Activation Energy Of Mg Research Articles

Fe/LiNO3/TiN co–modified MgO for enhanced thermochemical energy storage performance in MgO/Mg(OH)2 cycles

MgO/Mg(OH)2 thermochemical energy storage can convert solar energy and industrial waste heat into forms that are easier to store and transport by cyclic hydration/dehydration reactions. To achieve excellent energy storage performance, cyclic stability and optical absorption capability of MgO/Mg(OH)2, Fe/LiNO3/TiN co–modified MgO were synthesized for energy storage cycles by wet-mixing method. The energy storage performance, cyclic stability and dehydration kinetics of Fe/LiNO3/TiN co–modified MgO were studied in dual fixed–bed system and simultaneous thermal analyzer. The experimental results show that the Fe and LiNO3 in the composite not only increase the hydration conversion of MgO by 6.3 times at 30 min, but also reduce the dehydration temperature and dehydration activation energy of Mg(OH)2 by 99.4 °C and 64 %, respectively. TiN overcomes aggregation of MgO and sustains the excellent porous structure, which results in only a 9 % decrease in energy storage performance of Fe/LiNO3/TiN co–modified MgO after 15 energy storage cycles. Subsequently, TiN significantly strengthens the optical absorption performance of Mg(OH)2 and photothermal temperature under xenon-lamp simulated sunlight radiation. Furthermore, density functional theory calculation reveals that Fe and Li enhance the adsorption of H2O on MgO model and reduce energy barrier for hydration reactions by 62.76 %, while TiN effectively inhibits MgO clusters migration within the material and enhances the optical absorption coefficient. Hence, Fe/LiNO3/TiN co–modified MgO shows significant research value in MgO/Mg(OH)2 energy storage process.

Atomized Mg-Li spherical alloys: A new strategy for promoting reactivity of Mg

In this study, to improve the reactivity Mg, which is a frequently used fuel, Li was incorporated into it. Mg-Li alloys with 3 and 5 wt% Li, prepared via centrifugal atomization, displayed a high degree of sphericity and consisted of αMg. Thermal behaviors indicated that the initiation oxidization temperature of Mg-Li powders was ∼170–200°C lower than that of pure Mg, owing to the lower melting point of soluble Li than that of Mg. Li passes through oxide shells and initiates the oxidization of metals in the core. The apparent activation energy of Mg decreased significantly from ∼361.2 kJ/mol to ∼141.1 kJ/mol with the incorporation of Li. For combustion in air, the Li content should be 5 wt% to achieve increased combustion intensity compared to Mg powders. When Mg-Li alloys and Mg powders were composited with polytetrafluorethylene (PTFE) serving as an oxidizer, the burn rates of MgLi3/PTFE and MgLi5/PTFE were ∼7.1 and 12.1 m/s, respectively, which were ∼1.8 and 6.8 m/s higher than that of Mg/PTFE. The flame temperature of Mg-Li/PTFE (∼1537–1552℃) of 3–5 µm was slightly lower than that of Mg/PTFE (∼1637℃). Therefore, the prepared Mg-Li alloys are promising candidates for use as fuels for energetic materials.

Thermochemical energy storage drastically enhanced by zirconium oxide and lithium hydroxide for magnesium hydroxide

AbstractEnergy storage provides sustainable energy. Magnesium hydroxide (Mg(OH)2) is a promising material for thermochemical energy storage. Zirconium oxynitrate (ZrO(NO3)2) and lithium hydroxide (LiOH) are investigated to improve the heat‐storage efficiency of Mg(OH)2. Experimental results indicate that the heat‐storage rate can be significantly increased by 306‐fold. The dehydration conversion can be increased up to 99% with 122‐fold increase in 15.3 min. The onset temperature of Mg(OH)2 dehydration decreases from 337 to 228°C. The activation energy of Mg(OH)2 dehydration significantly reduces by 57.7%. The cycle ability of heat storage and heat release also remarkably improves. Moreover, the heat‐storage time significantly decreases. The crystal parameters, crystal volume, and platelet size of Mg(OH)2 notably diminish. The acting mechanism of these additives is proposed using high‐resolution transmission electron microscopy, X‐ray powder diffraction, and X‐ray photoelectron spectroscopy data. Moreover, the kinetic equations for the dehydration of excellent Mg(OH)2 composites are disclosed for computing simulation.

Mitigate self-compensation with high crystal symmetry: A first-principles study of formation and activation of impurities in GaN

Formation and activation energies of common impurities: silicon (Si), germanium (Ge), carbon (C), beryllium (Be), and magnesium (Mg) in wurtzite (wz-) and zincblende (zb-) GaN are investigated by first–principles calculations. Si and Ge are excellent donors with low activation energies (<30 meV). The acceptor activation energies of C and Be in wz-GaN (and zb-GaN) are found to be 590 and 205 meV (and 490 and 134 meV), respectively. Neither C nor Be is a qualified acceptor in general as the former has a large activation energy (>300 meV) and the latter has a strong self-compensation effect (the formation energy of Bei compensating donor is negative). Bei is energetically abundant in zb-GaN because it has a 0.66 eV lower formation energy than wz-GaN. This is attributed to the coherence of the symmetricity between the s-orbitals and the interstitial site. The acceptor activation energy of Mg in zb-GaN is found to be 153 meV, which is smaller than its value in wz-GaN, 226 meV. Vibrational analysis suggests that Mgi compensating donor is less favorable to form in zb- than wz-GaN, since the higher symmetricity of the interstitial site in zb-GaN renders a much smaller vibrational entropy. Contrarily, the differences are insignificant for the impurities residing on Ga sites due to the same tetrahedral symmetricity. The results evidence the importance of self-compensation effects, which can be mitigated by the higher crystal symmetry of zb-GaN.

Thermodynamic and Kinetic Properties of Mg(II) in LiCl-KCl Melt and the Formation of Zn-Mg Alloy on Liquid Zn Electrode

Thermodynamic and kinetic properties of Mg(II) in LiCl-KCl eutectic melt were investigated by electrochemical techniques. The diffusion coefficients of Mg(II) in the melt at various temperatures were studied by cyclic voltammetry, and the activation energy of Mg(II)/Mg was calculated to be (48.5 ± 3.7) kJ mol−1. The exchange current densities and reaction rate constants of Mg(II)/Mg at different temperatures from 773 K to 873 K were calculated by testing polarization curves on Mo electrode, and results showed that the exchange current densities became larger as the temperature increased. Equilibrium potentials of Mg(II)/Mg on Mo electrode and liquid Zn electrode were also tested by open circuit potential method. Subsequently, the apparent standard potential of Mg(II)/Mg, apparent standard Gibbs energy for the formation of MgCl2 and activity coefficients of Mg in the liquid Zn were calculated. Finally, metallic Mg was extracted in the form of Zn-Mg alloys by galvanostatic electrolysis on liquid Zn electrode. The deposit was analyzed by X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, and Mg2Zn11 was detected in the deposit.

Effect of varying thermal annealing temperatures on the surface and electrical properties of Mg-doped GaN

Gallium nitride (GaN)-based light emitting diodes (LEDs) have been widely used to produce blue light that, with phosphor, is converted into white light for every-day applications. Despite its high efficiency, the performance of GaN based LEDs is limited by the low electrical conductivity of magnesium (Mg)-doped GaN due to the high activation energy of Mg (140 – 200 meV) as well as the presence of magnesium-hydride (Mg-H) complex, which may passivate Mg as an acceptor. In this study, the efficacy of thermal annealing treatment at temperatures in the range of 550 - 850°C was investigated to activate metal-organic chemical vapor deposition (MOCVD) grown Mg-doped GaN. Changes in the roughness and surface morphology was observed between the varied annealing temperatures by atomic force microscopy (AFM) while X-ray diffraction (XRD) data acts as supporting information for structural quality. Hall effect characterization was conducted to determine electrical properties of the annealed samples. In this study, a correlation between the resistivity and surface roughness trends was observed through the acquisition of an insight into the lattice distortion affecting the material conductivity that occurred specifically at temperatures above 750 °C.

Microstructure and phase transition kinetics of Mg–Ni–Zn alloy phase change thermal storage materials

Metal and alloy heat storage materials are popular in energy storage fields due to their excellent thermal conductivity and high latent heat value. In this research, by changing the ratio to optimize Mg–Ni–Zn alloys, we study the microstructure and thermal properties of alloys with different composition ratios, and the relationship between alloy composition and properties was deeply analyzed. Besides, we also study the thermal analysis kinetics of the alloys. The results show that the Mg–Ni–Zn heat storage alloy mainly contains α-Mg, Mg2Ni, Mg7Zn3, MgZn2 and its eutectic structure. The increase of Zn content is beneficial to the increase of phase transition temperature and phase transition latent heat value, among which the maximum latent heat value of Mg–13%Ni–37%Zn alloy can reach 165 J g−1. In this research, a new type of ternary alloy heat storage material Mg–16%Ni–24%Zn alloy is proposed, and the activation energy of Mg–16%Ni–24%Zn alloy is 1014.99 kJ mol−1 by Coats-Redfern integral equation and extrapolation method. The mechanism function is the Mampel Power rule, and the integral form is G(α)=α32, which is a reference for subsequent researchers.

Quantification of evaporation parameter and determination of evaporation mechanism

Evaporation parameter of solute atoms segregated on surfaces is quantified and its physical meaning is revealed to be an integrated effect of segregation, diffusion, and evaporation. Evaporation activation energy is formulated and its minimum can be used to estimate the binding energy between solute and solvent atoms. Evaporation activation energy of Mg from the surface of Al-0.8 wt%Mg alloy is determined to be very close to the binding energy between Al atom and Mg atom, indicating segregated Mg is adsorbed on the surface. Excellent agreement between theoretical calculations and experimental results provides a quantificational approach to uncover the evaporation mechanism.

Mg-Doping of (111)B GaAs Thin Films Grown by Molecular Beam Epitaxy

In this work, the influence of the growth conditions in the incorporation of Mg p-type dopant atoms in epitaxial GaAs(100) and (111)B thin films was investigated. Hall effect measurements and photoluminescence spectroscopy were used to investigate the electrical and optical properties of the films, respectively. The doping level varied between 1016 and 1019 cm–3 and increased with the inverse of the growth temperature and with the temperature of the Mg evaporation cell, with similar behavior in both crystal orientations. The analysis of the Mg incorporation at several growth temperatures and Mg arrival rates established a value of Ed(111)B = 1.6 ± 0.1 eV for the thermal desorption activation energy of Mg in the GaAs(111)B, a value 33% higher than for the (100) case. However, when a more realistic incomplete ionization model for the acceptors is considered, the value of the thermal activation energy for desorption of Mg adatoms increases to 2.7 ± 0.8 eV and equals the value for the (100) surface. Two very ...

Effect of LiCe(BH4)3Cl with a high Li ion conductivity on the hydrogen storage properties of Li[sbnd]Mg[sbnd]N[sbnd]H system

The effect of LiCe(BH4)3Cl on the hydrogen storage properties of Mg(NH2)22LiH system was studied systematically, which has a high Li ion conductivity. The hydrogen desorption temperatures for LiMgNH system shift to lower temperatures by 0.05LiCe(BH4)3Cl doping with the onset temperature of dehydrogenation decreasing by 40 °C and the peak temperature decreasing by 30 °C. The Mg(NH2)22LiH–0.03LiCe(BH4)3Cl composite exhibits an improved comprehensive hydrogen storage properties, which can reversibly store about 5.0 wt% hydrogen at 160 °C, and released hydrogen as much as 8.6 times faster than that of the Mg(NH2)22LiH composite at 160 °C. The results indicated that the LiCe(BH4)3Cl-containing sample exhibited much better cycling properties than that of Mg(NH2)22LiH sample. XRD and FTIR results show that the structure of LiCe(BH4)3Cl does not change before and after hydrogen absorption/desorption, indicating it plays the catalytic effect. The hydrogen desorption activation energy of Mg(NH2)22LiH doped with 0.03LiCe(BH4)3Cl was reduced by 37.5%. The rate-controlling step of desorption shifted from the diffusion to the chemical reaction by the addition of LiCe(BH4)3Cl, indicating that the diffusion rates of small ions like H+, Li+ and Mg2+ in LiMgNH system are significantly enhanced, which could be well explained by the improved ionic conductivity of LiCe(BH4)3Cl doped sample.

Распад твердого раствора межузельного магния в кремнии

AbstractThe decomposition of a solid solution of interstitial magnesium Mg_ i in silicon is studied. Float-Zone dislocation-free single-crystal n -Si with a resistivity of ~8 × 10^3 Ω cm and oxygen and carbon contents of ~5 × 10^14 cm^–3 and ~1 × 10^15 cm^–3 is used in the experiments. The samples are doped using the diffusion sandwich method at T =1100°C followed by quenching. Decomposition of the supersaturated Mg_ i solid solution is studied by observing the kinetics of increasing the resistivity of doped samples resulting from their annealing in the range T = 400–620°C. It is found that the decomposition is characterized by an activation energy of E _ a ≈ 1.6 eV, which is close to the previously determined diffusion activation energy of Mg_ i in silicon. It is also shown that Si:Mg exhibits stable properties at temperatures not exceeding 400°C, which is important for its possible practical application.

New Field Effect Deep‐UV μLEDs Development

Mg‐doped AlGaN layers are needed for deep‐ultraviolet light emitting diodes (DUV‐LEDs). However because of the high activation energy of Mg in such layers, the ionization level of the Mg‐dopants is very low. In this paper a new design is proposed with a view to improve the wall plug efficiency (WPE) of DUV‐LEDs. The method consists in using a geometry combining a UV‐μLED and a gate covering the edges of the mesa. This approach is intended to by‐pass the problem of low ionization level of the dopants in p‐AlGaN. The simulated structures include a DUV‐LED mesa structure with Al0.5Ga0.5N/Al0.7Ga0.3N multiple quantum well (MQW) active region emitting around 265 nm. It also comprises a gate deposited on the sidewalls of the mesa. Simulations performed with Silvaco atlas will be presented. In particular we will show that by applying an appropriate voltage to the gate, holes can be efficiently injected into the active region. They reach the active region via an accumulation channel below the gate around the mesa, and then diffuse along the length of wells and easily recombine with electrons. Finally simulations show that the calculated WPE can be increased by a factor of 3 for an optimized structure geometry.

N-polar InGaN-based LEDs fabricated on sapphire via pulsed sputtering

High-quality N-polar GaN epitaxial films with an atomically flat surface were grown on sapphire (0001) via pulsed sputtering deposition, and their structural and electrical properties were investigated. The crystalline quality of N-polar GaN improves with increasing film thickness and the full width at half maximum values of the x-ray rocking curves for 0002 and 101¯2 diffraction were 313 and 394 arcsec, respectively, at the film thickness of 6μm. Repeatable p-type doping in N-polar GaN films was achieved using Mg dopant, and their hole concentration and mobility can be controlled in the range of 8 × 1016–2 × 1018 cm−3 and 2–9 cm2V−1s−1, respectively. The activation energy of Mg in N-polar GaN based on a temperature-dependent Hall measurement was estimated to be 161 meV, which is comparable to that of the Ga-polar GaN. Based on these results, we demonstrated the fabrication of N-polar InGaN-based light emitting diodes with the long wavelength up to 609 nm.

Dynamic recrystallization behavior of Mg–Li–Al–Nd duplex alloy during hot compression

In order to evaluate the dynamic recrystallization (DRX) behavior of as-cast Mg–9.15Li–3.23Al–1.18Nd duplex magnesium alloy, a series of compression test with a strain of 0.7 were performed in temperature range of 473–673K and strain rate range of 0.001–1s−1 on a Gleeble3500 thermo-mechanical simulator. The apparent characteristics of flow stress curves obtained in dual phase α+β were analyzed in term of different dependence of flow stress to temperature and strain rate and different microstructural evolutions. Results showed that the DRX process could easily occur in β-Li phase, whereas in the α-Mg phase was retarded. Meanwhile, the globularised morphology of the α-Mg in a matrix was the characteristic feature of the micrographs in the track microstructural evolutions of the α-Mg phase. Based on the regression analysis for the Arrhenius type equation of flow behavior, the apparent average activation energy of deformation was determined as Q=126.9285kJ/mol. The activation energy of Mg–Li alloys could be obviously reduced by adding Li. The kinetics of DRX evolution was calculated as XDRX=1−exp[−0.01689((ε−εc)/ε∗)1.8599]. The DRX kinetic model was validated by means of microstructure observation. As the evolution of DRX was sensitive to temperature and strain rate, DRX grains easily occurred at higher temperatures and lower strain rates.

Diffusion Mechanism and Kinetics of Diffusion Bonded Mg/Ni/Al Joint

Mg and Al were bonded successfully by means of diffusion bonding using Ni interlayer. The microstructure, diffusion mechanism and regulation of atom diffusion were investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis analysis (EPMA). The results showed that the joints consisted of Mg-Ni interface and Al-Ni interface, and there were Mg2Ni formed in the Mg-Ni interface and Al3Ni formed in the Al-Ni interface, respectively. Diffusion activation energy of Mg and Al were lower than that of Ni in the Mg-Ni and Al-Ni interface. The thickness (x) of Mg2Ni and Al3Ni can be expressed as x2=3.97×10-4 exp (-139600/RT) (t-t0) and x2=8.62×10-3 exp (-174200/RT) (t-t0) with heating temperature (T) and holding time (t).

Plastic Flow Characteristics of an Extruded Mg-Li-Zn-RE Alloy

Abstract The flow behavior of an extruded Mg-7.8Li-4.6Zn-0.96Ce-0.85Y-0.30Zr alloy was investigated by a thermo-mechanical simulator in the temperature range from 250 to 450 °C and the strain rate range from 0.001 to 10 s −1 . The results indicate that the flow stress-strain curves of the alloy are characterized by an increase of the flow stress until a maximum stress value followed by the flow softening being observed. Such flow behavior is a characteristic for hot working accompanied by dynamic recrystallization. The flow behavior of the alloy can be represented by a hyperbolic sine type equation during the whole deformation temperature; however, it is not proper to fit the flow stress equation using the power and the exponential equation. The stress exponent n in the hyperbolic sine equation is high and increases with increasing of deformation temperature. The hot deformation process of the alloy is mainly controlled by dislocation climb. The average hot deformation activation energy Q of the alloy is 148 kJ/mol, which is higher than both the self-diffusion activation energy of Mg (135 kJ/mol) and the lattice diffusion activation energy of β -phase (103 kJ/mol). The results discussed above can be attributed to the addition of rare-earth (RE).

Site preference of Mg acceptors and improvement of p-type doping efficiency in nitride alloys

We perform first-principles density functional calculations to investigate the effect of Al and In on the formation energy and acceptor level of Mg in group-III nitride alloys. Our calculations reveal a tendency for the Mg dopants to prefer to occupy the lattice sites surrounded with Al atoms, whereas hole carriers are generated in In- or Ga-rich sites. The separation of the Mg dopants and hole carriers is energetically more favourable than a random distribution of dopants, being attributed to the local bonding effect of weak In and strong Al potentials in alloys. As a consequence, the Mg acceptor level, which represents the activation energy of Mg, tends to decrease with increasing numbers of Al next-nearest neighbours, whereas it increases as the number of In next-nearest neighbours increases. Based on the results, we suggest that the incorporation of higher Al and lower In compositions will improve the p-type doping efficiency in quaternary alloys, in comparison with GaN or AlGaN ternary alloys with similar band gaps.

Thermal characterization of GaN-based laser diodes by forward-voltage method

An expression of the relation between junction temperature and forward voltage common for both GaN-based laser diodes (LDs) and light emitting diodes is derived. By the expression, the junction temperature of GaN-based LDs emitting at 405 nm was measured at different injection current and compared with the result of micro-Raman spectroscopy, showing that the expression is reasonable. In addition, the activation energy of Mg in AlGaN/GaN superlattice layers is obtained based on the temperature dependence of forward voltage.

Creep mechanism of as-cast Mg-6Al-6Nd alloy

The creep mechanism of as-cast Mg-6Al-6Nd alloy was studied. The stress exponent for creep is 5.8 under the applied stresses of 50–70 MPa at 175°C. The activation energy for creep is 189 kJ·mol−1 under the applied stress of 70 MPa in the range of 150–200°C. The true stress exponent and threshold stress for creep are calculated as 4.96 and 10.2 MPa, respectively. The true stress exponent indicates that its creep mechanism belongs to the dislocation climb-controlled creep, which is in agreement with the microstructure changes before and after creep. The high value for stress exponent is attributed to the interaction of Al11Nd3 phase with dislocations. The activation energy is more than the self-diffusion activation energy of Mg, which is attributed to the load transfer taking place from the matrix to Al11Nd3 phase during creep.

Crystal growth and p‐type conductivity control of AlGaN for high‐efficiency nitride‐based UV emitters

AbstractMicrostructural analysis was carried out to clarify the compositional dependence of the generation of dislocations in AlxGa1‐xN on an underlying AlN layer grown by metalorganic vapor phase epitaxy. When the film thickness is less than 1.5 μm, the threading dislocation density (TDD) increases with decreasing AlN molar fraction. However, when the film thickness exceeds 1.5 μm, TDD becomes maximum at x around 0.5. The growth of AlGaN on a grooved AlN template is effective in reducing TDD for all AlN molar fractions. TDD in AlGaN, which is close to binaries such as GaN and AlN, is a few 107 cm–2, while for the intermediate composition with x around 0.5, TDD is still at mid 108 cm–2. The activation energy of Mg in AlGaN is found to show a strong Mg concentration dependence with a negative one‐third power law in Al0.25Ga0.75N and Al0.5Ga0.5N as well as in GaN. Overdoping of Mg causes an increase in the activation energy for every composition; from this, the optimum Mg concentration for realizing the highest hole concentration can be deduced. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)