一种适用于雷达罩的频率选择表面新单元研究

从传统的频率选择选择表面(frequency selective surface,简称FSS)Y孔单元出发,提出了一种新单元.经优化设计,给出了Y孔单元和新单元的FSS结构参数,运用谱域Galerkin法对两种单元FSS的传输特性进行了数值计算.采用镀膜和光刻技术制作出相应的等效平板样件,在微波暗室对两种FSS等效平板在8—12GHz频段内的传输特性进行了测试,测试值与计算值基本一致.结果表明：与Y孔单元相比,新单元FSS在电磁波大角度(66°)入射时具有高的透过率,在电磁波大扫描角范围内(0°—66°)新单元FSS中心频率的漂移量更小.因此,所提出的新单元更适合于电磁波大角度入射下的应用,为FSS在曲面雷达罩上的应用提供了一种新单元. 关键词： 频率选择表面 中心频率 雷达罩     

Amorphization of ice near the melting point

In addition to reflections of the hexagonal phase of ice I _h, the intense diffuse scattering of X-rays mainly due to the amorphization of ice is revealed on the X-ray diffraction patterns of water ice samples prepared at liquid nitrogen (studied by the authors earlier) and samples prepared at T = ?10°C (this work). The measurements are performed in the temperature range from ?25 to 0°C. The existence of reflections of the crystalline phase and intense diffuse scattering on the X-ray diffraction patterns makes it possible make a conclusion about the coexistence of crystalline and amorphous structures of ice. Splitting of the first maximum on the electron-density radial distribution function is detected on the basis of an X-ray diffraction pattern recorded at T = ?3°C. This splitting is explained by an increase in the interatomic distances between the nearest-neighbor atoms located at different levels. Similar splitting was also detected on a radial distribution function constructed using an X-ray diffraction pattern recorded at ?10°C.     

Die mechanische Dämpfung in Eis-Einkristallen

The attenuation of multiply reflected acoustic pulses has been measured in ice single crystals for longitudinal and transversal waves. The frequency range extended from 5 to 200 Mc/s, the temperature from the melting point to ?120 °C. The attenuation in the range mentioned above is caused by scattering at microcrystalline units in the hexagonal ice.     

Thermistor controlled gunn oscillator at Ka-band

Frequency drift of Gunn oscillators is a major cause of concern in most of the Millimeter wave communication systems. This paper describes a simple and cost effective technique to arrest the frequency drift of a Ka band Gunn oscillator within 15 MHz for the operating temperature range of 0°C to 60°C as against a typical drift of about 50 to 100 MHz for free running Gunn oscillator for the same temperature range. At the ambient, the oscillator remains within ±1 MHz from switching on to stabilization. The temperature variation is sensed with a small thermistor bead placed close to the diode and a correction voltage is applied to the bias to compensate for the frequency drift. This compensation circuit also takes into account the non-linear behaviour of the thermistor and the Gunn oscillator with the temperature.     

Experimental investigation of the propagation velocity of ultrasonic waves and calculation of the elastic constants of nanocrystalline pyrocarbon

The dispersion of the propagation velocity of ultrasonic waves in pyrocarbon samples with a monotonically varying structure is studied in a frequency range of 0.5–25 MHz. The elastic constants of pyrocarbon samples that were obtained with the processing temperatures of 2100–3200°C are calculated on the basis of the measured values of propagation velocities of longitudinal and transverse ultrasonic waves.     

Signatures of the appearance of ice 0 in wetted nanoporous media at electromagnetic measurements

Ice 0 that is a new modification of crystalline ice, which can be formed only from supercooled water, is sought. To this end, the electric parameters of wetted nanoporous silicates SBA-15 and silica gel (Acros) for obtaining deeply supercooled water in pores are studied. Three electrical parameters of a medium are measured: the reflection coefficient of microwave radiation from the interface between the medium and air in a waveguide at a frequency of 12.4 GHz and the transmittance of radiation at a frequency of 94 GHz, the tangent of the dielectric loss angle at frequencies from 10 Hz to 100 kHz, and the characteristic electrical fluctuations in the frequency band of 1–100 Hz. Studies are performed at cyclic cooling and heating of the samples in the temperature range from + 20 °C to -150 °C. Sharp changes in all three parameters of the wetted silicates are revealed near the temperature range from -20 °C to - 24 °C. These changes can be attributed to the formation or destruction of ferroelectric ice 0.     

Acoustic studies of metal-ceramic composites with meso- and nanosize particles

The elasticity moduli and absorption coefficients of longitudinal ultrasonic waves with a frequency of 2–50 MHz depending on the steel concentration and sintering temperature of 1400–1700°C in vacuum were studied in samples of cermet-type composites on the basis of corundum and stainless steel. The results were discussed from the point of view of the elastic waves propagating in the fine-disperse two-phase medium at the presence of the intergrain and interphase boundaries, noticeably affecting the physical properties of the composite.     

Dielectric permittivity of nickel ferrites at microwave frequencies 1 MHz to 1.8 GHz

NiFe₂O₄ prepared via the sol–gel technique were pre-sintered at 900 °C and synthesized at different sintering temperatures from 1,000 °C to 1,200 °C at 100 °C intervals. The samples were characterized for microwave dielectric properties. These samples were measured using Agilent Impedance/Material Analyzer at frequencies 1 MHz to 1.8 GHz. Results showed a decrease in the dielectric constant and loss factor with frequency except at the turning point, around 150 MHz, where the loss factor showed a gradual increase. However, both the dielectric constant and loss factor increase with increasing sintering temperature. The grain size and density also increased with increasing sintering temperature, but the porosity and grain boundary density showed a decrease. This paper was presented at the International Conference on Solid State Science and Technology 2006, Kuala Terengganu, Malaysia, Sept. 4–6, 2006.     

Twin LC Resonator System for EPR Measurements Operating at Radio-Frequency

Two LC resonant circuits resonating at the same frequency (245 MHz) faced each other with reverse polarity (twin electron paramagnetic resonance (EPR) resonator). For the inductor of the LC circuit, a square single-turn one-loop coil (width, 38 mm) was fabricated. Each LC circuit was independently tuned using mechanical variable capacitors. A cylindrical phantom (diameter, 25 mm) including a 1 mM nitroxide radical physiological saline solution was located at the center of two coils (distance between these coils, 50 mm). Two resonant frequencies (the lower and the higher ones) were observed at each LC circuit of the twin EPR resonator with termination of the other LC circuit. The lower resonant frequency alone was observed when the powers from two LC circuits of the twin EPR resonator were combined by a 180° combiner. On the other hand, the higher frequency alone was observed when they were combined by a 0° combiner. EPR signals could be obtained using the 180° combiner (lower frequency) but not the 0° combiner (higher frequency).     

Measurements of the complex shear modulus of polymers under hydrostatic pressure usina the quartz resonator method

Abstract

The quartz resonator method measures the complex shear modulus or compliance of viscoelastic materials in the frequency range from 50 kHz to 140 MHz at temperatures between ?150°C and 300°C and pressures up to 1 GPa. This method can be applied to viscous fluids or polymer melts -even in their glassy or seminystalline regime.

The phase diagram of poly(diethylsiloxane) PDES (a mesophase polymer) was determined for two samples with different molecular weight at pressures up to 400 MPa and temperatures between 20°C and 100°C. Phase transitions are indicated by a sharp bend in the shear compliance although the volume effect of the mesophase-isotropic transition vanishes around 80 MPa.

The pressure dependence of the glass relaxation process (in PVAc), was studied by measuring the change of the complex shear modulus with pressure at constant temperatures between 95°C and 145°C and pressures up to 600 MPa. Additionally to the relaxation process, also the pressure dependence of the real part of the shear modulus in the glassy region can be determined for testing the dislocation concept in the meandermodell by W. Pechhold.     

Pulse compression technique for simultaneous HIFU surgery and ultrasonic imaging: a preliminary study

In an ultrasound image-guided High Intensity Focused Ultrasound (HIFU) surgery, reflected HIFU waves received by an imaging transducer should be suppressed for real-time simultaneous imaging and therapy. In this paper, we investigate the feasibility of pulse compression scheme combined with notch filtering in order to minimize these HIFU interference signals. A chirp signal modulated by the Dolph-Chebyshev window with 3-9 MHz frequency sweep range is used for B-mode imaging and 4 MHz continuous wave is used for HIFU. The second order infinite impulse response notch filters are employed to suppress reflected HIFU waves whose center frequencies are 4 MHz and 8 MHz. The prototype integrated HIFU/imaging transducer that composed of three rectangular elements with a spherically con-focused aperture was fabricated. The center element has the ability to transmit and receive 6 MHz imaging signals and two outer elements are only used for transmitting 4 MHz continuous HIFU wave. When the chirp signal and 4 MHz HIFU wave are simultaneously transmitted to the target, the reflected chirp signals mixed with 4 MHz and 8 MHz HIFU waves are detected by the imaging transducer. After the application of notch filtering with pulse compression process, HIFU interference waves in this mixed signal are significantly reduced while maintaining original imaging signal. In the single scanline test using a strong reflector, the amplitude of the reflected HIFU wave is reduced to −45 dB. In vitro test, with a sliced porcine muscle shows that the speckle pattern of the restored B-mode image is close to that of the original image. These preliminary results demonstrate the potential for the pulse compression scheme with notch filtering to achieve real-time ultrasound image-guided HIFU surgery.     

Broadband high-efficiency controllable asymmetric propagation by pentamode acoustic metasurface

We utilise the pentamode metasurface to realise broadband high-efficiency and controllable asymmetric transmission. The designed metasurface can manipulate the acoustic waves, as expected from the generalised Snell's law, and exhibits unique characteristics such as extraordinary broadband acoustic control, apparent negative refraction, and conversion from the propagating wave to surface mode. The asymmetric transmission features of positive refraction for the forward incidence (FI) and negative refraction for the reverse incidence (RI) can be realised within the range of 2600 Hz to 5600 Hz by controlling the incident angle from 0° to 35° with the transmission efficiency higher than 85.4% for the FI and RI. In addition, by further adjusting the angle of incidence in the range of 25° to 90°, asymmetric transmission characteristics can be expressed as surface wave transmission for the FI and transmitted wave transmission for the RI within the same frequency ranges.     

AC magnetic susceptibility at medium frequencies suggests a paramagnetic behavior of pure water

The AC magnetic susceptibility of water is studied in the frequency range of 1 kHz-1 MHz at a low magnetic field. The results show a paramagnetic behavior between 500 kHz and 1 MHz for samples at 37 °C and 25 °C, which is surprising considering that water is believed to be diamagnetic. AC magnetic susceptibilities for ice and different salt solutions were investigated as well: in such cases the susceptibility is always negative and therefore, such samples remain diamagnetic.     

Evidence for sound absorption in emulsions due to differing thermal properties of the two phases

The attenuation of low concentration indene in water emulsions was measured in the frequency range 5–145 MHz at 20°C. Due to the closeness in the densities and velocities of the two liquids, energy losses (in excess of those occuring in each phase separately) can be completely accounted for by the difference in the ratio of the expansivity to the specific heat of the two phases. This enables an estimate of the Sauter mean radius of the emulsions to be found.     

Conductivity of the solid electrolyte RbAg4I5 in the microwave region

The high frequency ionic conductivity of RbAg₄I₅ single crystals was measured in the range from 0.1 MHz to 8 GHz using a microwave reflection method. In the whole temperature region studied (30°C to 135°C) the bulk conductivity was found to be frequency independent and to coincide with the latest published values for the static conductivity. This result is in contradiction with values reported formerly in literature but agrees very well with recent measurements on the structurally similar solid electrolyte AgI.     

Temperature dependence of the magnetic properties and magnetoimpedance of nanocrystalline Fe<Subscript>73.5</Subscript>Si<Subscript>16.5</Subscript>B<Subscript>6</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> ribbons

The temperature dependences of the magnetic properties and the magnetoimpedance effect of soft magnetic nanocrystalline Fe_73.5Si_16.5B₆Nb₃Cu₁ alloy ribbons are studied in the temperature range 24–160°C. A high temperature sensitivity of the impedance and the magnetoimpedance effect of the ribbons are detected in the ac frequency range 0.1–50 MHz. At an ac frequency of 50 MHz, the change in the impedance reaches 0.2 Ω/°C (0.5%/°C) in the temperature range 85–160°C. When the temperature increases, a monotonically decreasing character of the dependence of the magnetoimpedance effect on the applied magnetic field changes into a dependence having an increasing initial segment. The effect of temperature on the magnetoimpedance properties of the soft magnetic nanocrystalline ribbons is shown to result from temperature-induced changes in their electrical conductivity, magnetization, and effective magnetic anisotropy.     

Messungen der Dielektrizitätskonstanten von reinem und NH4F-dotiertem Eis

The dielectric constant of pure and NH₄F doped ice single crystals has been measured at temperatures around — 10°C in the frequency range between 0.07 and 500 kc/s. The values measured on pure ice are in agreement with the Debye theory. At frequencies around 0.1 kc/s the dielectric constant?′ of doped crystals is higher than that of pure ice. Treating the frequency as a parameter, the dielectric constant?′ goes through a maximum and a minimum with rising NH₄F concentration. No dependence of the Debye dispersion frequency on the doping concentration could be detected.     

Deformation of the Raman scattering spectrum of Ih ice under local laser heating near 0°C

The Raman band of Nd:YAG laser second-harmonic scattering from the O-H stretching vibrations of hexagonal ice was observed to broaden asymmetrically by 90 cm^?1 near 0°C with the pulse repetition frequency increasing from 1 to 8 Hz. The center of this band was found to undergo simultaneously a Stokes shift by 25 cm^?1. The observed spectral features can be accounted for by the ice being heated by pulsed laser radiation through the electrocaloric effect, because the one-photon absorption mechanism produces a negligible contribution.     

Diffusion von Protonen (Tritonen) in Eiskristallen

The diffusion coefficient of tritons in ice crystals has been measured at different temperatures between 0°C and ?35°C. The applied method excludes any surface diffusion. The absolute value of the diffusion coefficient at ?7°C was determined as 2·10^?11 cm²/sec (±10%) and the activation energy as 13,5 kcal/mole (±8%). The activation energy of proton (triton)-diffusion is therefore consistent with the activation energy found for the dielectric and mechanical relaxation of protons in ice. The importance of this consistency relative to the diffusion mechanism is discussed.     

Electric Modulus Spectroscopic Studies of the Dielectric Properties of Carbon Nanotubes/Epoxy Polymer Composite Materials

The electrical properties of epoxy polymer/carbon nanotubes composites were characterized using impedance spectroscopy in the frequency range between 1 Hz and 10 MHz and temperature range between 25°C and 105°C. We report the analysis of the experimental data using the electric modulus formalisms to understand the dielectric relaxation mechanisms. The variation of the real and imaginary parts of the electric modulus versus frequency and temperature were suggestive of two relaxation processes, associated with dipolar relaxation and CNT-polymer interfaces. The Havriliak-Negami model of dielectric relaxation was used for modelling the relaxation processes, extracting the relaxation parameters.