Equivalence of the Impedance Method and the Method of Amplitude‐Frequency Characteristics for Investigations of Vibrations in Hydraulically Powered Supports

Two approaches to investigation of hydroelastic shock absorbers ensuring an efficient decrease in vibrations and noise in mobile machines have been presented. It has been shown that the parameters obtained by the method of amplitudefrequency characteristics more accurately correspond to the parameters of the physical model than the data computed with the impedance method.     

Working Standard of the Unit of Power of Electromagnetic Waves in the Frequency Range of 37.5–220 GHz

Measurement Techniques - We show the possibility of application of the bolometric converters of microwave power, included in the State Primary Standard of the unit of power of electromagnetic waves...     

Evanescent Wave Transport and Shot Noise in Graphene: Ballistic Regime and Effect of Disorder

We have investigated electrical transport and shot noise in graphene field effect devices. In large width over length ratio W/L graphene strips, we have measured shot noise at low frequency (f=600–850 MHz) in the temperature range of 4.2–30 K. We observe a minimum conductivity of $\frac{4e^{2}}{\pi h}$ and a finite and gate dependent Fano factor reaching the universal value of $\frac{1}{3}$ at the Dirac point, i.e. where the density of states vanishes. These findings are in good agreement with the theory describing that transport at the Dirac point should occur via evanescent waves in perfect graphene samples with large W/L. Moreover, we show and discuss how disorder and non-parallel leads affect both conductivity and shot noise.     

Characterization of the Properties of Barium–Strontium Titanate Films and Controlled Elements Based on Them in the Frequency Range of 1–60 GHz

Technical Physics Letters - The electrical properties of barium–strontium titanate films and tunable capacitors based on them are studied in a wide frequency range. The dielectric losses in...     

Frustrated Phase Separation in the Overdoped Regime of “123” High-Tc Superconductors

Recent inelastic neutron scattering measurements revealed the crystal-field (CF) spectra of the overdoped R_{1 – y} Ca_yBa₂Cu₃O₇ (R = Ho, Er; 0 < y < 0.25) high-T _c superconductors to consist of two spectral components associated with the optimally doped and the overdoped domains. Increase of the Ca concentration does not affect the local charge density of domains, but changes the spectral weight of the components. In the sense of this two-phase picture, which was established earlier for the underdoped region, there is a smooth crossover between the under- and overdoped parts of the phase diagram. The charge origin of the observed superposition effect is confirmed by the systematic analysis of the concentration dependences of the CF parameters for both Ho- and Er-based compounds. Therefore, the local charge inhomogeneity in the CuO₂ planes of high-T _c cuprates is a characteristic feature of the doping process, which depends neither on the way to introduce doping nor on the doping level.     

Modeling the Process of Liquid Flow in the System “Formation–Pipeline”

Journal of Engineering Physics and Thermophysics - A model of the process of nonstationary motion of a liquid in the conjugate system “formation–pipeline” has been constructed,...     

Heat Transfer in Turbulent Rayleigh–Bénard Convection Below the Ultimate Regime

A Rayleigh–Bénard cell has been designed to explore the Prandtl (Pr) dependence of turbulent convection in the cross-over range 0.7<Pr<21 and for the full range of soft and hard turbulences, up to Rayleigh number Ra?10¹¹. The set-up benefits from the favourable characteristics of cryogenic helium-4 in fluid mechanics, in situ fluid property measurements, and special care on thermometry and calorimetric instrumentation. The cell is cylindrical with diameter/height=0.5. The effective heat transfer Nu(Ra, Pr) has been measured with unprecedented accuracy for cryogenic turbulent convection experiments in this range of Rayleigh numbers. Spin-off of this study include improved fits of helium thermodynamics and viscosity properties. Three main results were found. First the Nu(Ra) dependence exhibits a bimodality of the flow with 4–7% difference in Nu for given Ra and Pr. Second, a systematic study of the side-wall influence reveals a measurable effect on the heat transfer. Third, the Nu(Pr) dependence is very small or null: the absolute value of the average logarithmic slope (d ln Nu/d ln Pr)_Ra is smaller than 0.03 in our range of Pr, which allows to discriminate between experiments with contradictory results [Ashkenazi et al., Phys. Rev. Lett. 83, 3641 (1999)] [Ahlers et al., Phys. Rev. Lett. 86, 3320 (2001)].     

End-of-Line Waveguide Thermistor Power Meters in the Frequency Range 16.7–37.5 GHz

Measurement Techniques - In this study, a multiprobe end-of-line converter, which is a part of a waveguide thermistor wattmeter, was developed. This converter functions to improve the power...     

Change of the Sign of the Effect with Increase in the Intensity of High‐Frequency Acoustic Excitation of a Turbulent Jet

Results of an experimental investigation of highfrequency acoustic excitation of turbulent jets for different intensities of the acoustic field are presented. It is shown that upon reaching a certain limiting level of excitation at high frequencies, the sign of the effect changes, i.e., at this level, highfrequency excitation leads to generation of turbulence in the jet rather than its suppression. Hence it follows that the highfrequency acoustic effect that suppresses turbulence is most efficient within an optimum range of frequencies and at an optimum level of excitation.     

Frequency modulation response due to the intensity modulation of fiber-grating Fabry–Perot lasers

A comprehensive study on the small-signal frequency modulation (FM) characteristics of a fiber-grating Fabry–Perot (FGFP) laser is numerically conducted. Fiber Bragg grating (FBG) is used as a wavelength selective element to control the properties of the laser output by controlling the external optical feedback (OFB) level. In addition to the external OFB level, the effect of other parameters such as temperature, injection current, cavity volume, nonlinear gain compression factor, linewidth enhancement factor, and fiber-grating (FG) parameters on FM characteristics of the laser are investigated. The study is performed by modifying a set of rate equations that are solved by considering the effects of external OFB and ambient temperature (T) variations. The temperature dependence (TD) of FM characteristics is calculated according to TD of laser parameters instead of using well-known Pankove relationship. Results show that the optimum external fiber length (L_ext) is 3.1?cm and the optimum range of working temperature for FGFP laser is within?±?2?°C from the FBG reference temperature (T_o). Also, it is shown that antireflection (AR) coating reflectivity and the linewidth enhancement factor have no significant effect on the FM response. The FM spectra peak amplitude is less than 5?dB with 5?mW output power. Good temperature stability is also obtained.     

Phase Relations in the Reduction of Solid Solutions in the Co–Mn–Ti–O System

The hydrogen reduction of spinel solid solutions in the Co–Mn–Ti–O system was investigated by a static method. Six phase regions were identified in which the gas phase is in equilibrium with various combinations of -Co, TiO₂ (rutile), and solid solutions of variable composition: Co _A Mn _B Ti_{3 – A – B} O₄ (spinel), Co _m Mn_{2 – m} TiO₄ (spinel), Co _N Mn_{1 – N} TiO₃ (ilmenite), and Co _n Mn_{1 – n} O (NaCl). The equilibrium compositions of the solid solutions and the corresponding oxygen partial pressures were determined, and the general trends of the reduction of spinel solid solutions in the Co–Mn–Ti–O system were established.     

Specific Features of the Magnetoelectric Effect in Permendur–Quartz–Permendur Structures in the Region of Electromechanical Resonance

Technical Physics Letters - The experimental frequency and field dependences of the magnetoelectric effect in three-layer permendur–quartz–permendur structures in the region of...     

Suppression of θ″ formation in the SiC whisker-reinforced Al-4 wt% Cu composites

Precipitation characteristics in a powder metallurgy (Al-4wt% Cu)-SiC whisker composite were investigated using transmission electron microscopy, differential scanning calorimetry, and macrohardness measurement. The results of macrohardness measurement show that the peak ageing is significantly retarded in the SiC whisker-reinforced Al-Cu alloys. It is shown that the suppression of formation plays an important role in the retarded age hardening. The suppression of formation is attributed to a high density of dislocations due to the difference in the thermal contraction between the whiskers and matrix. Numerical analysis was performed to estimate the misfit strain generated during cooling near the whiskers. The results indicate that a high density of dislocations should be developed by the relief of the misfit strain.     

Experimental Studies of the Influence of a High‐Frequency Electromagnetic Field on the Convection of Fluids in a Vertical Tube

Results of experimental studies of the influence of a highfrequency electromagnetic field on a change in the levels of fluids in a vertical tube are reported.     

Special features of the diamond crystallization in the Mg–Zn–B–C system

The diamond crystallization in the Mg–Zn–B–C system occurring in the diamond thermal stability region have been considered. The phase transformations, which take place during the preparation of the alloy–solvent for carbon and its structure, the diamond crystallization and properties of the resultant diamond crystals have been studied. The formation of the acceptor centers and inclusions in diamond crystals caused by the addition of boron into the growth system have been considered. It has been found that the use of the diamond powder produced in this system for abrasive machining surfaces of sapphire parts makes it possible to increase the machining efficiency and quality as compared with that of the powder produced in the Ni–Mn–C system.     

Calculation of the Free Energy of Fullerenes in the Gross–Neveu Model

Methods of field theory on Riemannian manifolds were used to study the Gross–Neveu model (incorporating the Hubbard model as a special case) at T 0. The generating functional of the Gross–Neveu model on a torus, S ² _r S ¹ , was obtained by the functional integration method. The model was regularized using the theory of zeta functions. Double sums were calculated using recurrent formulas. For the zeta function of the Dirac operator in the limit of s = 1, we obtained a polar singularity of the form 1/(s – 1), characteristic of the local limit of Green's function. The free energy density was computed as a function of the radius of the sphere r and inverse temperature using the Maple 6 pack. The results show no anomalies, indicating that there are no phase transitions to the principal order in 1/N. However, taking into account the kink–antikink configurations of the scalar field A(x) in calculations without the 1/N expansion may drastically change the phase structure of the model.     

Real-time measurement of in vitro enamel demineralization in the vicinity of the restoration–tooth interface

An X-ray attenuation method using photon counting (scanning microradiography) is presented for the real-time study of in vitro demineralization of dental tissues in the vicinity of the restoration–tooth interface. By repeated measurement of mineral content profiles during the course of demineralization, the pattern of lesion development and the rate of mineral loss can be studied. The method is illustrated by comparison of enamel demineralization near a polyacid-modified composite resin restoration, near a bis-GMA/TEGMA composite resin restoration, and in an unrestored control. The method has potential for study of the influence of restorative materials on susceptibility of tooth tissue to demineralization. ©©1999©Kluwer Academic Publishers     

Role of nickel in the oxidation of Fe–Cr–Ni alloys in air–water vapour atmospheres

Abstract

A series of experimental austenitic alloys has been produced in which the nickel content ranges from 14 to 43%, with constant levels of 20%Cr, 1%Mn and 0.5%Si. A combination of isothermal, discontinuous and cyclic oxidation testing has been used to elucidate the performance in dry air and in air with 10%, 45% or 62% water vapour at 700°C and 1000°C. Evaluation was by means of thermogravimetry, surface analysis with glow discharge optical emission spectroscopy and scanning electron microscopy.

Nickel is shown to have several roles: it accelerates the kinetics of chromia formation yet suppresses chromia spallation at 700°C. At 1000°C, it strongly decreases the breakaway oxidation and spalling associated with iron oxide formation. This effect is particularly marked in environments containing water vapour, where the material loss may be decreased 10-fold by an increase in the nickel content. Results correlate to thermodynamic and kinetic data which show nickel to increase the chromium activity and diffusivity in the alloy.     

On the influence of preloading in the nonlinear viscoelastic–viscoplastic response of carbon–epoxy composites

On an ongoing research for the nonlinear viscoelastic response of composites and polymers, a study of the influence of preloading applied to composite laminates subjected to creep–recovery loading is performed. In cases where high stress levels are applied, this response becomes highly nonlinear and has to be taken into account when designing composite parts. A major problem encountered in the experimental investigation of the nonlinear viscoelastic behaviour is the mode of the initial applied loading and its effect in the overall viscoelastic response of the test sample. The damage that occurs due to the instantaneous application of the load leads to an additional viscoelastic/viscoplastic strain component. In order to investigate this effect as well as to compare different preloading modes, as far as viscoelastic/viscoplastic response is concerned, a test program was initiated and the experimental data were investigated in the current study. A preloading mode is applied in each specimen prior to the creep–recovery testing at different applied stress levels. Useful results concerning the effect of preloading in the time dependent response of the material are concluded. Variation of the values of viscoplastic strain in respect to the preloading mode is also of great concern.