Pressure Of Mg Research Articles

Effect of Composition Adjustment on the Thermoelectric Properties of Mg3Bi2-Based Thermoelectric Materials.

Thermoelectric materials are widely used in refrigeration chips, thermal power generation, catalysis and other fields. Mg3Bi2-based thermoelectric material is one of the most promising thermoelectric materials. Herein, the Mg3Bi2-based samples were prepared by high temperature synthesis, and the influence of Mg/Sb content on the electrical transport properties and semi-conductivity/semi-metallicity of the materials has been studied. The results indicate that the efficiency of introducing electrons from excess Mg prepared by high temperature synthesis is lower than that introduced by ball milling, due to the high vapor pressure of Mg. The doping of Sb/Te at the Bi site would make it easier for the material to change from p-type conduction to n-type conduction. With the increase in Mg content, the semi-conductivity of the material becomes weaker, the semi-metallicity becomes stronger, and the corresponding conductivity increases. With the increase in Sb content, the samples exhibit the opposite changes. The highest power factor of ~1.98 mWm-1K-2 is obtained from the Mg3.55Bi1.27Sb0.7Te0.03 sample.

Effects of alloy elements (Mg, Zn) on the microstructure and mechanical properties of (TiC+TiB2)/Al composites

In this paper, 50 vol% (TiC + TiB2)/Al composites with different alloy elements (Mg, Zn) were prepared by combustion synthesis and hot-pressing method. The effects of Mg and Zn on the microstructure and mechanical properties of the composites were investigated. TiC and TiB2 particles in the composite were refined by the addition of Mg to a certain extent because of the decrease in the temperature of the reaction system caused by the high vapor pressure of Mg. Besides, the compressive strength of the composite with Mg was significantly improved, in which the solid solution strengthening of Mg atom and the interface strengthening by reducing the mismatch between α-Al and TiC/TiB2 contributed to this improvement. Compared with Mg, the refining effect of Zn on ceramic particles was not obvious and only the yield strength was increased by the solid solution strengthening.

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

In this work, ex situ MgB2 was mixed with 0.5 mol of Mg and sintered. The sintering conditions were varied over a temperature range of 600–1000 °C for 1, 3, and 7 h, respectively. The addition of Mg during the sintering increased the partial pressure of Mg and thus suppressed the decomposition of MgB2. Onset of critical temperature, Tc, was retained at ∼ 38 K even after the addition of Mg. By increasing the sintering temperature, magnetic critical current density, Jc at self-field, and 20 K of the ex situ samples increased consistently. With the addition of Mg for 1 h sintering, self-field Jc (20 K) was enhanced more than 20 times to 104 A cm−2 as the sintering temperature was increased. Such significant enhancement in the Jc is mainly due to the improved grain coupling aided by Mg during the short sintering.

Basic magnesium sulfate cement: Autogenous shrinkage evolution and mechanism under various chemical admixtures

Basic magnesium sulfate cement (BMSC) is a new type of whisker self-toughening cement-based material. Owing to its excellent mechanical properties, it has been applied to high-speed rail cover plates and military protective materials. However, the shrinkage deformation problem affects the application of BMSC in other fields. This article describes the effects of three chemical admixtures on the basic mechanical properties and autogenous shrinkage deformation of BMSC. Isothermal calorimetry (IC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were performed to study the influence of different chemical admixtures on the phase and microstructure of BMSC. Finally, based on the microscopic phase and structure analysis results, the autogenous shrinkage mechanism of BMSC under the action of different chemical admixtures was systematically analyzed. The results show that citric acid (CA) and amino trimethylene phosphonic acid (ATMP) can significantly improve the compressive strength of BMSC. Based on the phase analysis and microstructure analysis, ATMP and CA can induce more formation of the needle-like 5-1-7 phase. The results of shrinkage deformation show that the unmodified BMSC has obvious expansion, and the expansion rate increases with time. However, the BMSC modified by chemical admixtures shows small expansions at first, and then begins to show continuous shrinkage deformation. The expansion in the unmodified BMSC during crystal growth stems from the crystallization pressure of Mg(OH)2 as the main driving force. However, the formation of Mg(OH)2 is inhibited in the modified BMSC, so the shrinkage deformation of BMSC was mainly caused by the formation of the 5-1-7 phase.

Influence of substrate temperature on incorporation of magnesium into Zn1-xMgxO layers growth by molecular beam epitaxy

Plasma-assisted molecular beam epitaxy growth of ZnO and Zn1-xMgxO epilayers on Al2O3 (11–20) substrates is described. The influence of the substrate temperature on the growth rate and magnesium concentration in Zn1-xMgxO epilayers was studied in detail. It was found that the composition of magnesium in Zn1-xMgxO epilayers increased with increasing substrate temperature. The composition variation in the alloys as a function of the growth temperature could be explained by the difference of vapor pressure of Mg and Zn at high growth temperature. Theoretical approaches to surface evaporation of metal atoms under different growth condition are difficult. Here, we describe an uncomplicated method determine quantities experimental to obtain. These results can be extended to other types of ternary or quaternary materials grown by MBE technique.

High carrier concentration in Mg2Si1−xSbx (0 ≤ x ≤ 0.10) prepared by a combination of liquid-solid reaction, ball milling, and spark plasma sintering

Ternary compounds of Mg2Si1−xSbx (0 ≤ x ≤ 0.10) are prepared by a combination of liquid-solid reaction, ball milling, and spark plasma sintering. The carrier concentration of Mg2Si1−xSbx increases with the Sb content x and reaches 1.1 × 1021 cm−3 at x = 0.10, which is approximately ten times higher than that previously reported. The high carrier concentration is attributed to the facilitation of Sb-doping by ball milling and the suppression of Mg vacancy formation by short-time sintering. No decrease in the carrier concentration of Mg2Si0.90Sb0.10 is observed after annealing at 773 K for 100 h in a semi-closed system, which suggests that the compound is stable at 773 K under a high partial pressure of Mg.

First-principles study of Mg(0001)/MgO(1-11) interfaces

By means of first-principles density-functional calculations, we studied the surface energy of a nonstoichiometric MgO(1-11) slab, the interfacial energy and interfacial bonding characteristics of Mg-terminated and O-terminated Mg/MgO(1-11) interfaces with three stacking-site (TOP, HCP and FCC sites) models, and the effect of the thickness of Mg films on the O-terminated MgO(1-11) surface. The results indicate that the surface energies of the nonstoichiometric MgO(1-11) slab and interfacial energies of Mg/Mg(1-11) interface depend on Mg chemical potential. We found that the Mg-terminated MgO(1-11) surface is more stable than the O-terminated MgO(1-11) surface at high Mg chemical potential, and Mg/MgO(1-11) with FCC stacking-site model is the most stable configuration in the Mg/MgO(1-11) interfaces. The results of the electronic structure reveals that the interfacial bonding of Mg-terminated interface with FCC site model mainly consists of metallic bond and of the O-terminated interface with FCC site model is mainly ionic with a small degree of [Formula: see text]-type covalent bond. Although the interfacial energy of Mg-terminated Mg/MgO interface with FCC stacking-site model is slightly higher than that of O-terminated Mg/MgO interface, the molten Mg would epitaxially grow on the FCC sites of the Mg-terminated MgO(1-11) surface because of the high evaporation pressure of Mg at high temperature.

High Temperature Reproducible Preparation of Mg2Si Films on (001)Al2O3 substrates Using RF Magnetron Sputtering Method

ABSTRACTA RF magnetron sputtering method was used to prepare Mg2Si films at 300-400oC on (001) Al2O3 substrates from a Mg disc target with Si chips. Mg deposition was not detected at 400°C from a pure Mg disc target without Si chips due to the high vapor pressure of Mg. However, the amount of Mg deposition increased with the increase in Si/(Mg+Si) area ratio of the target surface together with the increase of the Si deposition. The obtained films had a stoichiometric composition of Si/(Mg+Si)=0.33 that consisted of the well crystalline Mg2Si single phase regardless of Si/(Mg+Si) area ratio of the target surface. This showed the existence of a “process window” against supply ratio of Si/(Mg+Si) for Mg2Si single phase films with a stoichiometric composition. This is considered to be due to the vaporization of the excess Mg prepared under the Mg excess condition as reported by Mahan et al. for Mg2Si films prepared at 200°C by ultra-high vacuum evaporation.

Activity Measurements of Mg in the Ternary Cu-Mg-Si System Using Thermogravimetric Knudsen Effusion Method

The high vapor pressure of Mg in comparison with Cu and Si enables the use of thermogravimetric Knudsen effusion method (KEM) to determine the thermodynamic properties of binary Cu-Mg, Mg-Si, and ternary Cu-Mg-Si alloys. In the current study, the weight loss of solid Mg with time has been determined at different constant temperatures between 705 K and 788 K (432 °C and 515 °C) by using KEM, and from these diagrams, the sublimation rate of Mg was calculated. By introducing the sublimation rates into the equation derived from the kinetic gas theory, the enthalpy change of sublimation reaction of Mg at the experimental temperatures was calculated to be 147.5 ± 6.5 kJ mol−1, which is close to the 143.8 ± 0.5 kJ mol−1 calculated using the thermodynamic data available in the literature. Similar procedure was also applied to the binary Cu-Mg, Mg-Si, and ternary Cu-Mg-Si alloys where the activities of Mg with respect to the Mg wt pct with WCu:WSi = 20:80 were calculated. The diversion points in the activity–composition diagrams gave the phase boundary compositions in the phase diagrams. The phase boundary compositions of Mg in the alloys determined using KEM were in good agreement with the known binary and the constructed ternary phase diagram using FactSage thermochemical software and databases.

Thermodynamic and Kinetic Destabilization of Magnesium Hydride Using Mg–In Solid Solution Alloys

Efforts to thermodynamically destabilize magnesium hydride (MgH2), so that it can be used for practical hydrogen storage applications, have been a difficult challenge that has eluded scientists for decades. This letter reports that MgH2 can indeed be destabilized by forming solid solution alloys of magnesium with group III and IVB elements, such as indium. Results of this research showed that the equilibrium hydrogen pressure of a Mg-0.1In alloy is 70% higher than that of pure MgH2. The temperature at 1 bar hydrogen pressure (T1bar) of Mg-0.1In alloy was reduced to 262.9 °C from 278.9 °C, which is the T1bar of pure MgH2. Furthermore, the kinetic rates of dehydrogenation of Mg-0.1In alloy hydride doped with a titanium intermetallic (TiMn2) catalyst were also significantly improved compared with those of MgH2.

Influence of Mg Vapor Pressure on the $MgB_2$/Carbon Fiber Fabricated by Physical Vapor Deposition method

We have fabricated the superconducting <TEX>$MgB_2$</TEX>/carbon fiber by physical vapor deposition method. Mg (Magnesium) and B (Boron) were simultaneously deposited on the carbon fiber using the RF-sputtering and thermal evaporation, respectively. To ensure the relatively high vapor pressure of Mg at the growth region and the subsequent phase stability of <TEX>$MgB_2$</TEX> at the deposition temperature, inverted funnel-like guide made of Mg-foil was employed while one side of the guide were open for the sputtered B flux. Mg vapor pressure should be controlled precisely to secure the complete reaction. The <TEX>$MgB_2$</TEX>/carbon fiber showed a uniformly deposited thin layer with dense and well-formed grains. The <TEX>$MgB_2$</TEX>/carbon fibers in this study showed <TEX>$T_c$</TEX>~37.5K, <TEX>$J_c$</TEX> ~ <TEX>$2{\times}10^4\;A/cm^2$</TEX> in the 20K, 0T.

CHARACTERIZATIONS OF THE CORE-SHELL STRUCTURED MgB2/CARBON FIBER SYNTHESIS BY RF-SPUTTERING AND THERMAL EVAPORATION

MgB2 /carbon fibers have been synthesized by the combination of RF-sputtering of B and thermal evaporation of Mg , followed by co-evaporation. First, boron layer was deposited by RF-sputtering on the carbon fiber with average diameter of 7.1 μm. Later this coated layer of B was reacted with Mg vapor to transform into MgB2 . Since the MgB2 reaction proceed with Mg diffusion into the boron layer, Mg vapor pressure and the diffusion time had to be controlled precisely to secure the complete reaction. Also the deposition rate of each element was controlled separately to obtain stoichiometric MgB2 , since Mg was evaporated by thermal heating and B by sputtering system. The sintered B target was magnetron sputtered at the RF-power of ~200 W, which corresponded to the deposition rate of ~3.6 Å/s. With the deposition rate of B fixed, the vapor pressure of Mg was controlled by varying the temperature of tungsten boat with heating element control unit between 100 and 900°C. The MgB2 layers with the thickness of 200–950 nm could be obtained and occasionally MgO appeared as a second phase. Superconducting transition temperatures were measured around ~38 K depending on the deposition condition.

Fabrication of &lt;emphasis emphasistype="italic"&gt;In-Situ&lt;/emphasis&gt; &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;${\rm MgB}_{2}$&lt;/tex&gt;&lt;/formula&gt; Films Using Evaporation and Sputtering Simultaneous Deposition Technique

The evaporation and sputtering simultaneous deposition (ESSD) system was developed for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in-situ</i> deposition of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films. Mg was evaporated from the nozzle of cylinder crucible which is heated by high frequency induction heating, while B was sputtered by RF-magnetron sputtering. The nozzle-substrate distance was small enough to keep high vapor pressure of Mg on the substrate. The Mg-flux coming out of the nozzle was controlled by means of regulating the temperature of Mg source, was monitored and was calculated in detail with the variation of the distance from its nozzle. The optimal deposition condition with the variations of substrate temperature, the temperature of Mg source and the power of RF-magnetron sputtering, was investigated. MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films with onset T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 14 K was obtained. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the film is lower than the bulk value of 39 K. Further works are in progress to improve the quality of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films by further optimization of deposition parameters and improvement of the ESSD system.

“Hysteresis” in interaction of nanocrystalline magnesium with hydrogen

The kinetics of hydriding/dehydriding of magnesium has been significantly improved during the recent years by the introduction of new methods of fabrication of nanocrystalline magnesium-catalyst composites. However, the equilibrium hydrogen pressure for decomposition of nanocrystalline MgH 2 was found to be somewhat lower than for conventional magnesium hydride. Moreover, the essential difference in equilibrium hydrogen pressure for absorption and for desorption of hydrogen by nanocrystalline magnesium was reported by many authors. This difference called “hysteresis” is a common phenomenon for hydrides, but it is not observed for magnesium powders with an ordinary particle size (larger than 1 μm). The aim of the present work was to elucidate why the hysteresis arises in magnesium–hydrogen system with a decrease in particle size. It is shown that the “hysteresis” observed for nanocrystalline magnesium is an apparent phenomenon which is due to a hindered nucleation during hydriding at a pressure close to equilibrium. The pressures measured for desorption represent the real equilibrium. The plateau pressure of Mg + H 2 ↔ MgH 2 equilibrium is lower for nanocrystalline magnesium than that for conventional magnesium. This result is explained in terms of smaller surface energy of magnesium hydride in comparison with magnesium.

Mass production of [formula omitted] alloy bulk by isothermal evaporation casting process

An innovative method, isothermal evaporation casting process (IECP), is developed to produce Mg 2 Ni alloy for mass production in this work. In the past, high vapor pressure of Mg was considered as a disadvantage for producing pure Mg 2 Ni alloy. However, this characteristic was used to develop a refinement procedure to separate primary Mg 2 Ni alloy from Mg / Mg 2 Ni eutectic matrix. Characteristics of as-cast specimens measured by X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and electron probe X-ray microanalyzer (EPMA) reveal that mass production of single phase Mg 2 Ni alloy was successfully fabricated by IECP. For every 4.0 kg of raw materials, 2.6 kg Mg 2 Ni alloy bulk was extracted at a yield of about 65%. The hydrogen storage capacity of the well-activated Mg 2 Ni alloy reaches 3.58 wt% at 300 °C under 40 atm H 2 atmosphere which is close to the theoretical capacity of 3.6 wt%.

Production of fine tantalum powder by preform reduction process using Mg–Ag alloy reductant

A preform reduction process (PRP) using Mg–Ag alloy reductant based on the magnesiothermic reduction of oxide by Mg vapor with reduced vapor pressure was investigated in order to produce fine, high-purity tantalum powder for electronic devices. A solid feed preform was fabricated by calcinating a slurry comprising a mixture of tantalum oxide powder, flux (e.g., CaCl 2), and binder. It was then placed on a stainless steel mesh suspended above the Mg–Ag alloy reductant and reduced by Mg vapor generated from the alloy while heating at a constant temperature of 1273 K. Tantalum powder with a purity of more than 99 mass% and a narrow particle size distribution ( D 10 = 0.2 μm; D 50 = 0.4 μm; D 90 = 0.9 μm) was successfully obtained under a specific condition. The nickel contamination of the powder from stainless steel is lowered by using the alloy reductant in comparison with using pure Mg reductant. It was found that the particle size of the powder decreased with the vapor pressure of Mg; therefore, it can be controlled by controlling the vapor pressure of Mg.

A short timescale for changing oxygen fugacity in the solar nebula revealed by high-resolution 26Al– 26Mg dating of CAI rims

Most rocky objects in the solar system, including the primitive chondrites and the terrestrial planets themselves, formed at oxygen fugacities ( f O2) near that of the Iron–Wüstite (IW) f O2 buffer. Conversely, the most ancient rocky objects of the solar system, the calcium aluminum-rich inclusions (CAIs), formed at f O2 values 5 orders of magnitude lower than the IW buffer in an environment more closely resembling a solar gas. High-resolution Mg isotope data and estimates for f O2 for rims on CAIs show that this shift from ∼solar to protoplanetary (chondritic) f O2 occurred in 100,000 to 300,000 yr for these objects. Magnesium isotopes show further that the rise in f O2 was accompanied by a rise in the partial pressure of Mg. These results establish that CAIs entered a region resembling where planet progenitors formed within 3 × 10 5 yr of their formation in the solar nebula.

Experimental investigation and thermodynamic calculation of binary Mg-Mn phase equilibria

The monotectic reaction L″=L′+δMn in the binary system Mg-Mn was measured at about 1200 °C using differential thermal analysis. This was possible by sealing the samples in arc welded Ta capsules, thus solving the problem of high vapor pressure of Mg at such high temperature and also suppressing any reaction of Mg with the environment. Problems associated with the reaction of Mn with the Ta crucible are discussed in detail. In addition, a thermodynamic assessment and calculation of the entire Mg-Mn phase diagram was performed, incorporating published experimental data on the Mn solubility in the Mg-rich corner.

Hydrogen sorption properties of Mg–1 wt.% Ni–0.2 wt.% Pd prepared by reactive milling

Despite the large differences in melting points and the high vapour pressure of Mg, we succeeded in the preparation of induction-melted Mg–1 wt.% Ni–0.2 wt.% Pd alloy. The alloy was then reactively ball milled in 10 bar hydrogen. The structural changes during milling were characterised by XRD and high resolution scanning electron microscopy. The hydrogen sorption properties have been systematically studied by gravimetric analysis in a wide temperature and pressure range for absorption (1–10 bar, 50–300 ° C) and desorption (250–350 ° C, 50 mbar–1 bar). The combined effects of Pd and Ni additions prove particularly useful in providing superior kinetics at very modest absorption conditions and excellent cyclic stability. These data are compared with those of nanocrystalline MgH 2 powders, obtained by intensive milling in argon atmosphere.

Effect of synthesis temperature on density and microstructure of MgB2superconductor at ambient pressure

The experimental thermodynamic of MgB2 synthesis process and phase compositions have been investigated by diffraction thermal analysis (DTA) technology and X-ray diffraction. The fabrication of MgB2 bulks and superconducting properties at the temperatures range from 600 to 800°C were reported. And microstructure of MgB2 bulks were observed by scanning electron microscope (SEM). A method was developed to determine the porosity of MgB2 and the highest density can be obtained in MgB2 prepared at 650°C at ambient pressure. It is found that the vapor pressure of Mg increases remarkably at high temperature, leading to the high porosity in MgB2 samples. MgB2 bulk with good superconducting property and fine microstructure was synthesized at 750°C.