Thermally stable hydrogen compounds obtained under high pressure on the basis of carbon nanotubes and nanofibers

Compounds containing 6.3–6.5 wt % H and thermally stable in vacuum up to 500°C were obtained by annealing graphite nanofibers and single-walled carbon nanotubes in hydrogen atmosphere under a pressure of 9 GPa at temperatures up to 45°C. A change in the X-ray diffraction patterns indicates that the crystal lattice of graphite nanofibers swells upon hydrogenation and that the structure is recovered after the removal of hydrogen. It was established by IR spectroscopy that hydrogenation enhances light transmission by nanomaterials in the energy range studied (400–5000 cm^?1) and results in the appearance of absorption bands at 2860–2920 cm^?1 that are characteristic of the C–H stretching vibrations. The removal of about 40% of hydrogen absorbed under pressure fully suppresses the C–H vibrational peaks. The experimental results are evidence of two hydrogen states in the materials at room temperature; a noticeable portion of hydrogen forms C–H bonds, but the most of the hydrogen is situated between the graphene layers or inside the nanotubes.     

Autoassociation du cyclohexanol. II - caracteristiques spectroscopiques des vibrateurs ν(OH) dans les complexes d'autoassociation.

From a thermodynamic model of associated solution, the molar extinction coefficients for the spectroscopically discernable ν(OH) vibrations of associated species of cyclohexanol in isooctane solutions at 25°C and 45°C are obtained. It is found that the molar extinction coefficients for the end hydroxyl group of open species and for the monomer band are very near (57 M^?1 cm^?1 at 25°C). For the hydrogen bonded hydroxyl groups, molar extinction coefficients at 25°C are found to be, in M^?1 cm^?1 units : 93 for the open dimer ($\text{ν}$ = 3515 cm^?1), 90 for the open trimer ($\text{ν}$ = 3440 cm^?1), 126 for the other i-mers ($\text{ν}$ = 3340 cm^?1).     

Study of the hydrogen solid solution in thulium

Solid solutions of hydrogen in thulium are investigated, mainly by X-ray determinations in the temperature range 20–500°C. Several observations lead us to distinguish between two variaties α and α¹. In the range of existence of the α¹ phase, below the transition temperature of 120°C, the hydrogen interstitials induce in the metal lattice a behaviour different from that of the α phase, as concerns the phase equilibrium, the additional resistivity, and the lattice anisotropy.Several simple models are proposed for the explanation of this anomaly, encountered also in other heavy rate earth-hydrogen systems.     

TPD-NMR study of hydrogen irreversibly adsorbed on Rh/TiO2

TPD-NMR studies are demonstrated which reveal the presence of three irreversibly bound hydrogen species on Rh/TiO₂, two assigned to species produced when rhodium metal is present on the surface and one to surface hydroxyl groups. The two surface species dependent on the presence of Rh desorb with maxima at around 120 arid 270°C, while the remaining species (assigned to hydroxyl groups) desorbs around 550°C.     

Isotope effects in paraelectric relaxation

Through measurements of the complex dielectric susceptibility in the temperature range between 0.3°K and a few °K we have studied the influence of the isotopic substitutions H→D and¹⁶O→¹⁸O on the relaxation time of the hydroxyl dipole in weakly doped KBr and RbCl. The H→D effect is present in both host lattices. The oxygen effect, however, is only for RbCl outside experimental error, indicating that in this host lattice the reorientation of the hydroxyl ion involves a considerable displacement of its center of mass.     

The measurement of cone length and range in etched plastics

Abstract

Group V impurities implanted at 400 keV into silicon have been detected in substitutional lattice positions by EPR. Three samples of VFZ, p-type 1200–1500 ohm-cm silicon from the same ingot were implanted with As⁷⁵, Sb¹²¹, and Sb¹²³, respectively. The EPR spectrum of each implanted substitutional impurity was observed after annealing the lattice damage. Only the isotope implanted in each sample was seen. Since only those donors which are electrically active can be observed, this technique measures the electrically active fraction of the implanted species. Upon annealing to 970°C, most of the antimony was active whereas only about 1/5 of the arsenic was observed. Comparisons with backscattering results indicate that between 350 and 600°C, ～95 per cent of the implanted antimony is substitutional but ～0 per cent is electrically active. The increase in electrical activity at 600°C is due to the rise of the Fermi level to the donor level as the residual lattice damage anneals. The paramagnetic damage centers observed were those also seen in oxygen-implanted silicon, Si-P3 and Si-Pl, but the Si-P3 center was not as well resolved and grows upon annealing to 200°C.     

Initial stages of hydrogen reduction of NiO(100)

The reduction of single crystal NiO(100) under hydrogen has been followed by AES, XPS and LEED for the pressure range of 1.0 × 10^?7 to 1.3 × 10^?6 Torr and for substrate temperatures of 150–350°C. The kinetics of reduction are controlled both by the rate of removal of lattice oxide at the surface and by the diffusion of subsurface oxygen to the oxygen-depleted surface. The rate of oxygen removal is first-order in surface oxide concentration and in hydrogen pressure. An induction period precedes the reduction reaction and its length is postulated to be controlled by surface defect concentration. The stoichiometric and reduced lattice oxygen species appear to be chemically identical and give a single symmetric XPS peak at 529.4 eV. Nickel spectra indicate a shift in XPS binding energies from those expected of the oxide to those of nickel metal early in the reduction process, although LEED indicates the NiO(100) surface lattice to remain the stable structure for surface reduced to approximately 20% of the stoichiometric oxygen concentration. Ni(100) island formation is observed, with Ni 〈010〉 and 〈001〉 directions along the NiO 〈010〉 and 〈001〉, respectively, but only after the NiO surface is severely depleted in oxygen.     

Hot electron studies of lattice damage created in silicon by implantation of 2.8 MeV protons

Abstract

In this paper we report the results of a study of the annealing properties of the ionized defect density associated with the damage created in the silicon lattice by implantation of 2.8 MeV protons at room temperature. In particular, the annealing of damage created by implanting to a level of 4.43 × 10¹² protons/cm² is reported. The resulting isochronal annealing curve covered the temperature range from 70°C to 460°C. Two major annealing stages are discussed, one a broad stage between 70°C to 200°C and the other an abrupt annealing stage between 440°C to 460°C. Between the temperature range 200°C to 440°C the number of ionized defects remained relatively constant. Above 460°C no detectable effects of the proton implantation remained.     

Temperature dependence of the hyperfine quadrupolar interaction in K2ZrF6

The thermal dependence of the hyperfine quadrupolar interaction of¹⁸¹Ta at Zr sites in K₂ZrF₆ has been studied by TDPAC between RT and 511 °C. Four phases of ever increasing symmetry have been found in the range RT-437 °C. The possible existence of a healing mechanism with an activation energy of 0.187±0.040 eV is discussed for the lattice recovery in the range 270 °C–335 °C. The results at temperatures above 437 °C can be understood assuming a thermal decomposition of the compound.     

Curie temperatures of CoPt ultrathin continuous films

The effects of layer thickness and thermal annealing on Curie temperature have been studied for CoPt ultrathin continuous layers in AlN/CoPt multilayer structures. It is found that there exists a critical thickness below which Curie temperature rapidly decreases due to the loss of spin-spin interactions in the vicinity of interface. After high temperature annealing, the in-plane lattice constant of CoPt film is increased and the exchange coupling parameter is decreased. Consequently, Curie temperatures decrease for some films with large thickness, compared with as-deposited state. Upon annealing at 600?^°C, CoPt undergoes ordering transformation, which also contributes to the degradation of the Curie temperature. However, when the CoPt film thickness is below 2?nm, the Curie temperature is increased after annealing. Especially for 1?nm thick film, the Curie temperature is strikingly increased from 173?^°C to 343?^°C after annealing at 600?^°C. This effect is attributed to the out-of-plane lattice deformation of CoPt thin layers in AlN/CoPt multilayer structures.     

Impedance spectroscopy study of gadolinia-doped ceria

The electrical properties of Gd_0.1Ce_0.9O_1.95 were investigated in various gas atmospheres using a.c. impedance spectroscopy. In dry oxygen, impedance spectra consisted of two arcs at low temperatures corresponding to bulk and grain boundary behaviour, respectively, and only one arc at high temperatures (T>550 °C). The results showed a high oxygen ion conductivity in oxygen atmosphere more than 10⁻² S/cm at 600 °C. In water atmospheres, the bulk arc in dry oxygen was reduced by 30 %, and an additional arc corresponding to the grain boundary effect appeared. It seems that the presence of water may decrease the bulk resistance, but causes an additional grain boundary resistance and thus increase in the total resistance. In hydrogen atmosphere, a conductivity enhancement up to one order of magnitude was observed. In hydrogen there is a reduction of Ce⁴⁺ to Ce³⁺ in the sample, which involves defect formation generating e.g. electrons, additional oxygen vacancies and at the same time protons in the lattice. These new charge carriers are responsible for a significant conductivity enhancement. Therefore, in a hydrogen atmosphere the material is a mixed (oxygen ion + proton + electron) conductor. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

New synthesis of high-quality storage phosphors

We present a single step synthesis method for the photostimulable X-ray storage phosphor BaFBr:Eu²⁺ which results in a highly sensitive powder with a relatively small average grain size of 5.4 μm. The starting chemical reagents are BaCO₃, NH₄F, NH₄Br and EuF₃. The reaction initiated by the decomposition of the ammonium materials leads to highly volatile hydrogen halide gases which react with the BaCO₃ to form BaFBr at temperatures up to 300 °C. Further heating results in the incorporation of Eu²⁺ and the formation of halide vacancies at temperatures in between 390 and 580 °C. The resulting photostimulated luminescence (PSL) efficiency is optimized after sintering at 800 °C. The reaction process is monitored by differential thermal analysis (DTA) and the reaction products are detected by mass-spectroscopy which confirms the proposed chemical reactions. Intermediate and final products are identified using X-ray diffraction. Photoluminescence (PL) and PSL spectra show the incorporation of Eu²⁺ into the lattice, as well as a PL peak at 470 nm which is not present in the PSL spectrum. This peak is shown to originate from O^2? in the lattice and directly affects the PSL sensitivity.     

Intermolecular Hydrogen Bonding of Poly(Imino Imino Ketone Ketone)

Molecular dynamics calculations were made of the distance from the hydrogen atom of the imino group to the distance of the oxygen atom of the carbonyl group of poly(imino imino ketone ketone) (PIIKK). The results showed that theoretically there was hydrogen bond formation between the hydroxyl group and the carbonyl group of PIIKK. The existence of the hydrogen bonding between the polymer chains of PIIKK was confirmed by infrared spectrum analysis. Due to the existence of the hydrogen bonding, the glass transition temperature increased to 273°C from 143°C for poly(ether ether ketone) (PEEK).     

CEMS study of defect annealing in Fe implanted AlN

An AlN thin film grown on sapphire substrate was implanted with 45 keV ⁵⁷Fe and ⁵⁶Fe ions at several energies to achieve a homogeneous concentration profile of approximately 2.6 at.%. in the AlN film. Conversion electron Mössbauer Spectroscopy data were collected after annealing the sample up to 900 °C. The spectra were fitted with three components, a single line attributed to small Fe clusters, and two quadrupole split doublets attributed to Fe substituting Al in the wurtzite AlN lattice and to Fe located in implantation induced lattice damage. The damage component shows significant decrease on annealing up to 900 °C, accompanied by corresponding increases in the singlet component and the substitutional Fe.     

Ultrasonic-assisted reduction for facile synthesis of ultrafine supported Pd nanocatalysts by hydroxyl groups on the surfaces of layered double hydroxides and their catalytic properties

Layered double hydroxide (LDH)-supported Pd nanocatalysts (Pd/LDH-OH) were prepared by ultrasonic-assisted reduction at 30 °C using an ultrasonic bath at a frequency of 25 kHz and an input power of 400 W for 30 min without the addition of any stabilizing reagent or chemical reductant, using LDH with a layered structure and interparticle mesoporosity as the reductant and carrier. This kind of pore structure allows ultrasound waves to spread inside the pore and make ultrasound directly act on the surface hydroxyl groups of LDH, producing highly reductive free radicals (H). The reductive free radicals rapidly reduced Pd²⁺ to Pd⁰, forming ultrafine Pd nanoparticles (PdNPs) with a particle size distribution of 1.85 nm–3.45 nm and an average particle size of 2.52 nm. The surface hydroxyl groups were converted to exposed oxygen groups after dissociation of hydrogen radicals, which is beneficial for anchoring and dispersing the resultant PdNPs. The resultant PdNPs were uniformly dispersed on the surface of the LDH carrier. The yield of the Suzuki coupling reaction between 4-bromotoluene and phenylboronic acid catalyzed by Pd/LDH-OH at 60 °C was 95.49% for 5 min and the TOF was 190.98 min⁻¹. After repeated for 5 times, the yield was maintained at 84.59%. The prepared Pd/LDH-OH nanocatalyst and the catalytic system are useful for Suzuki-Miyaura coupling reactions of N- and S-heterocyclic substrates. This provides an efficient and green approach for the preparation of supported nanopalladium catalysts.     

Hydrogen bonding in oxalic acid and its complexes: A database study of neutron structures

The basic result of carboxylic group that the oxygen atom of the -OH never seems to be a hydrogen bond acceptor is violated in the cases, namely urea oxalic acid and bis urea oxalic acid complexes, where the hydroxyl oxygen atom is an acceptor of a weak N—H… O hydrogen bond. The parameters of this hydrogen bond, respectively in these structures are: hydrogen acceptor distance 2.110 Å and 2.127 Å and the bending angle at hydrogen, 165.6° and 165.8°. The bond strength around the hydroxyl oxygen is close to 1.91 valence units, indicating that it has hardly any strength left to form hydrogen bonds. These two structures being highly planar, force the formation of this hydrogen bond. As oxalic acid is the common moiety, the structures of the two polymorphs, α-oxalic acid and β-oxalic acid, also were looked into in terms of hydrogen bonding and packing.     

Recrystallization of hexagonal silicon carbide after gold ion irradiation and thermal annealing

Silicon carbide (SiC) single crystals with the 6H polytype structure were irradiated with 4.0-MeV Au ions at room temperature (RT) for increasing fluences ranging from 1?×?10¹² to 2?×?10¹⁵ cm^?2, corresponding to irradiation doses from ~0.03 to 5.3 displacements per atom (dpa). The damage build-up was studied by micro-Raman spectroscopy that shows a progressive amorphization by the decrease and broadening of 6H-SiC lattice phonon peaks and the related growth of bands assigned to Si–Si and C–C homonuclear bonds. A saturation of the lattice damage fraction deduced from Raman spectra is found for ~0.8?dpa (i.e. ion fluence of 3?×?10¹⁴ cm^?2). This process is accompanied by an increase and saturation of the out-of-plane expansion (also for ~0.8?dpa), deduced from the step height at the sample surface, as measured by phase-shift interferometry. Isochronal thermal annealing experiments were then performed on partially amorphous (from 30 to 90%) and fully amorphous samples for temperatures from 200 °C up to 1500 °C under vacuum. Damage recovery and densification take place at the same annealing stage with an onset temperature of ~200 °C. Almost complete 6H polytype regrowth is found for partially amorphous samples (for doses lower than 0.8 dpa) at 1000 °C, whereas a residual damage and swelling remain for larger doses. In the latter case, these unrelaxed internal stresses give rise to an exfoliation process for higher annealing temperatures.     

Lattice location and dopant behavior of group II and VI elements implanted in silicon

Outdiffusion, lattice location and electrical behavior of Zn, Cd, Hg and Se, Te implanted into silicon at 50 keV were investigated by backscattering and channeling effect of 1 MeV He⁺ ions and by Hall effect and sheet resistivity measurements. All the species exhibited outdiffusion with thermal processing. A significant fraction of Zn, Cd and Hg, when implanted into a substrate of 350°C, occupied regular interstitial lattice sites, while 50–60 per cent of the Se and Te atoms were on substitutional lattice sites. Selenium implanted at room temperature and mercury implanted into a substrate of 350°C exhibited depth dependent lattice location. The implanted layers showed n-type behavior: the maximum value of number of carriers/cm² was less than the number of implanted ions/cm² in all cases. The highest electrical activity was observed for Se corresponding to 25 per cent of the substitutional component.     

Crystal lattice defects and Hall mobility of electrons in Si: Er/Si layers grown by sublimation molecular-beam epitaxy

The density of crystal lattice defects in Si: Er layers grown through sublimation molecular-beam epitaxy at temperatures ranging from 520 to 580°C is investigated by a metallographic method, and the Hall mobility of electrons in these layers is determined. It is found that the introduction of erbium at a concentration of up to ～5 × 10¹⁸ cm^?3 into silicon layers is not accompanied by an increase in the density of crystal lattice defects but leads to a considerable decrease in the electron mobility.     

Intrinsic defects, impurities and doping in ZnO nanorods grown at low temperature

We report on the defect properties of single-crystalline ZnO nanorods grown from solutions at temperatures below 90 °C. The nanorods can easily be doped by providing impurity precursors during growth. In the as-grown state the nanorods exhibit considerable lattice strain and distortions which compromise their electrical and optical properties. Upon annealing at moderate temperatures of <400 °C the lattice strain is converted into dislocation-type defects, and the dopant impurities become optically active. In the annealed state the near-bandgap photoluminescence quantum efficiency is improved more than 5 times and reaches ～16 % at room temperature. Thus with moderate annealing, interesting device applications become feasible for nanorods grown at T<90 ^°C.