Possibilities of atomic hydrogen storage by carbon nanotubes or graphite materials

Atomic hydrogen storage by carbon nanotubes (CNTs) and highly oriented pyrolytic graphite (HOPG) has been studied using a flow catalytic reactor and an ultra-high vacuum surface science apparatus including scanning tunneling microscope (STM), respectively. Defect sites on CNTs as adsorption sites of atomic hydrogen are introduced by oxidation pretreatment using La catalyst. Pd catalysts are then deposited on CNT surfaces for dissociation of H₂ into atomic hydrogen, which spills over to the defect sites. In the best case, 1.5 wt% of hydrogen is stored in the defective CNT with Pd particles at 1 atm and 573 K. In temperature programmed desorption (TPD) experiments, H₂ starts to desorb at 700–900 K depending on the annealing temperatures of CNTs prior to hydrogen storage. On the HOPG surface, hot atomic hydrogen produced by dissociation of H₂ using tungsten wire desorbs from graphite terraces at 400–700 K, which is much lower than that on CNTs. It is possible that one can decrease the desorption temperature by changing the method of H₂ dissociation.     

Adsorption equilibrium of hydrogen on graphene sheets and activated carbon

For obtaining the technical data to evaluate the performance of hydrogen storage by adsorption on graphene sheets (GS), analysis of adsorption equilibrium of hydrogen on the GS and the activated carbon were carried out based on the hydrogen adsorption data covering a wide temperature range. The GS and SAC-02 activated carbon, which respectively had a specific surface area about 300 m²/g and 2074 m²/g, were selected as adsorbents. Six adsorption isotherms of excess amounts of high purity hydrogen were measured at temperature from 77.15 K to 293.15 K and pressure up to 6 MPa. Parameters of Langmuir, Langmuir–Freundlich and Toth equations were set by non-linear fit against adsorption data, predicting accuracy of the equations was then evaluated by the accumulated relative errors between experimental data and those from the equations under different pressure regions. Absolute adsorption amounts determined by the modified equation were used to calculate the isosteric heat of adsorption.It shows that both adsorption isotherms of hydrogen on the GS and the activated carbon have the features of Type I, but the trend of isotherms varying over the pressure is different within the lower temperature region. Results from Langmuir equation have the largest error. Toth equation can much accurately predict the adsorption data with an overall accumulated relative error less than 4%. The value of the isosteric heat of hydrogen adsorption on the GS is about 5.06–6.37 kJ/mol, which is much higher than 4.05–5.52 kJ/mol for hydrogen on the SAC-02 activated carbon under the whole experimental condition. It reveals that interaction between hydrogen molecules and the graphene layer is stronger than that of hydrogen and carbon surface, and Toth equation could be appropriate to analyzing adsorption equilibrium for hydrogen on carbon based adsorbents.     

A facile assembly of polyimide/graphene core–shell structured nanocomposites with both high electrical and thermal conductivities

Polyimide/reduced graphene oxide (PI/r-GO) core–shell structured microspheres were fabricated by in-situ reduction of graphene oxide (GO), which was coated on the surface of PI microspheres via hydrogen bonding and π–π stacking interaction. The highly ordered 3D core–shell structure of PI/r-GO microspheres with graphene shell thickness of 3 nm was well characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM) and Raman spectra. The glass transition temperature (T_g) of PI/r-GO microspheres was slightly increased because of the interaction of r-GO and PI matrix while the temperature at 5% weight loss (T_5%) of PI/r-GO microspheres was decreased due to the side effect of reductant hydrazine hydrate. The PI/r-GO nanocomposites exhibited highly electrical conductivity with percolation threshold of 0.15 vol% and ultimate conductivity of 1.4 × 10⁻² S/m. Besides, the thermal conductivity of PI/r-GO nanocomposites with 2% weight content of r-GO could reach up to 0.26 W/m K.     

Geometries,stabilities, and electronic properties of YnSi (n = 2–14) clusters: Density-functional theory investigations

Using the density-functional approach, the geometries, stabilities, electronic properties, and magnetism of the Y_nSi (n = 2–14) clusters have been systematically investigated. The growth pattern for the different-sized Y_nSi (n = 2–14) clusters is Si-substituting Y_n+1 clusters and keeps the similar frameworks of the most stable Y_n+1 clusters. The Si atom substitutes the surface atom of the Y_n+1 clusters for n < 8. Starting from n = 8, the Si atom completely falls into the center of the Y-frame. The Si atom substitutes the center atom of the Y_n+1 clusters to form the Si-encapsulated Y_n geometries for n > 8. The calculated results show that doping of Si atom contributes to strengthening the stabilities of the yttrium framework. In addition, the relative stability of Y₁₂Si is the strongest among the investigated Y_nSi clusters, which might stem from its highest symmetry. Mulliken population analysis shows that charges always transfer from Y atoms to Si atom in all the Y_nSi (n = 2–14) clusters. Doping of Si atom decreases the magnetic moments of the most Y_n clusters. In particular, the magnetic moment does quench completely after doping Si in Y₁₃, which is ascribed to the disappearance of the itinerant 4d electron spin exchange effect. Finally, the frontier orbitals properties of Y_nSi are also discussed.     

Study on dynamic strain aging phenomenon of 3004 aluminum alloy

3004 Aluminum alloy has been subjected to tension test at a range of strain rates (5.56 × 10⁻⁵ to 5.56 × 10⁻³ s⁻¹) and temperatures (233–573 K) to investigate the effect of temperature and strain rate on its mechanical properties. The serrated flow phenomenon is associated with dynamic strain aging (DSA) and yield a negative strain rate dependence of the flow stress. In the serrated yielding temperature region a critical transition temperature, T_t, was found. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than T_t. According to the activation energy, it is believed that the process at the temperature region lower than T_t is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, however, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations.     

Phase change and optical band gap behavior of Se0.8S0.2 chalcogenide glass films

Se_0.8S_0.2 chalcogenide glass films have been prepared by thermal vacuum evaporation technique with thickness 583 nm. Annealing process at T ≥ 333 K crystallizes the films and nanostructured films are formed. The crystallite size was increased to 24 nm as the annealing temperature increased to 373 K. Orthorhombic crystalline system was identified for the annealed films. SEM micrographs show that films consist of two parallel surfaces and the thickness was determined by cross section imaging. The optical transmittance is characterized by interference patterns as a result of these two parallel surfaces, besides their average value at longer wavelength decreases as a result of annealing process. The band gap, E_g is red shifted due to crystallization by annealing. As the phase of the films changes from amorphous to crystalline in the annealing temperature range 333–363 K, a non sharp change of the band gap (E_g) is observed. This change was explained by Brus’s model of the energy gap confinement behavior of the nanostructured films. The optical refractive index increases suddenly when the system starts to be crystallized by annealing.     

Phase structure and hydrogen absorption property of LaMg2Cu

LaMg₂Cu alloy was prepared by inductive melting and then was annealed at 723 K for 10 h in 0.1 MPa argon atmospheres. X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the alloy consisted of LaMg₂Cu₂ phase, LaMg₃ phase and a few of unknown phases. The annealing treatment improved the equilibrium pressure and hydrogen absorption capacity of LaMg₂Cu alloy. The hydrogen absorption capacity of the as-cast and annealed alloys at 473 K were 2.86 and 3.33 wt.%, and the equilibrium pressure were 1 and 3 MPa, respectively. The enthalpy and entropy of LaMg₂Cu–H hydriding reaction were determined. LaMg₂Cu alloy could absorb hydrogen with rapid hydriding kinetics, the hydrogen absorption rate of LaMg₂Cu increased from 423 K to 498 K and the uptake time for hydrogen content to reach 90% of the maximum storage capacity for annealed alloy was less than 550 s at 498 K. The experimental curves of hydrogen absorption kinetics could be fitted with good accuracy by Jander equation. It suggested that the hydriding of LaMg₂Cu alloy was a three-dimensional diffusion-controlled process. And the activation energy and pre-exponential factor of LaMg₂Cu alloy were also calculated.     

Epitaxial matching of small metallic nanoclusters in large-misfit systems

The deposition at low energies of Cu and Au nanoclusters, respectively, on Au(0 0 1) and Cu(0 0 1) substrates is studied by constant-temperature molecular-dynamics simulations. Initially, clusters had icosahedral or Wulff shapes and their number of atoms ranged between 13 and 1289. The deposition energy and the temperature were, respectively, 17 meV/atom and 300 K. Atomic interactions are mimicked by a many-body potential based on the tight-binding model. A different behaviour of the clusters has been found as a function of the number of atoms and of the material. Below 100 atoms, Cu clusters align all their {2 0 0} planes with the substrate but do not achieve epitaxy since either their lattice structure becomes bcc or stacking faults arise. On the contrary, Au clusters with similar number of atoms grow epitaxially but hardly change the distances parallel and perpendicular to the interface in their unit cell. Cu clusters, for their part, fit the parallel distances to the Au lattice parameter. For larger clusters, in general, the alignment or epitaxy is not complete even in the cases of more favourable landing.     

Significant blue-shift in photoluminescence excitation spectra of Nd3+:LaF3 potential laser medium at low-temperature

Temperature-dependent optical properties of bulk Nd³⁺:LaF₃ crystals are reported. A blue-shift in the photoluminescence excitation (PLE) spectrum is observed at 30 K. The 173.2-nm emission peak wavelength at 300 K shifted to 172.8 nm at 30 K, consistent with the 6-nm blue-shift in transmission edge and 2437-cm⁻¹ increase in the lowest energy level of the 4f²5d configuration. Thermal broadening of the 5d–4f emission bands with increasing temperature is also observed as the dip at around 178.5 nm present at 30 K disappears at 300 K. A smaller spectral overlap between the PLE and emission spectra is observed as temperature is decreased. Our results suggest that absorption cross-section at the peak fluorescence wavelength is expected to decrease at 30 K.     

Temperature dependence of electronic band transition in Mn-doped SnO2 nanocrystalline films determined by ultraviolet-near-infrared transmittance spectra

Mn-doped SnO₂ (SMO) nanocrystalline films with the composition from 2.5 to 12.5% have been prepared on quartz substrates by pulsed laser deposition. The temperature dependence of electronic structures and optical constants in the SMO films have been investigated by transmittance spectra from 5.3 to 300 K. Optical response functions have been extracted by fitting the transmittance spectra in the photon energy range of 0.5–6.5 eV with the Adachi's model. It was found that the absorption edge presents a red-shift trend with increasing Mn composition, and the optical band gap (OBG) is varied between 4.22 and 3.44 eV. Moreover, as the Mn composition increases, the temperature dependence of OBG becomes weaker. The band gap narrowing value [(5.3 K)–(300 K)] has been reduced from 98 to 3 meV and linearly decreases with the Mn composition. The phenomena could be attributed to the transition from low doping level SnO₂ band-like states to Mn-related localized states. Moreover, the Urbach energy shows the degree of the structural disorder, which could be explained by an empirical formulas in different temperature regimes.     

Cathodoluminescence study of h- and c-GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N₂ in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN₃ as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h- and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.     

Preparation and characterization of LTA-type zeolite framework dispersed ruthenium nanoparticles and their catalytic application in the hydrolytic dehydrogenation of ammonia–borane for efficient hydrogen generation

The safe and efficient hydrogen storage and production are major obstacles to use hydrogen as an energy carrier. Therefore, significant efforts have been focused on the development of new materials for the chemical hydrogen storage and production. Of particular importance, ammonia–borane (NH₃BH₃) is emerging as one of the most promising solid hydrogen carrier due to its high gravimetric hydrogen storage capacity (19.6 wt.%) and low molecular weight (30.8 g/mol). ammonia–borane can release hydrogen gas upon catalytic hydrolysis under mild conditions. Herein, the discovery of a new catalytic material, ruthenium nanoparticles stabilized by ZK-4 zeolite framework, for this important reaction has been reported. This new catalyst system was prepared by borohydride reduction of ruthenium(III)-exchanged ZK-4 zeolite in water at room temperature. The characterization of the resulting material by advanced analytical tools shows the formation of ZK-4 zeolite dispersed ruthenium nanoparticles (2.9 ± 0.9 nm). The catalytic performance of the resulting supported ruthenium nanoparticles depending on activity, lifetime and reusability was demonstrated in the hydrolytic dehydrogenation of ammonia–borane. They were found to be highly active (initial TOF = 5410 h^?1), long-lived (TTO = 36,700) and reusable catalyst (retaining of >85% of initial activity in the 5th reuse) in this important catalytic reaction at room temperature under air.     

In situ fabrication and characterization of cobalt ferrite nanorods/graphene composites

Cobalt ferrite nanorods/graphene composites were prepared by a one-step hydrothermal process using NaHSO₃ as the reducing agent and 1-propyl-3-hexadecylimidazolium bromide as the structure growth-directing template. The reduction of graphene oxide and the in situ formation of cobalt ferrite nanorods were accomplished in a one-step reaction. The structure and morphology of as-obtained composites were characterized by field emission scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy, atomic force microscope, X-ray diffractometer, Fourier transform infrared spectra, X-ray photoelectron spectroscopy and Raman spectroscopy. Uniform rod-like cobalt ferrites with diameters of about 100 nm and length of about 800 nm were homogeneously distributed on the graphene sheets. The hybrid materials showed a saturation magnetization of 42.5 emu/g and coercivity of 495.1 Oe at room temperature. The electromagnetic parameters were measured using a vector network analyzer. A minimum reflection loss (RL) of − 25.8 dB was observed at 16.1 GHz for the cobalt ferrite nanorods/graphene composites with a thickness of 2 mm, and the effective absorption frequency (RL < − 10 dB) ranged from 13.5 to 18.0 GHz. The composites exhibited better absorbing properties than the cobalt ferrite nanorods and the mixture of cobalt ferrite nanorods and graphene.     

Cryogenic design and test results of 30-m flexible hybrid energy transfer line with liquid hydrogen and superconducting MgB2 cable

In this paper we present the development of a new hybrid energy transfer line with 30 m length. The line is essentially a flexible 30 m hydrogen cryostat that has three sections with different types of thermal insulation in each section: simple vacuum superinsulation, vacuum superinsulation with liquid nitrogen precooling and active evaporating cryostatting (AEC) system. We performed thermo-hydraulic tests of the cryostat to compare three thermo-insulating methods. The tests were made at temperatures from 20 to 26 K, hydrogen flow from 70 to 450 g/s and pressure from 0.25 to 0.5 MPa. It was found that AEC thermal insulation was the most effective in reducing heat transfer from room temperature to liquid hydrogen in ∼10 m section of the cryostat, indicating that it can be used for long superconducting power cables. High voltage current leads were developed as well. The current leads and superconducting MgB₂ cable passed high voltage DC test up to 50 kV DC. Critical current of the cable at ∼21 K was 3500 A. It means that the 30 m hybrid energy system developed is able to deliver ∼50–60 MW of chemical power and ∼50–75 MW of electrical power, i.e. up to ∼135 MW in total.     

Mechanical properties of aluminium–copper–lithium alloy AA2195 at cryogenic temperatures

Tensile testing was performed on a 4 mm thick sheet of the aluminum–lithium alloy AA2195 in T87 (solution treatment + water quenching + 7% cold work + peak aging) temper which was subjected to 7% cold working by combination of cold rolling and stretching, over a temperature range from ambient to liquid hydrogen (20 K) conditions. Properties were evaluated in longitudinal as well as transverse directions to characterize anisotropy with respect to strength and ductility. Strength and ductility were compared to the conventional aluminum alloy AA2219-T87, developed for similar cryogenic applications. Decreases in test temperature led to higher strengths with little or no change in ductility. As the temperature decreases, the differences between ultimate tensile strength as well as yield strength for two different combinations of cold roll and stretch studied in the present work, narrows down and become equal at 20 K.     

Development and parametric study of the convection-type stationary adiabatic demagnetization refrigerator (ADR) for hydrogen re-condensation

The adiabatic demagnetization refrigerator (ADR) system in this paper is composed of a conduction-cooled current cycling high-temperature superconducting (HTS) magnet system, a magnetic bed assembly, its heat exchange parts and an auxiliary precooling stage (a commercial GM cryocooler and a liquid nitrogen vessel). The whole magnetic refrigeration system including the conduction-cooled HTS magnet is cooled by the precooling stage to absorb the rejection heat of the ADR cycle. The packed bed type magnetic bed consists of tiny irregular powders of Dy_0.9Gd_0.1Ni₂ enclosed in a thin walled stainless steel container (22.2 mm in O.D., 0.3 mm in thickness and 40.0 mm in height). The precooled heat transfer fluid (helium) travels through the magnetic material when heat rejection is required; otherwise the helium stagnates within its pores (pseudo-adiabatic process). Flow of the heat transfer fluid substitutes for the function of a traditional heat switch, creating, essentially, a forced-convection type heat switch. The magnetic bed assembly is periodically magnetized and demagnetized at the center of the conduction-cooled HTS magnet which can stably generate both strong and alternating magnetic field from 0 T to 3.0 T (0–130 A) with an average ramp rate of 0.24 T s⁻¹. The cooling capacities of the ADR system at 20 K which is the normal boiling point (NBP) of hydrogen, are 11.1 J cycle⁻¹, 6.3 J cycle⁻¹ and 1.9 J cycle⁻¹ when the temperature spans are 1 K, 2 K and 3 K, respectively. We describe the detailed construction of the ADR system and discuss the test results with the operational parameters (the entrained helium pressure, the mass flow rate of helium and the operating temperature span) in the 20 K region.     

Magnetic refrigerator for hydrogen liquefaction

This paper reviews the status of magnetic refrigeration system for hydrogen liquefaction. There is no doubt that hydrogen is one of most important energy sources in the near future. In particular, liquid hydrogen can be utilized for infrastructure construction consisting of storage and transportation. When we compare the consuming energy of hydrogen liquefaction with high pressurized hydrogen gas, FOM must be larger than 0.57 for hydrogen liquefaction. Thus, we need to develop a highly efficient liquefaction method. Magnetic refrigeration using the magneto-caloric effect has potential to realize not only the higher liquefaction efficiency >50%, but also to be environmentally friendly and cost effective. Our hydrogen magnetic refrigeration system consists of Carnot cycle for liquefaction stage and AMR (active magnetic regenerator) cycle for precooling stages. For the Carnot cycle, we develop the high efficient system with >80% liquefaction efficiency by using the heat pipe. For the AMR cycle, we studied two kinds of displacer systems, which transferred the working fluid. We confirmed the AMR effect with the cooling temperature span of 12 K for 1.8 T of the magnetic field and 6 s of the cycle. By using the simulation, we estimate the efficiency of the hydrogen liquefaction plant for 10 kg/day. A FOM of 0.47 is obtained for operation temperature between 20 K and 77 K including LN₂ work input.     

Experimental study of one-stage VM cryocooler operating below 8 K

The Vuilleumier (VM) refrigerator, known as heat driven refrigerator, is one kind of closed-cycle Stirling type regenerative refrigerator. The VM refrigerator with power being supplied by liquid nitrogen was proposed by Hogen and developed by Zhou, which shows great potential for development below 10 K. This paper describes the experimental development of a VM cryocooler operating below 8 K, which was achieved by using liquid nitrogen as a heat sink of middle cavity. The regenerator was optimized by using a part of metallic magnetic regenerator material Er₃Ni to replace the lead sphere and a no-load temperature of 7.8 K was obtained. Then all the lead spheres were replaced by Er_0.6Pr_0.4 material and a no-load temperature of 7.35 K was obtained, which is the lowest temperature for this kind of refrigerator reported so far. The cooling power at 10 K is about 500 mW with a pressure ratio near 1.6 and a charge pressure of 1.8 MPa. Especially, the magnetic material Er_0.6Pr_0.4 was found to be a potential substitution for the conventional lead.     

Equilibrium,kinetic and thermodynamic studies on the adsorption of phenol onto graphene

Graphene, a new member of carbon family, has been prepared, characterized and used as adsorbent to remove phenol from aqueous solution. The effect parameters including pH, dosage, contact time, and temperature on the adsorption properties of phenol onto graphene were investigated. The results showed that the maximum adsorption capacity can reach 28.26 mg/g at the conditions of initial phenol concentration of 50 mg/L, pH 6.3 and 285 K. Adsorption data were well described by both Freundlich and Langmuir models. The kinetic study illustrated that the adsorption of phenol onto graphene fit the pseudo second-order model. The thermodynamic parameters indicated that the adsorption of phenol onto graphene was endothermic and spontaneous.     

Cryogenic flat-panel gas-gap heat switch

A compact additive manufactured flat-panel gas-gap heat switch operating at cryogenic temperature is reported in this paper. A guarded-hot-plate apparatus has been developed to measure the thermal conductance of the heat switch with the heat sink temperature in the range of 100–180 K. The apparatus is cooled by a two-stage GM cooler and the temperature is controlled with a heater and a braided copper wire connection. A thermal guard is mounted on the hot side of the device to confine the heat flow axially through the sample. A gas handling system allows testing the device with different gas pressures in the heat switch. Experiments are performed at various heat sink temperatures, by varying gas pressure in the gas-gap and with helium, hydrogen and nitrogen gas. The measured off-conductance with a heat sink temperature of 115 K and the hot plate at 120 K is 0.134 W/K, the on-conductance with helium and hydrogen gases at the same temperatures is 4.80 W/K and 4.71 W/K, respectively. This results in an on/off conductance ratio of 37 ± 7 and 35 ± 6 for helium and hydrogen respectively. The experimental results matches fairly well with the predicted heat conductance at cryogenic temperatures.