Alm65Pyr8Spe2 as inner core, mantle and outermost rim compositions. Inner portions of phengite have Si contents of up to 3.6 per formula unit (pfu), whereas rims are poorer in Si (3.2–3.4 pfu). We constructed a P-T pseudosection in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of the studied metapelite and contoured it by isopleths of various parameters such as the molar fractions of garnet components. Based on this contouring a P-T path was derived that starts at ultrahigh-pressure conditions. Garnet began to form at 25kbar and 490°C. Subsequently, temperatures increased and pressures decreased to finally reach P-T conditions of 13kbar and 565°C at which low-Si phengite, the outermost rim of garnet, Na-amphibole, epidote, quartz, magnetite, and rutile were in equilibrium. The P-T path is related to events in a subduction channel where the top of subducted oceanic crust, including the studied metasediments, was involved in an upwards-directed mass flow, resulting in the release of about 3wt.% H2O by garnet formation from hydrous minerals such as chlorite and lawsonite. In order to get hints at the interaction of such hydrous fluids, we have analyzed the trace and minor elements in phengite. The contents of B, Rb, Cs, and Tl (20, 397, 6.7 and 1.7ppm, respectively) are nearly constant over the entire Si range of potassic white mica. In contrast, the contents of Ba and Sr increase from 900 and 0.5 to 10500 and 14ppm, respectively, with decreasing Si content in phengite. We hypothesize that this result reflects early leaching of mobile elements during subduction and the later approach of the rock to a barite deposit within a melange ascending in the subduction channel.

Low core loss of non-Si quaternary Fe83.3B8P8Cu0.7 nanocrystalline alloy with high Bs of 1.7 T

The effect of replacement Si by P on the soft magnetic and structural properties of nanocrystalline Fe-Si-B-P-Cu alloys has been investigated. The nanocrystalline Fe83.3SiXB8P8−XCu0.7 (X = 0, 2, 4, 6) alloy ribbons consist of precipitated α-Fe phase and residual amorphous phase, and initial permeability of these alloy ribbons are enhanced with decreasing Si content. In particular, the nanocrystalline Fe83.3B8P8Cu0.7 (X = 0) alloy has both low core loss of 1.4 W/kg at 1.0 T – 50 Hz and high saturation magnetic flux density of 1.70 T. In addition, this alloy exhibits the most favorable nanocrystalline structure containing the homogeneously precipitated α-Fe grains with 14 nm in mean diameter. Therefore, it can be concluded that the soft magnetic properties and nanostructure of Fe-Si-B-P-Cu alloys are strongly affected by Si and P content. The Fe83.3B8P8Cu0.7 alloy with low core loss and high saturation magnetic flux density compared with a Fe amorphous alloy is suitable for a magnetic core material in electronic devices such as transformers, inductors and motors.

Local structures and Al/Si ordering in lanthanum aluminosilicate glasses explored by advanced 27Al NMR experiments and molecular dynamics simulations

The structures of 15 La-Al-Si-O glasses, whose compositions span 11-28 mol% La(2)O(3), 11-30 mol% Al(2)O(3), and 45-78 mol% SiO(2), are explored over both short and intermediate length-scales by using a combination of solid-state (27)Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. MAS NMR reveals Al speciations dominated by AlO(4) groups, with minor but significant fractions of AlO(5) (5-10%) and AlO(6) (≲3%) polyhedra present in all La(2)O(3)-Al(2)O(3)-SiO(2) glasses; the amounts of Al([5]) and Al([6]) coordinations increase for decreasing molar fraction of Si. The MD simulations reproduce this compositional trend, with the fractional populations of AlO(p) groups (p = 4, 5, 6) according well with the experimental results. The modeled La speciations mainly involve LaO(6) and LaO(7) polyhedra, giving a range of average La(3+) coordination numbers between 6.0 and 6.6; the latter increases slightly for decreasing Si content of the sample. Besides the expected bridging and non-bridging O species, minor contributions of oxygen triclusters (≤9%) and free O(2-) ions (≤4%) are observed in all MD data. The glass structures exhibit a pronounced Al/Si disorder; the MD simulations reveal essentially random SiO(4)-SiO(4), SiO(4)-AlO(p) and AlO(p)-AlO(q) (p, q = 4, 5, 6) associations, including significant amounts of AlO(4)-AlO(4) contacts, regardless of the n(Al)/n(Si) molar ratio of the glass. The strong violation of Al([4])-Al([4]) avoidance is verified by 2D (27)Al NMR experimentation that correlates double-quantum and single-quantum coherences, here applied for the first time to aluminosilicate glasses, and evidencing AlO(p)-AlO(q) connectivities dominated by AlO(4)-AlO(4) and AlO(4)-AlO(5) pairs. The potential bearings from distinct fictive temperatures of the experimental and modeled glass structures are discussed.

Effect of strontium on liquid structure of Al–Si hypoeutectic alloys using high-energy X-ray diffraction

High-energy X-ray diffraction experiments were performed using a synchrotron beam source to investigate the effect of strontium on the liquid atomic structure of Al–Si hypoeutectic alloys. The high-temperature liquid diffraction experiments were carried out on Al alloys with 3, 7, 10 and 12.5 (eutectic) wt.% Si, respectively, with 0 and 0.04 wt.% addition of Sr to each of the alloys. Further, the diffraction data for all the alloys were obtained at various melt temperatures (5–220 K) above the respective liquidus temperature. It was observed that the addition of 0.04 wt.% Sr results in significant change in the liquid structure parameters, such as structure factor, pair distribution function, radial distribution function, coordination number and packing density, at any given melt temperature of the alloy. Salient observations were that, for any specific alloy and temperature, addition of Sr significantly decreases coordination number and packing density. Further, with the addition of Sr in the liquid alloy, the atomic coordination number and packing density increases with decreasing temperature and decreasing Si content of the alloy. The results coupled with prior knowledge have enabled an in-depth understanding of the nucleation environment of the solidifying phases, specifically the role of Sr in delaying the clustering tendencies (nucleation) of the eutectic Si phase.

Ardennite, tiragalloite and medaite: structural control of (As5+,V5+,Si4+)O4 tetrahedra in silicates

AbstractSeveral silicate-minerals, such as ardennite – Mn2+4MgAl5[Si5(As5+,V5+)O22](OH)6, Z = 2, tiragalloite – Mn2+4[Si3As5+O12(OH)], Z = 4 and medaite – Mn2+6[Si5(V5+,As5+)O18(OH)], Z = 4 possess (V5+,As5+,P5+)O4 tetrahedra. Using electron-microprobe analysis (EMPA) and single-crystal X-ray diffraction methods, the crystal chemistry of ardennite from Salam-Château, Belgium and the Vernetto mine, Italy, tiragalloite from the Gambatesa mine, Italy, and medaite from the Molinello mine, Italy and the Fianel mine, Switzerland, were studied. Structure refinements converged to R1 values of 2.10–5.67%. According to chemical analysis, the Σ(As+V+P) content increases with decreasing Si content. Thus, Si replaces pentavalent cations in tetrahedral coordination. The (As5+,V5+,P5+,Si4+)O4 tetrahedra are categorized by their connections to SiO4 tetrahedra. The (As5+,V5+,P5+,Si4+)O4 tetrahedron of ardennite is isolated, and those of tiragalloite and medaite terminate a tetrahedral chain. The &lt;T–O&gt; of the isolated (As5+,V5+,P5+,Si4+)O4 tetrahedron shows a positive correlation with the mean ionic radius. For (As5+,V5+,P5+,Si4+)O4 tetrahedra with one T–O–T link, &lt;T–O&gt; and mean ionic radius are also correlated. In addition, the longest bridging T–O bond occurs between (As,V,P,Si)O4 and the adjacent SiO4 tetrahedron. The bridging O atom is over-bonded to satisfy the charge requirement of Σ(As+V+Si).

Composition dependency of the glass forming ability (GFA) in Mg–Ni–Si system by mechanical alloying

The pure elemental powder mixtures with the compositions of Mg 65Ni x Si 35− x ( x = 10, 20, 25, 33 at.%) were subject to high-energy ball mill, and the structures of the mixtures at different intervals of milling were characterised by X-ray diffraction (XRD). The compositional dependency of the glass forming ability (GFA) in Mg–Ni–Si system was evaluated based on the experimental results and the theoretical calculation. The compositional dependency of GFA in Mg–Ni–Si system can be understood well by comparing the enthalpies of the crystalline and amorphous phases based on the Miedema's theory for the formation enthalpy of alloys. Increasing the Ni/Mg ratio and/or decreasing Si content can improve the amorphous formability. The calculation results might be of great help in optimising the composition with high GFA in Mg–Ni–Si system.

High field investigation on the ferrimagnetic systems GdCo9−xSi4+x (−0.2 ⩽ x⩽ 0.2) and TbCo9Si4

GdCo9−xSi4+x (x = −0.2 to x = 0.2) and TbCo9Si4 exhibit the tetragonal LaFe9Si4 structure (space group I4/mcm). Magnetically these samples are ferrimagnetic with ordering temperatures close to 50K. The ferrimagnetic magnetization is found to decrease with decreasing Si content, thus, revealing an increasing cobalt sublattice magnetisation. The antiparallel coupling of the Gd- and Co-sublattice magnetisations breaks up at lower critical fields Hl well above 20 T. From the lower critical field Hl as well as from the slope ΔM/ΔH for fields above Hlthe inter-sublattice molecular coefficient can be determined which is approximately constant within this solid solution.