Structural and optical properties of MgyNi1–yHx gradient thin films in relation to the as-deposited metallic state

Thin Mg_yNi_1−yH_x films with a gradient in chemical composition are investigated by optical spectrophotometry, dc resistivity and X-ray diffraction measurements before and after exposure to hydrogen. In the metallic state crystalline Mg₂Ni is present for 0.6 ≤ y ≤ 0.8 and coexists with amorphous Mg and/or Ni. The hydride state is mainly characterized by the presence of Mg₂NiH₄ around the stoichiometric [Mg]/[Ni] = 2 composition and some MgH₂ on the Mg-rich side. The abrupt microstructural changes found in the as-deposited metallic state around the Mg–Mg₂Ni eutectic point correlate well with the compositional dependence of the optical properties in the hydride state. We conclude that the formation of the hydride depends directly on the detailed nature of the metallic parent phase. Furthermore, we demonstrate that high-throughput compositional screening via fiber-optic spectrophotometry is useful for hydride identification. When no structural long-range order is present, this provides a new tool for the search for hydrogen storage materials.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10−xCux (x = 0−4) alloys prepared by melt spinning

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg₂Ni-type Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg₂Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.     

Effect of Mg content on structure,hydrogen storage properties and thermal stability of melt-spun Mgx(LaNi3)100−x alloys

Amorphous Mg_x(LaNi₃)_100−x (x = 40, 50, 60, 70) alloys with ribbon shape (5 mm wide, 0.2 mm thick) have been prepared by rapid solidification, using a melt-spinning technique. Their microstructure, hydrogen storage properties and thermal stability were studied by means of XRD, SEM, PCTPro2000 and DSC analysis, respectively. The results indicated that when Mg_x(LaNi₃)_100−x alloys have been hydrogenated at 573 K under 2 MPa hydrogen pressure, LaH₃ phase is formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH₄ phase formed in the case of x (x = 40, 50, 60, 70), Mg₂NiH_0.3 phase formed in the case of x (x = 40, 50), and MgH₂ phase formed in the case of x = 70. Experimental data of hydrogen desorption kinetics, tested at 523 K, 573 K and 623 K, are in good agreement with Avrami–Erofeev equation. The maximum hydrogen absorption capacity is 2.71 wt.% for Mg₇₀(LaNi₃)₃₀ and 2.35 wt.% for Mg₇₀(LaNi₃)₃₀, the increase of hydrogen desorption capacity is in the order of x = 70 > x = 60 > x = 50 > x = 40. Based on DSC analysis, the activation energies for dehydrogenation of these samples are calculated to be 122 ± 2 kJ/mol (x = 40) > 101 ± 3 kJ/mol (x = 50) > 84 ± 5 kJ/mol (x = 60) > 64 ± 3 kJ/mol (x = 70), which are in agreement with the results of hydrogen desorption kinetics.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

Studies on de/rehydrogenation characteristics of nanocrystalline MgH2 co-catalyzed with Ti,Fe and Ni

In the present study, we have investigated the combined effect of different transition metals such as Ti, Fe and Ni on the de/rehydrogenation characteristics of nano MgH₂. Mechanical milling of MgH₂ with 5 wt% each of Ti, Fe and Ni for 24 h at 12 atm of H₂ pressure lead to the formation of nano MgH₂-Ti5Fe5Ni5. The decomposition temperature of nano MgH₂-Ti5Fe5Ni5 is lowered by 90 °C as compared to nano MgH₂ alone. It is also found that the nano MgH₂-Ti5Fe5Ni5 absorbs 5.3 wt% within 15 min at 270 °C and 12 atm hydrogen pressures. However, nano MgH₂ reabsorbs only 4.2 wt% under identical condition. An interesting result of the present study is that mechanical milling of MgH₂ separately with Fe and Ni besides refinement in particle size also leads to the formation of alloys Mg₂NiH₄ and Mg₂FeH₆ respectively. On the other hand, when MgH₂ is mechanically milled together with Ti, Fe and Ni, the dominant result is the formation of nano particles of MgH₂. Moreover the activation energy for dehydrogenation of nano MgH₂ co-catalyzed with Ti, Fe and Ni is 45.67 kJ/mol which is 35.71 kJ/mol lower as compared to activation energy of nano MgH₂ (81.34 kJ/mol). These results are one of the most significant in regard to improvement in de/rehydrogenation characteristics of known MgH₂ catalyzed through transition metal elements.     

PtxNi1−x nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane

We report on Pt_xNi_1−x (x = 0, 0.35, 0.44, 0.65, 0.75, and 0.93) nanoparticles as catalysts for hydrogen generation from hydrolysis of ammonia borane (NH₃BH₃). The Pt_xNi_1−x catalysts were prepared through a redox replacement reaction with a reverse microemulsion technique. The structure, morphology, and chemical composition of the obtained samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) equipped with energy dispersive X-ray (EDX), and inductively coupled plasma emission spectroscopy (ICP). The results show that the diameters of the Pt_xNi_1−x nanoparticles are about 2–4 nm, and the Pt atomic contents in the catalysts were 35%, 44%, 65%, 75%, and 93%, respectively. It is found that the catalytic activity toward the hydrolysis of NH₃BH₃ is correlated with the composition of the Pt_xNi_1−x catalysts. The annealing of Pt_0.65Ni_0.35 at 300 °C for 1 h increases the crystallinity of the nanoparticles, but shows almost the same activity as that without annealing. Among the as-prepared Pt_xNi_1−x nanoparticles, Pt_0.65Ni_0.35 displays the highest catalytic performance, delivering a high hydrogen-release rate of 4784.7 mL min⁻¹ g⁻¹ and a low activation energy of 39.0 kJ mol⁻¹.     

Microstructure characteristics,hydrogen storage kinetic and thermodynamic properties of Mg80–xNi20Yx (x = 0–7) alloys

Mg_80–xNi₂₀Y_x (x = 0–7) alloys were successfully prepared by using the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy measurements. The effects of yttrium content on the microstructure and hydrogen storage properties of the as-cast alloys were investigated. The alloys are composed of a primary phase of Mg₂Ni, lamella eutectic composites of Mg + Mg₂Ni, and some amount of YNi₃. The YNi₃ has completely transformed into in situ formed nanoscale YH₂ and YH₃, with the formation of Mg₂NiH₄. Pulverization of the alloy particles was observed during the hydrogen absorption and desorption cycles. However, the phase composition remains unchanged even after 20 hydrogenation cycles. Yttrium addition significantly improved the hydrogen-absorption kinetic performance of the alloy. In addition, pressure-composition-temperature measurements indicated that the entropy and enthalpy changes for the formation and decomposition of MgH₂ and Mg₂NiH₄ gradually decreased with the increase of yttrium content.     

Preparation of CuIn1−xGaxSe2 thin films by sputtering and selenization process

CuIn_1−xGa_xSe₂ polycrystalline thin films were prepared by a two-step method. The metal precursors were deposited either sequentially or simultaneously using Cu–Ga (23 at%) alloy and In targets by DC magnetron sputtering. The Cu–In–Ga alloy precursor was deposited on glass or on Mo/glass substrates at either room temperature or 150°C. These metallic precursors were then selenized with Se pellets in a vacuum furnace. The CuIn_1−xGa_xSe₂ films had a smooth surface morphology and a single chalcopyrite phase.     

Annealing effects on Zn1−xMgxO/CIGS interfaces characterized by ultraviolet light excited time-resolved photoluminescence

Annealed Zn_1−xMg_xO/Cu(In,Ga)Se₂ (CIGS) interfaces have been characterized by ultraviolet light excited time-resolved photoluminescence (TRPL). The TRPL lifetime of the Zn_1−xMg_xO/CIGS film increased on increasing the annealing temperature to 250 °C, whereas the TRPL lifetime of the CdS/CIGS film had little change by annealing at temperatures lower than 200 °C. This is attributed to the recovery of physical damages by annealing, induced by sputtering of the Zn_1−xMg_xO film. The TRPL lifetime abruptly decreased with annealing at 300 °C. The diffusion of excess Zn from the Zn_1−xMg_xO film into the CIGS interface is clearly observed in secondary ion mass spectroscopy (SIMS) depth profiles. These results indicate that excess Zn at the vicinity of the CIGS surface acts as non-radiative centers at the interface. The TRPL lifetime of the Zn_1−xMg_xO/CIGS film annealed at 250 °C reached values to be comparable to that of the as-deposited CdS/CIGS film. Performance of the Zn_1−xMg_xO/CIGS cells varied with the annealing temperature in the same manner as the TRPL lifetime. The highest efficiency of the Zn_1−xMg_xO/CIGS solar cells was achieved for annealing at 250 °C. The results of the TRPL lifetime on annealing show that the cell efficiency is strongly influenced by the Zn_1−xMg_xO/CIGS interface states related to the damages and diffusion of Zn.     

New trends to grow the n-CdZn (S1−xSex)2/p-CuIn(S1−xSex)2 heterojunction thin films for solar cell applications

The n-CdZn(S_1−xSe_x) and p-CuIn(S_1−xSe_x)₂ thin films have been grown by the solution growth technique (SGT) on glass substrates. Also the heterojunction (p–n) based on n-CdZn (S_1−xSe_x)₂ and p-CuIn (S_1−xSe_x)₂ thin films fabricated by same technique. The n-CdZn(S_1−xSe_x)₂ thin film has been used as a window material which reduced the lattice mismatch problem at the junction with CuIn (S_1−xSe_x)₂ thin film as an absorber layer for stable solar cell preparation. Elemental analysis of the n-CdZn (S_1−xSe_x)₂ and p-CuIn(S_1−xSe_x)₂ thin films was confirmed by energy-dispersive analysis of X-ray (EDAX). The structural and optical properties were changed with respect to composition ‘x’ values. The best results of these parameters were obtained at x=0.5 composition. The uniform morphology of each film as well as the continuous smooth thickness deposition onto the glass substrates was confirmed by SEM study. The optical band gaps were determined from transmittance spectra in the range of 350–1000 nm. These values are 1.22 and 2.39 eV for CuIn(S_0.5Se_0.5)₂ and CdZn(S_0.5Se_0.5)₂ thin films, respectively. J–V characteristic was measured for the n-CdZn(S_1−xSe_x)₂/p-CuIn(S_1−xSe_x)₂ heterojunction thin films under light illumination. The device parameters V_oc=474.4 mV, J_sc=13.21 mA/cm², FF=47.8% and η=3.5% under an illumination of 85 mW/cm² on a cell active area of 1 cm² have been calculated for solar cell fabrication. The J–V characteristic of the device under dark condition was also studied and the ideality factor was calculated which is equal to 1.9 for n-CdZn(S_0.5Se_0.5)₂/p-CuIn(S_0.5Se_0.5)₂ heterojunction thin films.     

Comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R = La,Nd and Sm) alloys

The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg₁₀NiR (R = La, Nd and Sm) alloys have been studied comparatively. It is found that the Mg₁₀NiNd and Mg₁₀NiSm alloys are in amorphous state but the Mg₁₀NiLa alloy is composed of an amorphous phase and minor crystalline La₂Mg₁₇ after melt-spinning. The alloys can be hydrogenated into MgH₂, Mg₂NiH₄ and a rare earth metal hydride RH_x. The rare earth metal hydride and Mg₂NiH₄ synergistically provide a catalytic effect on the hydrogen absorption–desorption reactions in the Mg−H₂ system. The hydrogen storage kinetics is not influenced by different rare earth metal hydrides but by the particle size of the rare earth metal hydrides.     

Effect of Ni content on the hydrogen storage behavior of ZrCo1−xNix alloys

ZrCo_1−xNi_x (x = 0, 0.1, 0.2 and 0.3) alloys were prepared and their hydrogen storage behavior were studied. ZrCo_1−xNi_x alloys of compositions with x = 0, 0.1, 0.2 and 0.3 prepared by arc-melting method and characterized by X-ray diffraction analysis. XRD analysis showed that the alloys of composition with x = 0, 0.1, 0.2 and 0.3 forms cubic phase similar to ZrCo with traces of ZrCo₂ phase. A trace amount of an additional phase similar to ZrNi was found for the alloy with composition x = 0.3. Hydrogen desorption pressure–composition–temperature (PCT) measurements were carried out using Sievert's type volumetric apparatus and the hydrogen desorption pressure–composition isotherms (PCIs) were generated for all the alloys in the temperature range of 523–603 K. A single sloping plateau was observed for each isotherm and the plateau pressure was found to increase with increasing Ni content in ZrCo_1−xNi_x alloys at the same experimental temperature. A van't Hoff plot was constructed using plateau pressure data of each pressure–composition isotherm and the thermodynamic parameters were calculated for desorption of hydrogen in the ZrCo_1−xNi_x–H₂ systems. The enthalpy and entropy change for desorption of hydrogen were calculated. In addition, the hydrogen absorption–desorption cyclic life studies were performed on ZrCo_1−xNi_x alloys at 583 K up to 50 cycles. It was observed that with increasing Ni content the durability against disproportionation of alloys increases.     

Reversible de-/hydriding characteristics of a novel Mg18In1Ni3 alloy

The present work demonstrates the reversible hydrogen storage properties of the ternary alloy Mg₁₈In₁Ni₃, which is prepared by ball-milling Mg(In) solid solution with Ni powder. The two-step dehydriding mechanism of hydrogenated Mg₁₈In₁Ni₃ is revealed, namely the decomposition of MgH₂ is involved with different intermetallic compounds or Ni, which leads to the formation of Mg₂Ni(In) solid solution or unknown ternary Mg–In–Ni alloy phase. As a result, the alloy Mg₁₈In₁Ni₃ shows improved thermodynamics in comparison with pure Mg. The Ni addition also results in the kinetic improvement, and the minimum desorption temperature is reduced down to 503 K, which is a great decrease comparing with that for Mg–In binary alloy. The composition and microstructure of Mg–In–Ni ternary alloy could be further optimized for better hydrogen storage properties.     

Structural characterization and electrochemical hydrogen storage properties of Mg2Ni1−xMnx(x = 0, 0.125, 0.25, 0.375) alloys prepared by mechanical alloying

Mg₂Ni_1−xMn_x(x = 0, 0.125, 0.25, 0.375) electrode alloys are prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures are characterized by XRD and SEM. XRD analysis results indicate that the substitution of Mn for Ni could inhibit the formation of MgNi₂ phase with the increases of x from 0 to 0.375. Replacing Ni with Mn can also promote the formation of the amorphous phase when x increases from 0 to 0.25 for the MA alloys milled for 48 h. The new phase Mg₃MnNi₂ is formed only when x = 0.375 after 48 h of milling. This new phase belongs to the face-centered cubic lattice (Fd-3m) with the lattice constant a being 1.1484 nm. Estimated from the peaks broadening, the crystallite size and lattice strain of Mg₃MnNi₂ phase are 15.6 ± 3.6 nm and 1.09 ± 0.34%, respectively. Curve fit of XRD shows that amorphous and nanocrystalline Mg₂Ni coexist in the Mg₂Ni_1−xMn_x (x = 0, 0.125, 0.25) alloys milled for 48 h. The SEM observation reveals that all the MA alloys particles are mainly flaky and show cleavage fracture morphology and these particles are agglomerates of many smaller particles, namely subparticles. Electrochemical measurements indicate that all MA alloys have excellent activation properties. The discharge capacities of MA alloys increase with the prolongation of milling time. For 16 h of milling, with the increase of Mn content, the discharge capacities of Mg₂Ni_1−xMn_x (x = 0, 0.125, 0.25, 0.375) MA alloys monotonously decrease. For 24 h of milling, the discharge capacities of the Mg₂Ni_1−xMn_x (x = 0, 0.125, 0.25, 0.375) alloys also show a rough tendency to decrease with the increase of Mn content except Mg₂Ni_0.875Mn_0.125 MA alloy. On the other hand, for 48 h of milling, as the rise of Mn content from x = 0.125 to 0.375, the discharge capacities increase. Mg(OH)₂ is formed during charge/discharge cycles in the KOH solution for all MA alloys. After 48 h of milling, the substitution of Mn for Ni for x = 0.25 improves the cycle stability at the expense of decreasing the discharge capacity. In contrast, Mg₃MnNi₂ phase is relatively stable during charge/discharge cycles and therefore can significantly enhance the cycle stability under simultaneously maintaining a high discharge capacity.     

Structural and photoluminescence study of the quaternary alloys system CuIn(SxSe1−x)2

The full composition range CuIn(S_xSe_1−x)₂ alloy system has been studied using 40 mm length crystal cuts from 10 mm diameter ingots grown by the classical Bridgman method. X-ray diffraction diffractographs show that the CuIn(S_xSe_1−x)₂ compounds have a chalcopyrite structure for each composition x, they exhibit an expansion on the unit cell characteristics by the tetragonal distortion which depends linearly on the electronegativity of the atoms. The photoluminescence spectra is investigated as a function of various compositions, temperature and excitation intensities. Photoluminescence spectra shows a wide variation in the dominant peak location and an overall blue shift with the increase of sulphur content. Photoluminescence CuInS₂ and CuIn(S_0.72Se_0.28)₂ have been studied in detail.