Ga Samples Research Articles

Iron isotopes in ancient and modern komatiites: Evidence in support of an oxidised mantle from Archean to present

The mantle of the modern Earth is relatively oxidised compared to the initially reducing conditions inferred for core formation. The timing of the oxidation of the mantle is not conclusively resolved but has important implications for the timing of the development of the hydrosphere and atmosphere. In order to examine the timing of this oxidation event, we present iron isotope data from three exceptionally well preserved komatiite localities, Belingwe (2.7Ga), Vetreny (2.4Ga) and Gorgona (0.089Ga). Measurements of Fe isotope compositions of whole-rock samples are complemented by the analysis of olivine, spinel and pyroxene separates. Bulk-rock and olivine Fe isotope compositions (δ57Fe) define clear linear correlations with indicators of magmatic differentiation (Mg#, Cr#). The mean Fe isotope compositions of the 2.7–2.4Ga and 0.089Ga samples are statistically distinct and this difference can be explained by greater extent of partial melting represented by the older samples and higher mantle ambient temperatures in the Archean and early Proterozoic relative to the present day. Significantly, samples of all ages define continuous positive linear correlations between bulk rock δ57Fe and V/Sc and δ57Fe and V, and between V/Sc and V with TiO2, providing evidence for the incompatible behaviour of V (relative to Sc) and of isotopically heavy Fe. Partial melting models calculated using partition coefficients for V at oxygen fugacities (fO2s) of 0 and +1 relative to the fayalite-magnetite-quartz buffer (FMQ) best match the data arrays, which are defined by all samples, from late Archean to Tertiary. These data, therefore, provide evidence for komatiite generation under moderately oxidising conditions since the late Archean, and argue against a change in mantle fO2 concomitant with atmospheric oxygenation at ~2.4Ga.

Nanoscale inhomogeneities: A new path toward high Curie temperature ferromagnetism in diluted materials

Room temperature ferromagnetism has been one of the most sought after topics in today's emerging field of spintronics. It is strongly believed that defect- and inhomogeneity- free sample growth should be the optimal route for achieving room-temperature ferromagnetism and huge efforts are made in order to grow samples as "clean" as possible. However, until now, in the dilute regime it has been difficult to obtain Curie temperatures larger than that measured in well annealed samples of (Ga,Mn)As ($\sim$190 K for 12% doping). In the present work, we propose an innovative path to room-temperature ferromagnetism in diluted magnetic semiconductors. We theoretically show that even a very small concentration of nanoscale inhomogeneities can lead to a tremendous boost of the critical temperatures: up to a 1600% increase compared to the homogeneous case. In addition to a very detailed analysis, we also give a plausible explanation for the wide variation of the critical temperatures observed in (Ga,Mn)N and provide a better understanding of the likely origin of very high Curie temperatures measured occasionally in some cases. The colossal increase of the ordering temperatures by nanoscale cluster inclusions should open up a new direction toward the synthesis of materials relevant for spintronic functionalities.

Temperature dependent magnetostrains in polycrystalline magnetic shape memory Heusler alloys

Abstract The transformation-induced strains with and without applied magnetic field and magnetostriction loops at different temperatures have been measured for polycrystalline ferromagnetic and metamagnetic shape memory Heusler alloys such as Ni Mn Ga, Ni Fe(Co) Ga and Ni Mn Sn, respectively. The results evidence a magnetic field oriented growth of martensitic variants during the martensitic transformation in Ni Mn Ga sample, whereas this effect is not evident in the other two alloys. In the Ni Mn Sn alloy, the isotropic volume magnetostriction dominates the anisotropic domain magnetostriction.

Modification in Structure, Phase Transition, and Magnetic Property of Metallic Gallium Driven by Atom–Molecule Interactions

The present work supports a novel paradigm in which the surface structure and stacking behavior of metallic gallium (Ga) were significantly influenced by the preparation process in the presence of organic small molecules (ethanol, acetone, dichloromethane, and diethyl ether). The extent of the effect strongly depends on the polarity of the molecules. Especially, a series of new atom-molecule aggregates consisting of metallic Ga and macrocyclic hosts (cyclodextrins, CDs) were prepared and characterized by various techniques. A comprehensive comparative analysis between free metallic Ga and the Ga samples obtained provides important and at present rare information on the modification in structure, phase transition, and magnetic property of Ga driven by atom-molecule interactions. First, there is a notable difference in microstructure and electronic structure between the different types of Ga samples. Second, differential scanning calorimetry analysis gives us a complete picture (such as the occurrence of a series of metastable phases of Ga in the presence of CDs) that has allowed us to consider that Ga atoms were protected by the shielding effect provided by the cavities of CDs. Third, the metallic Ga distributed in the aggregates exhibits very interesting magnetic property compared to free metallic Ga, such as the uniform zero-field-cooled and field-cooled magnetization processes, the enhanced responses in magnetization to temperature and applied field, and the fundamental change in shape of magnetic hysteresis loops. These significant changes in structural transformation and physical property of Ga provide a novel insight into the understanding of atom-molecule interactions between metallic atoms and organic molecules.

Stable magnetic-field-induced strain above 1% in polycrystalline Ni–Mn–Ga

Polycrystalline Ni–Mn–Ga samples were directionally solidified and trained in order to lower the twinning stress. Polyurethane was used to fill the cracks. Magnetic-field-induced strain (MFIS) was measured in a rotating magnetic field over more than 10 6 cycles. The operating temperature was varied between 0 and 45 °C. With increasing temperature, the MFIS increased from 0.6% to 1.2%. After cooling to room temperature, an enhanced MFIS of 1.0% was retained and maintained over another 10 6 cycles.

Gramicidin A backbone and side chain dynamics evaluated by molecular dynamics simulations and nuclear magnetic resonance experiments. II: nuclear magnetic resonance experiments.

Motional properties are important for understanding protein function and are accessible to NMR relaxation measurements. The goal of this study is to investigate the internal dynamics occurring in gramicidin A (gA) channels in order to provide benchmark experimental data for comparison with the results of molecular dynamics simulations. We therefore synthesized several (15)N isotope-enriched gA samples, covering all backbone residues as well as the Trp indole side chains for NMR relaxation experiments. On the basis of the (15)N NMR spectra for labeled gA samples incorporated in sodium dodecylsulfate (SDS) micelles, we determined T(1), T(2), and heteronuclear NOE values for backbone and indole (15)NH groups. The results indicate that the SDS-incorporated gA channel is a constrained structure without an especially "floppy" region. The NMR observables, particularly those for backbone groups, are predicted well by the molecular dynamics simulations in the accompanying article (DOI 10.1021/jp200904d ).

Luminescence dynamics in Ga(AsBi)

The temporal evolution of the spectrally resolved luminescence is measured for a Ga(AsBi) sample at low temperatures. The results are analyzed with the help of kinetic Monte Carlo simulations incorporating two disorder scales attributed to alloy disorder and Bi- clustering. An average time of 5 ps is identified as the upper limit for carrier capture into the Bi clusters whereas the extracted hopping rate associated with alloy fluctuations is much faster than the transitions between the individual cluster sites.

Microstructure and properties of Ni–Mn–Ga alloys produced by rapid solidification and pulsed electric current sintering

Ni–Mn–Ga alloys were compacted using pulsed electric current sintering (PECS) at 850–875 °C (50 MPa, 8 min) of flake-like powders made from the rapidly quenched melt-spun ribbons. Two kinds of ribbons were used: one made with a relatively slow wheel speed (6 m/s; average grain size ∼14 μm), and another with a faster wheel speed (23 m/s; average grain size ∼5 μm). Both sets of flake-like powders consisted of a mixture of non-modulated martensite (NM) and seven-layered modulated martensite (7M) structure. The amount of NM was greater in the slower speed material, while the other one exhibited mostly the 7M structure. These crystal structures were inherited by the sintered samples. In the compacts having the NM structure the multi-step martensitic reaction overlapped with the magnetic transition, and the Curie temperatures during heating and cooling differed from each other. In the compacts having mainly 7M structure the Curie point was about 100 °C and the martensitic transition took place in the paramagnetic state, while the intermartensitic one occurred in the region of 60–85 °C. This material demonstrated good magnetic properties and saturation magnetization, at best ∼50 emu/g. Mechanical properties of the compacts were good, and comparable to those of the polycrystalline Ni–Mn–Ga samples in compression.

Transparent Conducting ZnO:Ga Epitaxial Films Prepared on Epi-GaN/Al<sub>2</sub>O<sub>3</sub> (0001) by MOCVD

High-quality Gallium-doped zinc oxide (ZnO:Ga) films have been prepared on epi-GaN/sapphire (0001) substrates by the metalorganic chemical vapour deposition (MOCVD) method. The relative amount of gallium doping was varied from 0 to 8% (atomic ratio). The structural, electrical and optical properties of the ZnO:Ga films have been investigated in detail, as a function of Ga content. All the prepared samples have the wurtzite structure of pure ZnO with a strong (0002) preferred orientation. The microstructure for the surface of films was markedly influenced by the amount of Ga doping. The resistivity decreases continuously with adding Ga content and reaches to the value of 8.4×10-3 Ω•cm at 8%. The average transmittance for the deposited ZnO:Ga samples in the visible range was over 75%.

ChemInform Abstract: Acid Strength of Silica-Supported Oxide Catalysts Studied by Microcalorimetric Measurements of Pyridine Adsorption.

Microcalorimetric measurements of the differential heat of pyridine adsorption were used to probe the distribution of acid strength on a series of silica-supported oxide catalysts. Depositing oxides of the following cations onto silica increased the acid strength of the catalyst: Ga{sup 3{plus}}, Zn{sup 2{plus}}, Al{sup 3{plus}}, Fe{sup 3{plus}}, Fe{sup 2{plus}}, Mg{sup 2{plus}}, and Sc{sup 3{plus}}. The acid strength distribution curves for the supported oxide samples showed either two or three regions of constant heat of adsorption while silica had an energetically homogeneous surface. The Ga, Al, and Sc samples were found to have both Bronsted and Lewis acidity while the remaining samples showed only Lewis acidity. Incremental adsorption of pyridine indicated that the initial region of highest heat corresponds to strong Lewis acidity while intermediate heats seemed to be due to weaker Lewis acid sites or a combination of Lewis and Bronsted acid sites. The final region of lowest heat was due to H-bonded pyridine on silica. Estimates of the entropies of adsorption were determined, providing information about the mobility of the adsorbed pyridine molecules. The initial differential heat of adsorption increases proportionally to the Sanderson electronegativity of the added oxide.

Clustering effects in Ga(AsBi)

The photoluminescence from a Ga(AsBi) sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.

Effect of gallium as an additive in hydrodesulfurization WS2/γ-Al2O3 catalysts

Abstract The effect of gallium in W/Ga(x)–γ-Al2O3 catalysts was investigated in the hydrodesulfurization of dibenzothiophene. The γ-Al2O3 carrier was modified by the gallium addition prior to W impregnation and further calcination at 723 K. An electrophoretic study was carried out on the calcined Ga samples at 0, 0.29, 0.55, 0.84, 1.09, 1.43, 1.71 and 2.22 wt.% of Ga and it indicated that Ga addition affected significantly the surface of alumina. The presence of at least two gallium species was proposed, at low gallium content GaAl2O4 species decreased the isoelectric point (IEP) and probably the formation of Ga2O3 at higher contents increased again the IEP. Raman spectroscopy showed that the gallium incorporation had a strong effect in the formation of WOx species at the surface of catalysts. The highest amount of irregular WOx particles calculated by the Raman (O W O + W O)/W–O–W band intensities ratio was found on the W/Ga(1.09)–γ-Al2O3 catalyst. The presence of gallium induced higher activity on the dibenzothiophene hydrodesulfurization reaction in all catalysts tested. The highest Raman ratio and the highest activity were found for the W/Ga(1.09)–γ-Al2O3 catalyst, suggesting that better dispersed W species could be responsible for the highest HDS activities.

Thermophysical properties of coexistent phases of plutonium

Plutonium is the element with the greatest number of allotropic phases. Thermally induced transformations between these phases are typically characterized by thermal hysteresis and incomplete phase reversion. With Ga substitutional in the lattice, low symmetry phases are replaced by a higher symmetry phase. However, the low temperature martensitic phase transformation (δ→α') in Ga stabilized δ-phase Pu is characterized by a region of thermal hysteresis which can reach 200°C in extent. These regions of thermal hysteresis offer a unique opportunity to study thermodynamics in inhomogeneous systems of coexistent phases. The results of thermophysical properties measured for samples of inhomogeneous unalloyed and Ga alloyed Pu will be discussed and compared with similar measurements of their single phase constituents.

∼2.7 Ga crust growth in the North China craton

Numerous U–Pb zircon geochronological data show that rocks of the Precambrian basement from the North China craton (NCC) have two age peaks at 1.8–1.9 and ∼2.5 Ga, respectively. Whether the most prominent peak at ∼2.5 Ga really registered a period of major crustal growth in the NCC thus making it different from the global major crustal growth period of ∼2.7 Ga is uncertain. In this paper, we carried out in situ U–Pb and Hf isotopes of zircons and whole rock chemical and Sr–Nd isotopic analyses for six Precambrian samples from the NCC in order to understand crustal growth and reworking. The samples have U–Pb zircon ages of 1.8–1.9, ∼2.5 and ∼2.7 Ga, respectively. The two samples with U–Pb zircon ages of ∼2.7 Ga have zircon Hf model ages (T Hf DM) and/or whole rock Nd model ages (T Nd DM) close to their U–Pb zircon ages. It is clear that they represent juvenile crust generation, thus providing direct evidence that ∼2.7 Ga was a period of new crustal addition. The samples with U–Pb zircon ages of ∼1.8 and ∼2.5 Ga also have T Hf DM and/or T Nd DM similar to those of the ∼2.7 Ga samples, indicating that these samples also have a mantle extraction time of 2.7 ± 0.1 Ga and their ∼1.8 and ∼2.5 Ga ages may represent periods of reworking of the 2.7 ± 0.1 -Ga crust rather than juvenile crust generation. The reworking is manifested either as metamorphism or as lower crustal-derived magmatism. Specifically, the ∼2.5-Ga tonalitic gneiss sample is thought to be derived by melting of the ∼2.7 Ga lower crust. Since many ∼2.5-Ga tonalite–trondhjemite–granodiorites (TTGs) and calc–alkaline granitoids in the NCC contain ∼2.7-Ga xenocrystic and inherited zircons, we postulate that they are probably also derived mainly from the 2.7 ± 0.1-Ga crust. Our data imply that the quantity of ∼2.7 Ga continental crust in the NCC is probably greater in extent than that exposed today.

Crack growth of 10M Ni–Mn–Ga material in cyclic mechanical loading

The 10M martensitic Ni–Mn–Ga single crystal materials are usually applied in the magneto-mechanical actuators. Therefore, it is important to know the possible effect of the long-term cyclic shape changes on their structure and behavior. This can be evaluated with the mechanical fatigue testing. In the present study, the single crystal 10M Ni–Mn–Ga samples of different compositions were applied to strain-controlled uniaxial mechanical cycling in the multivariant state at ambient temperature. The experiments revealed distinctive changes of the twin variant structure, especially in the mobile twin area, density of twin boundaries, and in the tendency for fatigue crack growth. Characterization of the crack surface showed that the cracks in the microscale grow in a step-wise manner on specific crystallographic planes, i.e, twin boundary planes, but that the macroscopic crack does not occur only along crystallographic directions.

Grain growth in Ni–Mn–Ga alloys

The influence of annealing temperature and time on grain growth in polycrystalline Ni–Mn–Ga samples near the stoichiometric composition Ni 2MnGa was investigated. Grain growth was only observed for compositions with a Ni-content below 50 at.%. The existence of constitutional vacancies as a possible origin for the different grain growth behavior was excluded by positron annihilation spectroscopy (PAS). In order to activate grain boundary motion and hence grain growth in Ni 50Mn 29Ga 21 the samples were annealed and deformed in situ in compression up to various strain levels. A sharp threshold to initiate grain growth is observed between 8% and 10% of compressive strain.

Catalytic activity of mesoporous Ga-SBA-15 materials in α-pinene isomerisation: Similarities and differences with Al-SBA-15 analogues

A complete investigation of the catalytic activity of Ga-SBA-15 materials in α-pinene isomerisation is presented here. The performances of solids with different Si/Ga ratios prepared either by grafting or direct synthesis are compared with those of Al-SBA-15 analogues. In the case of Ga samples prepared by direct synthesis, α-pinene conversion can reach 90%, in relatively mild experimental conditions (353 K, 1 h). Camphene and limonene represent more than 50% of the products formed. In the case of samples prepared by post-synthesis, α-pinene conversion decreases with the Ga content, but remains much higher than with the corresponding Al-SBA-15 materials. In this latter case, heavy compounds are the main products and can correspond to more than 50% of the isomerised α-pinene. Undeniably Ga-SBA-15 solids allow better yields of camphene + limonene and limit the production of heavy undesirable by-products.

Frequency response of acoustic-assisted Ni–Mn–Ga ferromagnetic-shape-memory-alloy actuator

A prototype of Ni–Mn–Ga based ferromagnetic-shape-memory-alloy (FSMA) actuator was designed and built; an acoustic-assist technique was applied to the actuator to enhance its performance. A piezoelectric stack actuator was attached to the Ni–Mn–Ga sample to generate acoustic energy to enhance twin-boundary mobility and, hence, reduce the magnetic threshold field required for activating twin-boundary motion. The dynamic response of the acoustic-assist FSMA actuator was measured up to 1 kHz actuation. The acoustic assistance improves the actuator performance by increasing the reversible magnetic-field-induced strain (MFIS) by up to 100% (increase from 0.017 to 0.03 at 10 Hz) for drive frequencies below 150 Hz. For frequencies above 150 Hz, the acoustic-assist effect becomes negligible and the resonant characteristic of the actuator takes over the actuator response. Even though the acoustic assist does not improve the actuation at high frequencies, the MFIS output of 5% can be obtained at the resonant frequency of 450 Hz without acoustic assistance. The FSMA actuator is shown to be ideal for applications that require large strain at a specific high frequency

XRD and IR structural investigations of a particular breathing effect in the MOF-type gallium terephthalate MIL-53(Ga)

The gallium terephthalate Ga(OH)[O(2)C-C(6)H(4)-CO(2)].xA (A = HO(2)C-C(6)H(4)-CO(2)H) was hydrothermally synthesized in water under mild conditions (210 degrees C, 3(1/2) h) in the presence of terephthalic acid. The compound was characterized by powder X-ray diffraction, TGA, IR and BET method. This compound is isostructural to the MIL-53 type, previously observed with the trivalent cations Cr, Fe, Al, In. It exhibits a three-dimensional metal-organic framework built up from infinite chains of trans corner-sharing GaO(4)(OH)(2) octahedra (viamu(2)-hydroxo bonds) linked to each other through the terephthalate linkers. It results in the formation of lozenge-shape channels structure running parallel to the infinite files of gallium-centered octahedra. After activation, the compound is able to adsorb one molar equivalent of water at room temperature under ambient air (MIL-53(Ga){H(2)O}). Different hydrogen bond interactions are observed for the encapsulated water within the channels. In one tunnel, pairs of water species with strong hydrogen-bond interactions were observed whereas in the adjacent tunnel, only a continuous linear and weakly hydrogen bonded network occurs. The dehydrated form is obtained upon heating the MIL-53(Ga) solid at 80 degrees C together with the shrinkage of the channels (MIL-53(Ga)_lt. This form is stable up to 220 degrees C and then the open structure MIL-53(Ga)_ht is visible, but starts to decompose from 350 degrees C. Such a breathing effect was previously reported with cations such as Cr or Al but in the case of Ga, the stability domain of the narrow pore structure MIL-53(Ga)_lt is larger (160 degrees C instead of 20-30 degrees C for Al, for instance). The BET surface area was 1140 +/- 114 m(2).g(-1). The phase transitions were characterized by IR spectroscopy at different temperatures, which confirms the stability domain of the narrow close form (specific band at 1016 cm(-1)) of MIL-53(Ga) and then the pore opening (shifted band toward 1024 cm(-1)) together with the structure collapse. An identical behaviour is also discussed for the aluminum MIL-53 analogue. A comparison between the behaviour of the Al, Ga and Fe samples is presented.

Ga-Modified Nanostructured ZnO: Characterization and Application in Dye-Sensitized Solar Cells

ZnO has received great attention in many applications due to its electronic and optical properties. We report on the preparation of ZnO and gallium-containing ZnO (ZnO:Ga) nanoparticles by the precipitation method. The nanoparticles have the wurtzite structure and a high crystallinity. Gallium ions are present as Ga3+, as evidenced by the binding energies through XPS. Porosity and surface area of the powder increased under increasing gallium level, explained by the smaller particle size of ZnO:Ga samples compared with ZnO. The estimated optical band gap of ZnO was 3.2 eV, comparable to ZnO:Ga.