Reactive Mechanical Alloying Synthesis of Nanocrystalline Cubic Zirconium Nitride

Zirconium nitride powders with rock salt structure （γ-ZrNx） are prepared by mechanical milling of a mixture of Zirconium and hexagonal boron nitride （h-BN） powders. The products are analysed by x-ray diffraction （XRD）, scanning electron microscopy （SEM）, and Raman spectroscopy （ITS）. The formation mechanism of γ-ZrNx by ball milling technique is investigated in detail. N atoms diffuse from amorphous BN （a-BN） into Zr to form Zr（N） solid solution alloy, then the Zr（N） solid solution alloy decomposes into γf-ZrNx. No ZrB2 is observed in the as-milled samples or the samples annealed at 1050° C for 2h.     

Blue photoluminescence in Ti-doped alkaline-earth stannates

Blue photoluminescence properties of Ti-doped alkaline-earth stannates, A₂(Sn_1−xTi_x)O₄ (A=Ca, Sr, Ba) (x=0.005-0.15), were examined at room temperature. These stannates showed intense broad emission bands peaking at 445 nm for Ca₂SnO₄, at 410 nm for Sr₂SnO₄, and at 425 nm for Ba₂SnO₄ under UV excitation. Emission intensities were relatively insensitive to Ti concentration and no sharp concentration quenching was observed. Mixing alkaline-earth ions in the crystal structures did not increase the emission intensities in the A₂(Sn_1−xTi_x)O₄ system. The excitation spectra of these stannates exhibited broad bands just below the fundamental absorption edges, implying that luminescence centers do not consist of the component elements in the host materials. It was suggested that the isolated TiO₆ complexes are possible luminescence centers in these materials, as previously proposed in other Ti-doped stannates such as Mg₂SnO₄ and Y₂Sn₂O₇.     

Preparation of amorphous composites with polymer-derived silicon nitride matrix reinforced by three-dimensional silica fiber

Perhydropolysilazane, a low viscosity preceramic polymer with good infiltration efficiency and high char yield, was used to prepare amorphous composites with polymer-derived silicon nitride matrix reinforced by three-dimensional silica fiber, and the mechanical properties and microstructures were investigated. The composites without fiber coating showed a typical brittle fracture behavior with a smooth fracture surface, and a flexural strength of just 33.5 MPa. While the composites with fiber coating exhibited a non-brittle fracture behavior with distinct fiber pull-out in the fracture surface, and a high flexural strength of 144.9 MPa. It was the controlled fiber/matrix interface by precoating treatment that contributed to the high mechanical property of the composites.     

De Haas–van Alphen effect and magnetization in dodecaborides HoB12, ErB12 and TmB12

High-quality HoB₁₂, ErB₁₂ and TmB₁₂ single crystals with the fcc UB₁₂ structure are grown; field dependencies of their magnetization are measured up to 14 T; angular dependences of the magnetization in the (1 0 0) and (1 1 0) planes are calculated, and properties of their Fermi surfaces (FS) are studied using the de Haas–van Alphen (dHvA) effect. The angular dependences of the dHvA frequencies in the (1 0 0) and (1 1 0) planes are obtained and values of their effective cyclotron masses are determined. The oscillation frequencies lie in the range (11.7–127.3)×10² T, and cyclotron masses within the range (0.4–1.45)m₀. For ErB₁₂ and TmB₁₂, we found branches in the oscillation spectrum that originated from FS fragments with spins along and opposed to the field, and the energy of the exchange interaction is estimated. Its value for the γ-branch in ErB₁₂ is equal to 8.2 meV, and for the γ-, β- and θ-branches in TmB₁₂; these values are 14.5, 28.5, and 18.3 meV, respectively.     

Spectroscopic detection of biological NO with a quantum cascade laser

Two configurations of a continuous wave quantum cascade distributed feedback laser-based gas sensor for the detection of NO at a parts per billion (ppb) concentration level, typical of biomedical applications, have been investigated. The laser was operated at liquid nitrogen temperature near λ=5.2 μm. In the first configuration, a 100 m optical path length multi-pass cell was employed to enhance the NO absorption. In the second configuration, a technique based on cavity-enhanced spectroscopy (CES) was utilized, with an effective path length of 670 m. Both sensors enabled simultaneous analysis of NO and CO₂ concentrations in exhaled air. The minimum detectable NO concentration was found to be 3 ppb with a multi-pass cell and 16 ppb when using CES. The two techniques are compared, and potential future developments are discussed. Received: 1 November 2000 / Revised version: 19 January 2001 / Published online: 20 April 2001     

Preparation and Photocurrent Performance of Highly Ordered Titania Nanotube Implanted with Ag/Cu Metal Ions

Ag/Cu-doped titania nanotubes （Ag/TiNT, Cu/TiNT） are prepared by a metal vapor vacuum arc implanter. A scanning electronic microscope is employed for microstruetural characterization. The photo-current performance of doped titania nanotubes under UV and visible light is tested by an electrochemical workstation CS300UA, the results show that the absorption edge of both Ag/TiNT and Cu/TiNT samples shifts to the visible light region and the band gap becomes narrower. Ag/TiNT possesses better photo-current ability than Cu/TiNT under UV and visible light. Titania doped with Ag and Cu metal ions is also studied based on the linearized augmented planewave method implemented by WIEN2k package, the result becomes better with the experimental performance.     

Violet luminescence in Ge nanocrystals/Ge oxide structures formed by dry oxidation of polycrystalline SiGe

Nanocrystals of Ge surrounded by a germanium oxide matrix have been formed by dry thermal oxidation of polycrystalline SiGe layers. Violet (3.16 eV) luminescence emission is observed when Ge nanocrystals, formed by the oxidation of the Ge segregated during the oxidation of the SiGe layer, are present, and vanishes when all the Ge has been oxidized forming GeO₂. Based on the evolution of the luminescence intensity and the structure of the oxidized layer with the oxidation time, the recombination of excitons inside the nanocrystals and the presence of defects in the bulk oxide matrix are ruled out as sources of the luminescence. The luminescence is attributed to recombination in defects at the Ge sub-oxide interface between the Ge nanocrystals and the surrounding oxide matrix, which is GeO₂.     

Synthesis of lanthanum nickelate perovskite nanotubes by using a template-inorganic precursor

A new route to obtain metal oxide nanotubes is presented: an inorganic coordination complex precursor containing the metal ions and impregnated into alumina membrane templates yield hollow tubular nanostructures of LaNiO₃ by calcination at 600 °C as characterized by powder X-ray diffraction (XRD). Scanning electron microscopy (SEM) shows that the resulting nanotubes have 200 nm in diameter in good agreement with the template pore. Transmission electron microscopy (TEM) and dark field transmission electron microscopy (DF-TEM) show that the nanotubes with 10-20 nm walls and internal separations are composed of 3-5 nm crystals.     

Double-hysteresis-like loops in Mn-doped (Pb,La)(Zr,Ti)O3 ceramics

PLZT12/40/60 ceramics doped with different Mn content were prepared by a conventional mixed oxide method. The shrinkage of some local crystal cells was confirmed by XRD diffraction. The effect of Mn doping on the hysteresis loop was investigated. A peculiar double-hysteresis-like loop was found and various possible mechanisms were discussed. Random local strains caused by shrunk crystal cells can satisfactorily explain the double hysteresis characteristics. The dependence of the double-hysteresis-like loops on temperature was also investigated. Due to the heat activation of polar moments in shrunk crystal cells, the double-hysteresis-like loops can transform into a normal one when the temperature is rising.     

Computational study on transamination of alkylamides with NH3 during metalorganic chemical vapor deposition of tantalum nitride

DFT calculations were employed to investigate transamination during metalorganic chemical vapor deposition (MOCVD) of transition metal nitrides films, such as titanium nitride (TiN) and tantalum nitride (TaN). The calculated energetics and rate constants for the ligand exchange of tert-butylimidotris(dimethylamido) tantalum (TBTDMT) with NH₃ demonstrated that NH₃ addition to form the ammonia adduct, TBTDMT·NH₃, proton transfer and dissociation of dimethylamine to afford net transamination of the dimethylamido ligand are facile even at low temperature (∼300 °C). The transamination of the tert-butylimido ligand, however, was relatively slow at those temperatures but became facile at temperatures appropriate for CVD growth (∼600 °C). Rapid transamination is consistent with lower temperature for growth of TaN by MOCVD in the presence of NH₃, efficient removal of carbon-containing ligands, and incorporation of higher levels of nitrogen in the resulting films.