An experimental study of the development dynamics of the chaotic oscillation mode in a deterministic self-oscillatory microwave system

A possibility of experimentally obtaining an increased-output-power noise generator operating in the 3-cm wavelength range is considered. The noise generator was obtained by changing the self-excited oscillator, based on an avalanche-transit diode, to the chaotic generation mode by an external microwave signal action. Spectra of obtained chaotic generator oscillations with different parameters of both the generator and the external signal are presented. The statistical estimate of the quality of generated noise is given.     

Luminescence behavior of silicon and carbon nanoparticles dispersed in low-polar liquids

ABSTRACT: A comparative photoluminescence analysis of as-prepared and chemically modified (by alkyl chains -C18H37) silicon and carbon nanoparticles dispersed in low-polar liquids is reported. Influence of the low-polar liquid nature and ambient temperature on photoluminescence of the nanoparticles has been investigated from the point of view of their possible application as thermal nanoprobes.     

Multilayer AlN/AlGaN/GaN/AlGaN heterostructures for high-power field-effect transistors grown by ammonia MBE on silicon substrates

We report preliminary results on the transfer of the ammonia MBE technology of AlN/AlGaN/GaN/AlGaN heterostructures to silicon substrates. Optimization of the growth conditions allowed the number of macroscopic cracks in the epilayers to be reduced and ensured the growth of heterostructures with two-dimensional electron gas, which are suitable for the creation of field-effect transistors. The saturation current of prototype devices based on the heterostructures grown on silicon substrates are comparable with the analogous parameter of devices grown on sapphire and exhibits no decrease related to thermal scattering at high bias voltages.     

Contact between rough wavy surfaces allowing for the mutual effect of the asperities

The paper proposes a model of contact interaction between wavy rough surfaces allowing for the mutual effect of microasperities. The model is intended to calculate the contact parameters under heavy loadings and, correspondingly, at large contour areas. The model in question assumes that the contact waves deform elastically and the asperities deform plastically.     

Effect of cooling rate upon hardening on the kinetics of low-temperature decomposition of martensite in medium-carbon steel

The effect of the cooling rate upon hardening on the microstructure, microhardness, and kinetics of low-temperature decomposition of martensite in a medium-carbon steel has been studied. It has been shown that, as the cooling rate increases, the microstructure of hardened steel also increases. The martensite decomposition occurs in two stages; at the first stage, the rate of decomposition is higher than that at the subsequent stage. The rate of martensite decomposition at the first stage of aging increases as the cooling rate increases.     

30～200kV、上升沿1.5ns电压脉冲作用下常压空气中逃逸电子的产生(英文)

A pulse generator with a voltage rise time of～1.5 ns and voltage amplitude variable from 30 kV to 200 kV was designed for generating runaway electron beams in atmospheric pressure air with different interelectrode gaps.The influence of the voltage amplitude and gap length on the generation was studied.In the gas diode geometry under study,the gap voltage at which the generation of a runaway electron beam begins was determined.Decreasing the voltage pulse amplitude does not increase the beam current pulse width measured with a time resolution of～0.1 ns.It is shown that the escape of beam electrons to the downstream of the foil is sync in time with the voltage drop across the gap,and that the delay of beam current generation increases gradually from 1.1 ns to 2.6 ns as the voltage pulse amplitude across the gap decreases from～100 kV to 40 kV.     

Selectivity,Kinetics, and Efficiency of Reversible Anion Exchange with TcO4− in a Supertetrahedral Cationic Framework

[ThB₅O₆(OH)₆][BO(OH)₂]·2.5H₂O (Notre Dame Thorium Borate‐1, NDTB‐1) is an inorganic supertetrahedral cationic framework material that is derived from boric acid flux reactions. NDTB‐1 exhibits facile single crystal to single crystal anion exchange with a variety of common anions such as Cl^?, Br^?, NO₃^?, IO₃^?, ClO₄^?, MnO₄^?, and CrO₄^2?. More importantly, NDTB‐1 is selective for the removal of TcO₄^? from nuclear waste streams even though there are large excesses of competing anions such as Cl^?, NO₃^?, and NO₂^?. Competing anion exchange experiments and magic‐angle spinning (MAS)‐NMR spectroscopy of anion‐exchanged NDTB‐1 demonstrate that this unprecedented selectivity originates from the ability of NDTB‐1 to trap TcO₄^? within cavities, whereas others remain mobile within channels in the material. The exchange kinetics of TcO₄^? in NDTB‐1 are second‐order with the rate constant k₂ of 0.059 s^?1 M^?1. The anion exchange capacity of NDTB‐1 for TcO₄^? is 162.2 mg g^?1 (0.5421 mol mol^?1) with a maximum distribution coefficient K_d of 1.0534 × 10⁴ mL g^?1. Finally, it is demonstrated that the exchange for TcO₄^? in NDTB‐1 is reversible. TcO₄^? trapped in NDTB‐1 can be exchanged out using higher‐charged anions with a similar size such as PO₄^3? and SeO₄^2?, and therefore the material can be easily recycled and reused.     

Physicochemical Behavior of Impurity Atoms Formed by Nuclear Transmutation in the Course of Post-Radiation Annealing of Irradiated Cadmium Metal

The mechanism of impurity atom transfer during annealing of irradiated cadmium targets was studied. Structural transformations of the irradiated metal are accompanied by accelerated migration of Sn and In. It was found that the migration rate of impurity atoms depends on the nuclear history (radiation dose, type and energy of the bombarding particles), and the transfer rate decreases during post-radiation annealing of the metal. To explain the abnormal behavior of impurities, the transfer mechanism was proposed, which takes into account irregularity of the metal crystal structure after irradiation and structural transformations, and also possible stabilization forms of transmutation nuclides in the irradiated cadmium (at the substitution and intercalation positions and at the crystallite grain boundary).     

Power performance and scalability of AlGaN/GaN power MODFETs

The scalability of power performance of AlGaN/GaN MODFETs with large gate periphery, as necessary for microwave power devices, is addressed in this paper. High-frequency large-signal characteristics of AlGaN/GaN MODFETs measured at 8 GHz are reported for devices with gatewidths from 200 μm to 1 mm. 1-dB gain compression occurred at input power levels varying from -1 to +10 dBm as the gatewidth increased, while gain remained almost constant at -17 dB. Output power density was ~1 W/mm for all devices and maximum output power (29.9 dBm) occurred in devices with 1-mm gates, while power-added efficiency remained almost constant at ~30%. The large-signal characteristics were compared with those obtained by dc and small-signal S-parameters measurements. The results illustrate a notable scalability of AlGaN/GaN MODFET power characteristics and demonstrate their excellent potential for power applications