Towards a 1 MW, 170 GHz gyrotron design for fusion application

The electrical design of different components of 1 MW, 170 GHz gyrotron such as, magnetron injection gun, cylindrical interaction cavity and collector and RF window is presented in this article. Recently, a new project related to the development of 170 GHz, 1 MW gyrotron has been started for the Indian Tokamak. TE_34,10 mode is selected as the operating mode after studied the problem of mode competition. The triode type geometry is selected for the design of magnetron injection gun (MIG) to achieve the required beam parameters. The maximum transverse velocity spread of 3.28% at the velocity ratio of 1.34 is obtained in simulations for a 40 A, 80 kV electron beam. The RF output power of more than 1 MW with 36.5% interaction efficiency without depressed collector is predicted by simulation in single-mode operation at 170 GHz frequency. The simulated single-stage depressed collector of the gyrotron predicted the overall device efficiencies >55%. Due to the very good thermal conductivity and very weak dependency of the dielectric parameters on temperature, PACVD diamond is selected for window design for the transmission of RF power. The in-house developed code MIGSYN and GCOMS are used for initial geometry design of MIG and mode selection respectively. Commercially available simulation tools MAGIC and ANSYS are used for beam–wave interaction and mechanical analysis respectively.     

RF behavior of triple-frequency high power fusion gyrotron

The RF behavior of high power, triple frequency (170-, 127.5-, and 85 GHz) gyrotron for fusion application is presented in this paper. The operating mode selection is discussed in detail for each corresponding frequencies and TE_34,10, TE_25,8 and TE_17,5 modes are selected as the operating mode for 170 GHz, 127.5 GHz and 85 GHz operation of the device, respectively. The interaction cavity geometry and beam parameters are finalized by the cold cavity analysis and beam-wave interaction simulations. Considering the beam parameters and the beam launching positions in cavity (beam radius), the design of Magnetically Tunable MIG (MT-MIG) is also presented. Results of MT-MIG confirm the beam launching with desired beam parameters at the beam radius corresponding to the selected operating modes for all three frequencies. The CVD diamond window is also designed for RF power transmission. The beam-wave interaction simulations confirm more than 1 MW power at all three frequencies (170-, 127.5-, and 85 GHz).     

Crystal growth and structural properties of the spinel-type Li1 + xMn2 − xO4 (x = 0.10, 0.14)

Single crystals of the lithium-rich lithium manganese oxide spinels Li_1 + xMn_2 − xO₄ with x = 0.10 and 0.14 have been successfully synthesized in high-temperature molten chlorides at 1023 K. The single-crystal X-ray diffraction study confirmed the cubic Fd3¯m space group and the lattice parameters of a = 8.2401(9) Å for x = 0.10 and a = 8.2273(10) Å for x = 0.14 at 300 K, respectively. The crystal structures have been refined to the conventional values R = 3.7% for x = 0.10 and R = 3.1% for x = 0.14, respectively. Low-temperature single-crystal X-ray diffraction experiments revealed that these single crystal samples showed no phase transition between 100 and 300 K. The electron-density distribution images in these compounds by the single-crystal MEM analysis clearly showed strong covalent bonding features between the Mn and O atoms due to the Mn–3d and O–2p interaction.     

Structure and electrochemical properties of Sm0.5Sr0.5Co1 − xFexO3 − δ cathodes for solid oxide fuel cells

Crystal structure, thermal expansion coefficient, electrical conductivity and cathodic polarization of compositions in the system Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ with 0 ≤ x ≤ 0.9 were studied as function of Co / Fe ratio and temperature, in air. Two phases, including an Orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9, were observed in samples of Sm_0.5Sr_0.5Co_1 − xFe_xO_3 − δ at room temperature. The adjustment of thermal expansion coefficient (TEC) to electrolyte, which is one of the main problems of SSC, could be achieved to lower TEC values with more Fe substitution. High electrical conductivity above 100 S/cm at 800 °C was obtained for all specimens, so they could be good conductors as cathodes of IT-SOFC. The polarization behavior of SSCF as a function of Fe content was evaluated by means of AC impedance using LSGM electrolyte. It was discovered that the Area Specific Resistance (ASR) of SSCF increased as the amount of substitution of Fe for Co increased. When the amount of Fe reached to 0.4, the highest ASR was obtained and then the resistance started decreasing above that. The electrode with a composition of Sm_0.5Sr_0.5Co_0.2Fe_0.8O_3 − δ showed high catalytic activity for oxygen reduction operating at temperature ranging from 700 to 800 °C.     

Adsorption of molecular SiO2 on a clean Si(1 0 0) surface

We have investigated the adsorption of molecular (gaseous) SiO₂ on a clean Si(1 0 0) p(2 × 2) reconstructed surface using density functional theory based methods. The SiO₂ molecule is found to be chemisorbed on various sites on the Si surface and the most energetically favourable structure is on top of the dimers. The minimum energy pathways for the various adsorption channels indicate that the reaction is barrierless in all cases. The corresponding vibrational spectrum is also calculated and the adsorbed molecules are, as expected, found to have red-shifted vibrational frequencies. The energetically favourable adsorption sites and adsorption energies are comparable to the results found for SiO.     

Adsorption of Xe atoms on the TiO2(1 1 0) surface: A density functional study

The interaction of xenon atoms with the TiO₂(1 1 0) surface of rutile has been studied by density functional theory methods. Five different possible adsorption sites on the relaxed and clean TiO₂(1 1 0) surface and on two different type of oxygen vacancies possible on this oxide substrate have been considered. In the case of the defect-free substrate, and when compared with a previous study concerning the adsorption of Ar atoms also on TiO₂(1 1 0), the xenon atom, as a larger and easier polarizable species, is shown to have a deeper physisorption well, as expected. Likewise, Xe atoms prefer to be bounded to positions nearby the outermost titanium atoms as found previously for Ar. This is in agreement with most studies concerning rare gases adsorbed on transition metal surfaces. In the case of the reduced surfaces, it is found that the interaction is more favourable in the protruding rows. The interaction is dominated by dispersion forces and DFT + dispersion energies are 3.5–5 times larger than the non-corrected DFT values and Xe-surface distances are smaller. Finally, an interesting correlation is obtained for the calculated interaction energies and the Xe–Ti distance.     

Coverage effects on the adsorption of sulfur on Co(0 0 0 1): A DFT study

The adsorption of sulfur on Co(0 0 0 1) was studied using density functional theory calculations at coverage from 0.11 ML to 1.0 ML. Calculated results indicate that atomic S favors in hollow sites with bond S–Co dominated at lower coverage and at higher coverage the strong adsorbate S–S interaction leads to the formation of S₂ species. It was shown that the adsorption energy generally increases (gets weaker) with the coverage in a near linear fashion for the most stable configurations. In addition, modification of the surface electronic properties has been discussed and some discrepancy are found between our calculations and the findings of O adsorption on Au(1 1 1) and Pt(1 1 1) surfaces.     

Synthesis and characterization of layered Li1−x(Ni0.5Co0.5)1−yFeyO2(0 ≤ (1 − x) ≤ 1 and 0 ≤ y ≤ 0.2) cathodes

Layered Li(Ni_0.5Co_0.5)_1−yFe_yO₂ cathodes with 0 ≤ y ≤ 0.2 have been synthesized by firing the coprecipitated hydroxides of the transition metals and lithium hydroxide at 700 °C and characterized as cathode materials for lithium ion batteries to various cutoff charge voltages (up to 4.5 V). While the y = 0.05 sample shows an improvement in capacity, cyclability, and rate capability, those with y = 0.1 and 0.2 exhibit a decline in electrochemical performance compared to the y = 0 sample. Structural characterization of the chemically delithiated Li_1−x(Ni_0.5Co_0.5)_1−yFe_yO₂ samples indicates that the initial O3 structure is maintained down to a lithium content (1 − x) ≈ 0.3. For (1 − x) < 0.3, while a P3 type phase is formed for the y = 0 sample, an O1 type phase is formed for the y = 0.05, 0.1 and 0.2 samples. Monitoring the average oxidation state of the transition metal ions with lithium contents (1 − x) reveals that the system is chemically more stable down to a lower lithium content (1 − x) ≈ 0.3 compared to the Li_1−xCoO₂ system. The improved structural and chemical stabilities appear to lead to better cyclability to higher cutoff charge voltages compared to that found before with the LiCoO₂ system.     

A low noise 2 × 16 Ge:Sb focal-plane array: Paving the way for large format FPAs for far IR astronomy

Future astronomical instruments call for large format and high sensitivity far infrared focal-plane arrays to meet their science objectives. Arrays as large as 128 × 128 with sensitivities equal to or better than 10⁻¹⁸ W/√Hz are set as targets for the far IR instruments to be developed within the next 10 years. These seemingly modest goals present a not-so-modest quantum leap for far IR detector technology whose progress is hampered by a number of complexities; chief among them the development of low noise readouts operating at deep cryogenic temperatures and a viable hybridization scheme suitable for far IR detectors. In an effort to incrementally develop large-format photoconductor arrays, we have fabricated a 2 × 16 Ge:Sb array using the SBRC190 readout – a cryogenic 1 × 32 CTIA readout multiplexer initially developed for SOFIA’s AIRES instrument. In this paper we report the results of the extensive parametric tests performed on this array showing an impressive noise performance of 2.2 × 10⁻¹⁸ W/√Hz and a DQE of 0.41 despite some design limitations. With such an encouraging performance, this prototype array will serve as a platform for our future developmental effort.     

Improved electrical parameters of vacuum annealed Ni/4H-SiC (0 0 0 1) Schottky barrier diode

The reported work has been focused on the improvement of electrical parameters of Schottky diode using vacuum annealing at mild temperature in Ar gas ambient. Nickel Schottky barrier diodes were fabricated on 50 μm epitaxial layer of n-type 4H-SiC (0 0 0 1) substrate. The values of leakage current, Schottky barrier height (?_B), ideality factor (η) and density of interface states (N_SS) were obtained from experimentally measured current–voltage (I–V) and capacitance–voltage (C–V) characteristics before and after vacuum annealing treatment. The data revealed that ?_B, η and reverse leakage current for the as-processed diodes are 1.25 eV, 1.6 and 1.2 nA (at ?100 V), respectively, while for vacuum annealed diodes these parameters are 1.36 eV, 1.3 and 900 pA (at same reverse voltage). Improved characteristics have been resulted under the influence of vacuum annealing because of lesser number of minority carrier generation due to incessant reduction of number of available discrete energy levels in the bandgap of 4H-SiC substrate and lesser number of interface states density at Ni/4H-SiC (0 0 0 1) interface.     

Interaction of NO with the O-rich RuO2(1 1 0) surface at 300 K

The interaction of NO with the O-rich RuO₂(1 1 0) surface, exposing coordinatively unsaturated O-bridge, O-cus, and Ru-cus atoms, was studied at 300 K by thermal desorption spectroscopy (TDS) and high-resolution electron energy-loss spectroscopy (HREELS). The conclusions are validated by isotope substitution experiments with ¹⁸O. During exposure to NO an O···N–O surface group (NO₂-cus) is formed with O-cus. Additionally, a smaller number of empty Ru-cus sites are filled by NO-cus. If one warms the sample to 400 K, NO₂-cus does not desorb but decomposes into O and NO again, the latter being either released into gas phase or adsorbed as NO-cus. With O-bridge such a surface group is not stable at 300 K. Our experiments further prove that O-cus is more reactive than O-bridge.     

Structure and conductivity investigations of alkaline earth substituted uranium oxide,U1 − xMxO2 ± δ (M = Mg,Ca, Sr) for solid oxide fuel cell applications

Alkaline earth substituted UO₂ (U_1 − xM_xO_2 ± δ; M = Mg, Ca, Sr; 0.1 ≤ x ≤ 0.525) with fluorite structure was synthesized in reducing atmosphere. Structure and conductivity properties of U_1 − xM_xO_2 ± δ fluorites were investigated for possible application in solid oxide fuel cells (SOFC). At room temperature and ambient atmosphere the materials are stable; however they decompose at an oxygen partial pressure p_O2 > 10^− 4 atm and temperatures higher than 600 °C. The total conductivity measured for the best conducting U_1 − xM_xO_2 ± δ material with M = Ca and x = 0.177 is as high as 3 S/cm at p_O2 < 10^− 4 atm at 600 °C. The relatively low ionic transference number (t_i∼0.02) is disadvantageous for potential use as electrolyte material for SOFC applications. The high conductivity and possible depolarization effects suggest potential use as anode materials in SOFC.     

Entropy-driven adsorption of carbon nanotubes on (0 0 1) and (1 1 1) surfaces of CeO2 islands grown on sapphire substrate

According to the aim to compose combinatorial material by adsorption of carbon nanotubes onto the structured CeO₂ surface the interaction of the armchair (5,5) and zigzag (8,0) nanotubes with the (0 0 1) and (1 1 1) surfaces of CeO₂ islands have been investigated by theoretical methods. The thermodynamics of the adsorption were studied at the low surface coverage region. The interaction energy between the nanotube and the different CeO₂ surfaces shows significant increase when the size of the interface reaches 7–8 unit cells of CeO₂ and it remains unchanged in the larger interface region. However, the entropy term of the adsorption is significantly high when the distances of CeO₂ islands are equal to 27 nm (adsorption of armchair (5,5) nanotube) or 32 nm (adsorption of zigzag (8,0) nanotube). This property supports adsorption of nanotubes onto CeO₂ surfaces which possesses a very specific surface morphology. A long-wave vibration of nanotubes was identified as background of this unexpected phenomenon. This observation could be applicable in the development of such procedures where the nanotube adsorption parallel to the surface is aimed to perform.     

Metastable helium atom scattering from Ni(1 1 0) surface

The survival probability (SP) of metastable helium atoms (He¹) during scattering from the clean, alkalated and oxygen-adsorbed Ni(1 1 0) surfaces has been examined in the kinetic energy range of 50–400 meV. The measurements were carried out using a time-of-flight technique and a pulsed-discharge type metastable helium atom source. The SP is nearly constant for a kinetic energy (E_kin) of 50–100 meV and decreases exponentially with the increase in E_kin at 100–400 meV. It has been shown that the SP at E_kin=100–400 meV depends on the repulsive part of the He¹-surface interaction potential.     

Metallization of Ge(0 0 1)-p(2 × 1) surface as result of thermal fluctuations

We present a theoretical study of the metallization of Ge(0 0 1)-p(2 × 1) surface which is observed in experimental data. We have considered the connection between thermal fluctuation of this surface structure and its metallic properties. To this end we have performed long-time MD-DFT simulations. The obtained results show that thermal fluctuation of the Ge(0 0 1)-p(2 × 1) structure may cause its metallization which in not necessary connected with a flip-flop motion of dimer atoms. It was shown that the metallization of the Ge(0 0 1)-p(2 × 1) surface takes place when the dimer buckling angle is reduced to around 11°. In the case of our simulations the considered surface system remained in the metallic state for 25% of the simulation time. We have also found that the metallic state of the fluctuating Ge(0 0 1)-p(2 × 1) surface is built up by dangling bonds of the dimer atoms shifted up (D_up) and down (D_down).     

Preparation and characterization of Fe3O4(1 1 1) nanoparticles and thin films on Au(1 1 1)

Fe₃O₄ nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe₃O₄ was formed by annealing α-Fe₂O₃(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe₂O₃(0 0 0 1) structures into Fe₃O₄ nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe₃O₄ initially appears as nanoparticles at low coverages, and forms thin films at ～2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe₃O₄ particles with periodicities of ～50 and ～42 Å, respectively. As the Fe₃O₄ particles form more continuous films, the ～50 Å feature was the predominant superstructure observed. The Fe₃O₄ structures at all coverages show a hexagonal unit cell with a ～3 Å periodicity in the atomically resolved STM images.     

AFM study of a SiC film grown on Si(1 0 0) surface using a C2H4 beam

A 3C-silicon carbide (SiC) thin film grown on a Si(1 0 0) surface using an ethylene (C₂H₄) molecular beam has been studied by atomic force microscopy. At the center of the irradiation area of the ethylene beam, the shape of the SiC islands was rectangular, the average length of which was 74.5 nm and the average height was 13.1 nm. Each SiC island consists of the SiC particles with the average diameter of 17 nm. Just inside of the boundary region of the beam irradiation, the average size and height of the islands decreased to 50.1 and 8.2 nm, respectively. Just outside of the boundary region, the average size and height decreased to 17.7 and 5.1 nm, respectively. The average reaction probabilities at the above three points were estimated to be 0.14, 0.27 and 2.7%, respectively. New growth mode of the crystal growth is proposed (particles gathering island mode).     

A quantitative explanation for the apparent anomalous temperature dependence of OH + HO2 = H2O + O2 through multi-scale modeling

Kinetic models for complex chemical mechanisms are comprised of tens to thousands of reactions with rate constants informed by data from a wide variety of sources – rate constant measurements, global combustion experiments, and theoretical kinetics calculations. In order to integrate information from distinct data types in a self-consistent manner, a framework for combustion model development is presented that encapsulates behavior across a wide range of chemically relevant scales from fundamental molecular interactions to global combustion phenomena. The resulting kinetic model consists of a set of theoretical kinetics parameters (with constrained uncertainties), which are related through kinetics calculations to temperature/pressure/bath-gas-dependent rate constants (with propagated uncertainties), which in turn are related through physical models to combustion behavior (with propagated uncertainties). Direct incorporation of theory in combustion model development is expected to yield more reliable extrapolation of limited data to conditions outside the validation set, which is particularly useful for extrapolating to engine-relevant conditions where relatively limited data are available. Several key features of the approach are demonstrated for the H₂O₂ decomposition mechanism, where a number of its constituent reactions continue to have large uncertainties in their temperature and pressure dependence despite their relevance to high-pressure, low-temperature combustion of a variety of fuels. Here, we use the approach to provide a quantitative explanation for the apparent anomalous temperature dependence of OH + HO₂ = H₂O + O₂ – in a manner consistent with experimental data from the entire temperature range and ab initio transition-state theory within their associated uncertainties. Interestingly, we do find a rate minimum near 1200 K, although the temperature dependence is substantially less pronounced than previously suggested.     

Investigation on the microscopic features of layered oxide Li[Ni1 / 3Co1 / 3Mn1 / 3]O2 and their influences on the cathode properties

Three kinds of samples of Li[Ni_1 / 3Co_1 / 3Mn_1 / 3]O₂ were prepared respectively from direct solid-state reaction method, combustion method and co-precipitation route and their microscopic structural features have been investigated using Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), magnetic susceptibility measurement and X-ray photoelectron spectroscopy (XPS). The microscopic features such as uniform distribution of transition metal ions at 3b-site and the site-exchange ratio between lithium and nickel were found to be significantly dependent on the synthetic routes. The electrochemical properties of three samples were monitored using 2016 coin-cell by galvanostatic charge–discharge cycling test and cyclic voltammetry, which showed that the microscopic structural features are deeply related with the electrochemical performance. The obtained results also suggested that the combustion method may become a much simple alternative synthetic route to the complicate co-precipitation method.     

Synthesis and characterization of La4Ni3−xCoxO10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2) for solid oxide fuel cell cathodes

The cobalt-doped lanthanum–nickel oxide system, La₄Ni_(3−x)Co_xO_10±δ (0.0 ≤ x ≤ 3.0, Δx = 0.2), was investigated as possible cathode materials for intermediate-temperature solid-oxide fuel cells. X-ray diffraction shows the presence of two structural phases in the series belonging to Bmab for 0.0 ≤ x ≤ 0.2, 0.8 ≤ x ≤ 2.0 and 2.6 ≤ x ≤ 3.0 and Fmmm for 0.4 ≤ x ≤ 0.6 and 2.2 ≤ x ≤ 2.4. All compositions are oxygen-deficient (δ < 0). Electrical conductivity measurements show a systematic decrease in the conductivity as cobalt content increases from x = 0.0 to 2.0, and reverses for x > 2.0. AC impedance measurements of the x = 0.4 composition in symmetrical cells with LSGM as an electrolyte show improved electrode performance over the parent nickelate La₄Ni₃O_9.78. Long-term thermal stability studies show the x = 0.4 composition to be more stable than the x = 3.0 phase after annealing at 1173 K in air for 1 week making this material a viable candidate for cathodes in solid oxide fuel cells.