Vapor 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.

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 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.

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.

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.

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.

Ion beam synthesis of Mg2Si

The semiconducting metal silicides and Mg2Si in particular have been extensively studied in the last few years because of the following reasons: the materials have the potential to be successfully integrated in the optoelectronic silicon technology, the materials are nontoxic and the constituting elements are found in a large amount in nature, and lastly the formation of nanocrystals in a perticular matrix offers the possibility of semiconducting structures with new properties. The Mg-Si phase diagram [1] shows that Mg2Si is the only silicide in this system and it melts congruently at 1085 ◦C. Therefore, crystal growth from the melt should be possible, but one has to be careful about the considerable evaporation of the Mg component as the boiling point of pure Mg is very close to the compound melting temperature. Growth by chemical vapor transport is hardly possible because of the lack of stable magnesium halides and the growth of Mg2Si bulk material has been accomplished exclusively from a melt [2–5]. The compound Mg2Si has, similar to Si, a cubic structure with a larger lattice parameter. The lattice mismatch for the Mg2Si (111) face on the Si (111) face is 1.9%. Due to the comparatively small lattice mismatch high quality epitaxial growth of Mg2Si onto monocrystalline silicon appears to be possible [6]. However thin film formation is difficult because of the low condensation and high vapor pressure of Mg and that is why thin film preparation and studies are scarce. The first reports on Mg2Si thin film formation [7, 8] note that the silicidation proceeds rather rapidly in Mg/Si thin film substrates at relatively low temperatures and silicide columns matching the monocrystalline silicon substrate are formed. There has been an attempt to fabricate Mg2Si by laser melting of magnesium film deposited on a Si substrate [9]. The synthesized layers contain both silicides and Si crystallites. Mahan et al. [10] have prepared Mg2Si films, textured in the (111) direction by molecular-beam epitaxy. They find that intended reactive deposition of magnesium onto silicon substrate at temperatures from 200 to 500 ◦C results in no accumulation of magnesium. However, codeposition of magnesium and silicon at 200 ◦C, using a magnesium-rich flux ratio, gives stoichiometric Mg2Si films. A solidphase growth technique with preliminary formation of templates was used by Galkin et al. [11] to prepare continuous Mg2Si films with a thickness of the order of 20 nm on Si (111) substrates. The very low condensation coefficient of magnesium as well as the limited Mg2Si films thickness due to the barrier behavior of Mg2Si can be overcome using the ion-beam synthesis (IBS) method, reported in this paper. The samples were prepared by IBS, followed by rapid thermal annealing (RTA). The implantation of 24Mg+ ions was performed using an ion accelerator type ILU-4, allowing a high current density. As an ion plasma source 4N pure Mg, heated at 500 ◦C, was used. The vapor pressure of Mg at this temperature is about 10−2–10−1 Torr. During the implantation, in order to avoid amorphization [12], the substrates were heated to a temperature of about 230 ◦C by means of the incident ion beam (current density 10–12 μA · cm−2). The mass separated 24Mg+ ions were implantated into n-type Si wafers with (100) orientation and resistivity 4.3–4.5 ·cm. Four types of samples were prepared. The first two types were fabricated by two-step ion implantation with two different doses—5 × 1016 and 1 × 1017 cm−2, with energies 15 and 40 keV respectively. The other two types of samples were prepared by one-step implantation of two different higher doses—2 × 1017 and 4 × 017 cm−2 with energy 40 keV. The Mg implantation profiles were simulated by SRIM (Stopping and Range of Ions in Matter) and the initial profiles of the implanted Mg ions were estimated as:

CaO-MgO-Al 2o 3-SiO 2 liquids: chemical and isotopic effects of Mg and Si evaporation in a closed system of solar composition

A method is shown for calculating vapor pressures over a CMAS droplet in a gas of any composition. It is applied to the problem of the evolution of the chemical and Mg and Si isotopic composition of a completely molten droplet having the composition of a likely refractory inclusion precursor during its evaporation into the complementary, i.e. modified solar, gas from which it originally condensed, a more realistic model than previous calculations in which the ambient gas is pure H 2(g). Because the loss rate of Mg is greater than that of Si, the vapor pressure of Mg (g) falls and its ambient pressure rises faster than those of SiO (g) during isothermal evaporation, causing the flux of Mg (g) to approach zero faster and MgO to approach its equilibrium concentration sooner than SiO 2. As time passes, δ 25Mg and δ 29Si increase in the droplet and decrease in the ambient gas. The net flux of each isotope crossing the droplet/gas interface is the difference between its outgoing and incoming flux. δ 25Mg and δ 29Si of this instantaneous gas become higher, first overtaking their values in the ambient gas, causing them to increase with time, and later overtaking their values in the droplet itself, causing them to decrease with time, ultimately reaching their equilibrium values. If the system is cooling during evaporation and if mass transfer ceases at the solidus temperature, 1500 K, final MgO and SiO 2 contents of the droplet are slightly higher in modified solar gas than in pure H 2(g), and the difference increases with decreasing cooling rate and increasing ambient pressure. During cooling under some conditions, net fluxes of evaporating species become negative, causing reversal of the evaporation process into a condensation process, an increase in the MgO and/or SiO 2 content of the droplet with time, and an increase in their final concentrations with increasing ambient pressure and/or dust/gas ratio. At cooling rates <∼3 K/h, closed-system evaporation at P tot ∼ 10 −3 bar in a modified solar gas, or at lower pressure in systems with enhanced dust/gas ratio, can yield the same δ 25Mg in a residual CMAS droplet for vastly different evaporated fractions of Mg. The δ 25Mg of a refractory residue may thus be insufficient to determine the extent of Mg loss from its precursor. Evaporation of Mg into an Mg-bearing ambient gas causes δ 26Mg and δ 25Mg of the residual droplet to fall below values expected from Rayleigh fractionation for the amount of 24Mg evaporated, with the degree of departure increasing with increasing fraction evaporated and ambient pressure of Mg. δ 26Mg and δ 25Mg do not depart proportionately from Rayleigh fractionation curves, with δ 25Mg being less than expected on the basis of δ 26Mg by up to ∼1.2‰. Such departures from Rayleigh fractionation could be used in principle to distinguish heavily from lightly evaporated residues with the same δ 25Mg.

Crystal growth of MgB 2 from Mg–Cu–B melt flux and superconducting properties

A new method for the preparation of single crystals of the superconducting intermetallic MgB 2 compound from a Mg–Cu–B melt flux is presented. The high vapour pressure of Mg at elevated temperature is a serious challenge of the preparation process. The approximate thermodynamic calculations of the ternary Mg–Cu–B phase diagram show a beneficial effect of Cu, which extends the range of formation of MgB 2 to lower temperatures. Within the as-solidified Mg–Cu–B melt flux the MgB 2 compound forms plate-like single crystals up to a size of 0.2×0.2×0.05 mm 3 or alternatively rims peritectically grown around Mg B 4 particles. AC-susceptibility measurements were conducted with specimens selected from different parts of the as-solidified flux containing MgB 2 particles. Peritectically formed MgB 2-particles display the highest transition temperature of T c=39.2 K and a relatively narrow transition width of Δ T c=1.3 K. Other sections of the sample exhibit various superconducting transitions from T c=39 K to 7.2 K. This variation of T c is attributed to a finite homogeneity range of the MgB 2 compound whereas significant Cu solid solubility in MgB 2 can be excluded.

Growths of MgB 2 thin films by pulsed laser deposition

The recent discovery of metallic-like superconductivity in bulk MgB 2 material [1] opened a new class of possible applications in current transport, magnetic field devices and electronic devices. Fabrication of thin films in situ is essential in order to explore the device applications of this material, and in spite of a few reports [2–4], this still remains a challenge. The main reasons for this are the high vapour pressure of Mg even at low temperatures, and the high susceptibility of Mg to oxidation. A number of c-axis oriented MgB 2 films were grown on Al 2O 3-R cut, and the stoichiometry of the films was measured by ICP as a function of deposition pressure and laser fluence. It was found that the relative concentration of Mg on the substrate varies considerably with pressure and laser fluence. A relative concentration of around 53 at% Mg, which is necessary in order to form stoichiometric MgB 2 phase, can be obtain only in a narrow range of deposition pressures and laser fluences. The films obtain under these conditions were characterised by X-ray diffraction and magnetic moment in applied fields up to 9T.

Oxidation of magnesium

AbstractPure Mg was oxidized in 20 kPa O2, at temperatures between room temperature and 400°C, for times ranging from 2 min to 60 h. The oxide layer was analysed by XPS. The samples were pre‐treated by mechanical polishing and ion etching. Oxidation was carried out in a furnace connected to the XPS instrument. Magnesium was also oxidized in CO2 at 150°C and in water in order to gain information about the binding energies of the elements in MgCO3 and Mg(OH)2. The binding energies of Mg 2p, O 1s and C 1s of these compounds, as well as MgO, are reported. The thickness of the oxides formed during oxidation for 15 min at room temperature and at 150°C are 1.5 and 1.7 nm, respectively. At 300°C the growth of the oxide is well fitted by an inverse logarithmic law. The fact that the oxide formed in a dry atmosphere up to 300°C is extremely thin indicates that it has a low defect density. The influence of the humidity on the corrosion properties of Mg is discussed. The influence of the high vapour pressure of Mg on its oxidation properties is also mentioned. Copyright © 2002 John Wiley &amp; Sons, Ltd.

MOCVD of MgAl 2O 4 thin films using new single molecular precursors: application of β-hydrogen elimination to the growth of heterometallic oxide films

We have deposited the MgAl 2O 4 thin films on Si(1 0 0) substrates in the temperature range of 270∼600°C using newly developed single molecular precursors of Mg[(μ-O t Bu) 2AlMe 2] 2 and Mg[Al(O t Bu) 4] 2 by MOCVD. Polycrystalline, crack-free stoichiometric MgAl 2O 4 thin films were successfully grown on at as low as 400°C. This growth temperature was much lower than that of conventional CVD, and this is the first report of the MgAl 2O 4 growth using Mg[(μ-O t Bu) 2AlMe 2] 2. During film deposition, the vapor pressure of Mg[Al(O t Bu) 4] 2 was decreased due to oligomerization of the precursor itself. When Mg[(μ-O t Bu) 2AlMe 2] 2 was used, however, the vapor transport problem of the precursor was solved by introducing more volatile alkyl group. β-Hydrogen elimination was employed in the growth mechanism. The synthesized precursors and the as-grown films were characterized with NMR, XRD, XPS, RBS, and SEM, and the gas-phase by-products of the CVD reaction were collected and identified by gas chromatography.

Solid solution of Mg in Ti by mechanical alloying

Solid solution of 9 wt% Mg in Ti has been achieved by mechanical alloying in a clean atmosphere. The equilibrium solubility of magnesium in titanium is very low and processing of these alloys by conventional routes is not possible due to the high vapour pressure of Mg. Alloys of Ti and Mg have only been previously achieved by vapour quenching. Contamination was avoided by processing under a high purity argon atmosphere in sealed vials. Lattice parameter measurements and SEM observations suggested that all the Mg was in solid solution after 48 h milling. This has been confirmed by high resolution TEM which shows extremely fine Ti grains in which Mg is homogeneously distributed with no clustering or precipitation.