Electromagnetic Materials

FOREWORD

CONTENTS

No abstract received.

The Phase-Switched Screen is a novel technique for the dynamic control of radar cross-section. It relies on the modification of the signal scattered from an object in such a way as to place the signal energy outside the bandwidth of a receiving system. The paper includes a theoretical discussion of the new technique and examples of its application in practice.

No abstract received.

The resonant frequency of long conductive fibers embedded in an anisotropic composite as a function of layer thickness is investigated both numerically and experimentally. An empirical law is suggested to fit the results obtained. The law involves a critical thickness value, below which the layer can no longer be treated as a bulk material for a fiber of given length and thickness. The values of the critical thickness were found from numerical results for two particular composites.

Self-adaptive material systems (SAMs) and their effective constitutive characterization are reviewed. The effect of a PEC shield for the SAM support system on the optimal control of the SAM is analysed.

No abstract received.

No abstract received.

No abstract received.

No abstract received.

No abstract received.

No abstract received.

Mechanical alloying is a powerful tool for the fabrication of amorphous and nano-structured powder materials. Many magnetic materials fabricated by mechanical alloying have shown many unique properties. Our recent work has shown that mechanically alloyed fine nano-structured powders have the potential as microwave absorbers. In this work, we have fabricated micron and sub-micron Fe_100-xM_x with M = 3d element or 4f element. SEM, TEM, VSM and Mössbauer spectroscopy were used for powder characterization. Microwave properties were investigated using HP Vector Network Analyzer. The EM wave absorbing properties of the mechanically alloyed powders were compared with commercial Carbonyl Iron powders and pure Fe powder. Results show that mechanical alloyed Fe-based powders may be promising for microwave absorbing applications.

We conducted dielectric measurements on composites comprising bundles of single-walled carbon nanotubes embedded in an epoxy matrix. The complex permittivity spectra exhibit dielectric resonance and electronic conduction behavior. Subsequently, we model the system based on two mechanisms: dielectric response and conduction. The former is described by the theory of ferroelectric resonance and the latter by the motion of the conducting electrons. Comparison between theoretical analysis and experimental permittivity data shows good agreement.

We have been studying melt-spun Sm-Co 1:7.5 type alloys with boron substitution that present a nanocomposite microstructure and high coercivities in both as-spun and short time annealed ribbons. In this work, we examine four different compositions, namely Sm(Co_0.82Fe_0.1Zr_0.04B_0.04)_7.5, Sm(Co_0.73Fe_0.1Cu_0.09Zr_0.04B_0.04)_7.5, Sm(Co_0.70Fe_0.1Cu_0.12Zr_0.04B_0.04)_7.5 and Sm(Co_0.70Fe_0.1Ni_0.12Zr_0.04B_0.04)_7.5 in order to understand the role of Cu or Ni in the development of microstructure and high coercivity. Melt-spun ribbons have been obtained from arc-melted bulk samples and were subsequently annealed in argon atmosphere for 10-60 min at 800-870°C. In asspun ribbons, the hexagonal TbCu₇ crystal structure type has been determined from X-ray diffraction patterns, while fcc-Co has been identified as a secondary phase. In annealed ribbons, the 1:7 phase transforms into 2:17 and 1:5 phases. TEM pictures show a homogeneous nano-crystalline microstructure with average grain size of 30-80 nm. Coercivity values of 15-27 kOe are obtained from hysteresis loops traced at non-saturating fields of 50 kOe. The coercivity decreases with temperature, but it is high enough to maintain values higher than 5 kOe at 380°C. After long annealing, the Co phase increases. Although the grains grow larger (50-120 nm), with favorable annealing conditions, the microstructure remains uniform, while loop squareness is improved and the coercivity is not degraded. Samples with Cu have high coercivity values, and replacement of Cu by Ni has increased the magnetization and improved the loop squareness without affecting adversely the coercivity.

This paper discusses several new research directions and applications of contemporary magnetic materials and magnetism, including: (1) high frequency magnetic materials; (2) microwave magnetic materials; (3) nano-magnetic materials and mesocopic magnetism; (4) widespread existence of magnetism and interdisciplinary research.

The static and dynamic magnetic properties of BaCo_xZn_1-xFe₁₆O₂₇ have been systematically studied. Results show that BaCo_xZn_1-xFe₁₆O₂₇ is a potentially good candidate for electromagnetic materials with low reflectivity at microwave frequency.

By making careful choice of chemical compositions and preparatory conditions, poly crystalline high saturation magnetisation nickel-zinc ferrites with fixed quantity of titanium and varied quantities of indium have been studied to examine their utility for high frequency power applications. Preliminary results indicate definite improvements in resistivity and saturation magnetisation, thus contributing to minimise core losses- an important requirement for the defined purpose. Further measurements are underway. The present results are explained by discussing the possible mechanisms responsible for the observed positive variations.

The magnetic properties of Li_0.5-x/2 Co_x Fe_2.5-x/2 O₄ (where x=0.1,0.2,0.3,0.4,0.5,0.6,0.7) ferrites have been studied. The x-ray diffraction studies reveal the single phase formation of the ferrite samples. The SEM micrographs show that the grain size decreases up to x=0.4 and increases for x>0.4. The grain size for all the samples lies in the range 0.2 μm and 6.7μm. The permeability studies at room temperature show that the values lie in the range of 5.5 to 20.6. The AC susceptibility measurements show a single domain structure. Usually the single domain structure is present in case of grain size < 0.1 μm. Both these variations suggest the presence of non- magnetic grain boundaries (NMGB) which involves deposition/ segregation of non magnetic particles over the surface of grains which reduce effective grain size. An attempt has been made to calculate the effective grain size using the data obtained from the measurements of initial permeability with temperature and the grain size obtained from the SEM micrographs.

A new compound with a chemical formula SrTi₂Zn₂Fe₁₀O₂₂ has been synthesized using analar grade reactants by solid-state diffusion technique at 1173°K for 100 hour. The formation of the compound is checked by X-ray diffractometry with filtered Cu radiation. The structural result shows that the compound is in a single hexagonal phase without traces of unreacting phases of the reactants. The observed unit cell dimensions are a = 5.891 Å and c = 46.064 Å. The compound is studied magnetically by using Gouy’s balance and the result shows that it is ferrimagnetic at room temperature (Curie temperature T_C=334°K). From the paramagnetic behaviour of the compound above the critical temperature, Curie molar constant is worked out and is found to be 42.67, matching with the expected value for stable oxygen states of the cations in the molecule. Electrical conductivity measurement shows slight deviation from linearity near Curie temperature with activation energy 0.44 eV. In addition to these, the compound is analyzed by thermo-gravimetry, scanning electron microscopy and infrared absorption. The thermo-gravimetric analysis shows the compound is thermally stable upto 1273°K; the microstructural study (SEM) shows clearly hexagonal platelets with average particle size 2.263 μm where as the infrared absorption gives two absorption peaks as observed in spinel structures in the range 400 cm⁻¹ to 4600 cm⁻¹, which may be due to octahedral and tetrahedral complexes present in the compound.

Current developments in the field of power electronic devices are leading to miniaturization. However, if storage inductors or small transformer are needed, the final dimension of a device is determined by these components. A size reduction is usually possible by increasing the operating frequency and the initial permeability, if ferrite with high magnetic performances is available. The ferrites producers and user are more and more interested in quickly and complex characterization of the ferrite cores. To obtain the characteristics of soft ferrites operating in the domain of the high frequency and having low hysteresis and power losses a new and complete experimental set-up was designed and used. Many papers report the dependence of the magnetization processes in soft ferrite cores on excitation frequency [1], [2]. Especially the microstructure, the resistivity and the permittivity could influence the distortion in the RL circuits with MnZn or Ni-Zn ferrite cores [3]. In some ferrites the pores and impurities can impede the movement of domain walls and the coherent rotations leading in magnetization processes. In this case the magnetization processes will be influenced by the frequency above the resonance ferrimagnetic frequency [4]. In ferrites with low porosity in static process or at low frequency the movement of domain walls will lead the magnetization process [5]. At high frequencies the wall can't fallow the field and the movements of the wall are irreversible due to the damping [6]. By using the Fourier techniques for the spectral analysis we have studied the digitized output signal in a secondary coil of a transformer with a Ni-Zn-Cu ferrite core. The cores were samples with different microstructure sintered at various temperatures [7]. The differences observed between the spectral coefficients at various excitation frequencies is discussed in relation with the two main magnetization processes. The behaviour is related to the microstructure and to the experimental hysteresis loops of the samples. In the same time different waveforms for the magnetic field strength were used to obtain minor and major hysteresis loops.

We study the effects of interfacial intermixing on electronic structure and magnetic properties of Cr/Cu by using the method of ab initio full potential linear muffin-tin orbital (FP-LMTO). The formation of CrCu in the subsurface layer is energetically favorable. The antiferromagnetic ordering of in-plane in Cr/Cu is found to be energetically favorable more than that of out-plane. In order to simulate the interfacial intermixing in Cr/Cu, we use the system of 50:50 CrCu alloy layer on the surface or at the interface, as well as the sandwich of Cr/(Cr_xCu_1-x)_n/Cu (where x = 0.25, 0.50, and 0.75 for n=1, 2, and 3, respectively). Our result for the energetics of surface alloying shows clearly that the Cr-Cu intermixing has a strong effect on the Cr magnetization.

In the last several years, many efforts were spent in developing the new section of electrodynamics -electrodynamics of materials with negative index of refraction. The first experiments in this area [1, 2] were carried out by a group of physicists at the University of California, San Diego (UCSD, USA). These works demonstrated unusual electrodynamic properties of some composite materials. These characteristics can be formally explained if it is assumed that the given materials possess the negative index of refraction n. This material presents itself as a combination of small metallic elements, disposed in space in some geometric order, forming structure resembling a sort of crystal. It is possible to consider such a type of structure at wavelengths noticeably larger than the size of its elements and the distance between them. The experiments by the authors of the specified work were made in the centimeter range of wavelengths, but the sizes of the elements explored in the composites and the distances between elements are typically of the order of 7-10 mm.

No abstract received.

Alternative possibilities of how to create an electromagnetic device being able to reconstruct near-field images of a source with sub-wavelength resolution (so-called perfect lens) are considered. It is shown that there is a variety of such means not involving backward-wave (double-negative, left-handed, or Veselago) materials or periodical backward-wave structures.

We have found that the wire medium (array of long conducting cylinders), one of the component of the famous realization of Veselago media, exhibits strong spatial dispersion even in the very large wavelength limit. Our analysis reveals that the conventional description of this medium by means of a local dispersive uniaxial dielectric tensor is not complete, leading to unphysical results for the propagation of electromagnetic waves at all frequencies. Since non-local constitutive relations have been usually considered in physics as a second-order approximation, meaningful in the short wavelength limit, the aforementioned result is important in a much more general sense for the theory of electromagnetic materials.

We study negative refractive index metamaterials from theoretical, numerical, and experimental point of views. Theoretically we calculate the time domain power (Poynting vector) transmitted at a boundary between a right-handed medium and a frequency dispersive left-handed medium, both homogenous and isotropic, for case of a multi-frequency signal. Numerically we perform full wave 3D FDTD simulations of the known rod and split-ring lattice in various configurations. Experimentally, we study the shifting of a Gaussian beam and the deflection of power inside a T-junction filled with an L-shaped metamaterial. In all three approaches, we find that materials that exhibit negative refraction not only theoretically exist, but can be built in the form of metamaterials made of rods and split-rings.

No abstract received.

Work done in collaboration with Weijia Wen, Weikun Ge

With the help of computer simulations, we have studied the properties of some electromagnetic (EM) wave functional materials. Some of these materials have been successfully fabricated in laboratory with experimentally measured optical properties in good agreements with theoretical predictions.

In this study, we present the results of the characterisation of a left-handed material performed in free space between 7 and 16 GHz with an increasing incidence angle. We demonstrate that this material has a negative optical index depending on the incidence. The value of this index is comprised between −0.1 and −0.5 for an incidence angle between 5 and 25°. The bidimensional structure of the material is the origin of this dependency.

No abstract received.

No abstract received.

The last few years have seen a growing interest in the analysis of heterogeneous materials. These investigations lead to many applications as for example the development of absorbing materials or frequency selective surfaces. To the diversity of the applications corresponds a variety of approaches used in the modeling of the fields scattered by heterogeneous materials. We have mainly investigated the case of 2D gratings of dielectric and magnetic fibers embedded in a multilayered medium placed or not over a perfectly conducting plane.

We present a method for fabricating thin films with three-dimensional electromagnetic band gaps at optical frequencies (i.e., photonic band gaps). The films consist of periodic tetragonal arrays of nanometre scale square helices, and are produced using a thin film deposition method known as Glancing Angle Deposition (GLAD). This technique relies on advanced substrate motion and extreme flux incidence angles to synthesize porous thin films with a chiral microstructure. Periodic organization of the material is achieved by pre-patterning the substrate surface with a seed topography using electron beam lithography, whilst the helical microstructure of the film provides periodicity in the third dimension, perpendicular to the substrate. The square spiral GLAD films closely match a near-ideal photonic band gap architecture, as proposed by Toader and John. By varying the substrate seeding period and the pitch of the square helices, we can potentially tailor the films to a variety of frequency response specifications. In addition, by deliberately leaving out seeds in the substrate pattern we can engineer line defects in the film. These properties will be crucial for using the GLAD films in electromagnetic components.

No abstract received.

The antenna systems with steerable capabilities are of significant importance for many current communication technologies such as satellite tracking for mobile communication, airport traffic control, radar systems etc. The unique magnetic behavior of ferrites and garnets at microwave frequencies can offer tenability to such systems. Garnets are usually considered as the best material for microwave device applications due to their excellent temperature stability and low magnetic and dielectric losses. On the other hand, lithium ferrites have shown the advantage of having high permittivity and high Curie temperatures. Keeping in view, the importance of garnet and some merits of ferrite materials an attempt has been made to make a comparative study in between these two materials, for tunable antenna applications. The paper describes the investigations carried out on the patch antenna, fabricated on LiTi ferrite and YIG material substrates. The resonant frequency of the antennas for both the cases was found to be changed significantly for about 300 MHz, with the external applied field of 2.5 mT. The transmitting power parameters (S₂₁) were found to be in good agreement with return loss characteristic (S₁₁). The gain offered by the ferrite was found superior to that observed for garnet based antenna. These finding can be of immense use for fabrication of device of specific applications.

The control of non-ionizing Electromagnetic (EM) radiation is becoming significant in many EM absorption and shielding applications in engineering and medicine. In this paper, design and development of novel composite materials and coatings employing distributed materials such as microcoil, chiral materials, ferromagnetic liquids, nonlinear metal oxides, ferroelectric tunable materials, voltage controllable dielectric and conducting polymers and microencapsulated balloons containing microcoil and ferromagnetic liquid are presented. One of the principal objectives of this effort is to develop both passive and smart (active) composites and coatings, with fast control architecture using Neural Networks, to control EM radiation over a very wide band range from very low frequencies -- where the wave is primarily magnetic in character-- to very high frequencies in the GHz range.

Results on both passive and active coatings on simple planar samples and complex 3D objects are presented which shows absorption in the order of −25 to −30 dB and EMI shielding effectiveness in the order of −60 to −110 dB for a wide range of frequency. The primary focus of the smart coatings is to employ Electroactive Polymeric (EAP) materials that would interact with the electromagnetic radiation, and perform a signature modification and evasion operation that has many applications.

A tunable microstrip ring resonator was designed and fabricated at resonance frequency 9.30 GHz in the X Band using the thin metallic film on the bulk polycrystalline ferrite substrate. Temperature stability of fabricated ring resonator have been studied by keeping it in the room temperature range.

No abstract received.

This paper presents analysis and behavior of ferrite beads with reference to the impedance, frequency, and spread spectrum modulation.

The first section of this paper will address the basic principles of EMI, ferrite chips, and will explain the characteristics of ferrites associated with EMI at low and higher frequencies. While the section two discusses reducing EMI with spread spectrum modulations.

At the present time it is actually developed the scientific-practical direction to the materials formation, which properties are changed under the external influence. Such materials are available to react on the temperature, pressure, moisture, electric and magnetic fields changes and so on. All of these are related to the kind of the so called "smart materials". One of such materials is the magnetoelastic (ME), which are available to change the form, dimensions and the elastic properties under the external magnetic field influence [1,2], to them are connected and the polymeric magnetic gels [3-6].

On the basis of such materials can be made the magneto-elastic thin-wall sensors, which can be considered as the magnetic analogs of the surface acoustical sensors, changing their properties in the function of the different influences of the external environment parameters. For example, for the distance pressure measurement, temperature, viscosity of the liquid and so on [7,8].

No abstract received.

Magnetic polymers on the base of epoxy composition and polyethylene within magnetic powder were obtained. Different forms of magnets can be obtained from these materials, while magnetic field may be concentrated outside or within sample forms. The obtained polymer magnets were elastic and strong. They had high adhesion, and stability to influence of seawater, petroleum, etc.

Conventional ferroelectric and ferromagnetic ceramics are crystalline ceramics. The amount of crystalline phase is almost closed to hundred percent. The introduction of a small amount glass phase or other second phase will much reduce the dielectric permittivity and the magnetic susceptibility of the materials. However, in many cases such as microwave applications, high permittivity and high susceptibility are not preferable to the requirements. In many cases, very complicated and comprehensive requirements of electric, magnetic, mechanic behaviors are of much importance to the specific applications. The processing requirements to the materials such as co-firing of the material with other materials and/or electrodes, sometimes dominates the selection of materials. A new group of microcrystalline ferroelectric and ferromagnetic glass ceramics is introduced to meet such special requirements. Most major ferroelectrics and ferromagnetics can be prepared as glass ceramics by using sol-gel technology. The glass content of the glass ceramics can be adjusted in very wide range from less than a few percent to more than ninety percent. The size of ferroic crystallites can be controlled in the range from nanometer sizes to micrometer sizes. The electric and magnetic behaviors of the glass ceramics will mostly inherit from the crystalline components, while the mechanical and thermal processing behaviors will be mostly depending on the glass components. The electric, magnetic, elastic, and thermal behaviors of the glass ceramics can be tailored in much wider range compare to crystalline counter parts. The ferroic glass ceramic can be prepared in the form of bulk ceramics, thick and thin films. There are a lot of advantages of glass ceramics, such as low densification temperatures, fine and dense microstructures, low specific density, low acoustic impedance, low and adjustable thermal expansion, good mechanical strength and toughness, better temperature and frequency stability of dielectric/magnetic behaviors. Discuss of the idea to develop the ferroelectric and ferromagnetic glass ceramics will be given in this presentation. Examples of the processing and microwave applications of the materials will be also given.

Ferroelectric thin films like Ba_0.5Sr_0.5TiO₃ are useful in several high-density electronic and tunable microwave device applications, because of its high dielectric constant and low loss. The reported values for dielectric constant and dielectric loss of Ba_0.5Sr_0.5TiO₃ thin films were varied from different research groups. We address to improve the dielectric properties of Ba_0.5Sr_0.5TiO₃ (BST) thin films by doping with other low loss oxide. We choose Al₂O₃ as dopant due its low microwave dielectric loss. The Al₂O₃-BST thin films were fabricated by pulsed laser deposition technique. The Structural phase composition were determined by X-ray diffraction (XRD). The characterization of dielectric properties for Al₂O₃-BST thin films were carried out by using non-destructive microstrip dual resonator. The dielectric loss are significantly reduce with the Al₂O₃-doping. Consequently, the figure of merit for Al₂O₃-BST thin films were founds to improvement with the content of Al₂O₃. Thus the Al₂O₃-BST thin films showed the potential to be exploited for tunable microwave devices applications.

Recently a transparent dielectric substrate based on Ba(Sn,Mg,Ta)O₃ material has been developed for microwave applications. We measured the real relative permittivity of this transparent substrate at frequency of 3.2 GHz using a split post dielectric resonator technique at temperatures from 230 K to 300K, and a high resolution cryogenic post resonator at 9.2GHz from 38K to 88K. The Transmission Mode Q factor Technique was used for data processing to ensure high accuracy of the measurements. Measured values of ε_r’ and tanδ of transparent BSMTO turned out to be 25.2 and 3.45×10⁻⁵ at room temperature respectively. The Ba(Sn,MgTa)O₃ transparent substrate may be useful for miniaturised planar antenna arrays and other microwave applications where transparent substrates are needed.

BaZr_xTi_1-xO₃ (BZT, x=0.20, 0.25, 0.30 and 0.35) ferroelectric thin films were deposited on Pt/Ti/SiO₂/Si and (100) LaAlO₃ substrates via a pulsed laser deposition (PLD). The deposited BZT thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrical measurement. The BZT thin films were of single phase perovskite structure. The films deposited on the Pt/Ti/SiO₂/Si substrate were polycrystalline, with a zero field dielectric constant of 641, 507, 485 and 419, and a maximum dielectric tunability of 59%, 68%, 65% and 53%, for the BaZr_xTi_1-xO₃ films of x=0.20, 0.25, 0.30 and 0.35, respectively. The films deposited on the LaAlO₃ substrate were epitaxial. BaZr_0.25Ti_0.75O₃ thin film on LaAlO₃ possessed a zero field dielectric constant of 413, a dielectric loss of 0.01 and a maximum dielectric tunability of 18%.

Yttrium Vanadate is a birefringent crystal material used in optical isolators and circulators with potentials for application in cryogenic microwave devices. As microwave properties of YVO₄ are not known, we measured the complex permittivity at frequency of 25 GHz using the Hakki-Coleman dielectric resonator technique in the temperature range from 13 K to 80K. The ε_r of YVO₄ turned out to be similar to that of sapphire, one of popular dielectric materials used at microwave frequencies. Measured loss tangent of YVO₄ was of the order of 10⁻⁶ at cryogenic temperatures. As Yttrium Vanadate is easy to fabricate and machine, it may replace the expensive sapphire in some microwave applications.

Good quality bilayers of ferromagnetic/superconductor (F/S) and superconductor/ferromagnetic (S/F) heterostructures were prepared by pulsed laser deposition. Detailed magnetization studies on these structures were carried out using a SQUID magnetometer. Notwithstanding, the identical growth condition, the epitaxial sequence seems to greatly influence the magneto-transport properties of these structures. The superconducting property of the first structure(F/S) is retained, while in the latter (S/F) structure, it is seriously impaired. We suggest that the contrasting behaviour of the two, otherwise similar, structures originates from the predominance of either of the two competing phenomena at the interface, namely, Andreev reflections and quasiparticle tunneling.

In this paper, we have reported the optical properties of vacuum evaporated CdTe thin films. Thin films have been prepared from the polycrystalline powder of CdTe on well-cleaned glass substrate under the vacuum of 10⁻⁶ Torr. The thicknesses of the films were calculated by the quartz crystal monitor (57nm to 575nm). The transmittance characteristics of films were studied by the spectrophotometer. The films were analyzed by an X-ray diffractometer. Lattice constants of the films were determined from the X-ray diffraction pattern using a Nelson-Relay plot. The optical constants (the refractive index n and the absorption constant k) and absorption coefficient α were measured on CdTe thin films in the wavelength range 190nm to 2500 nm. In these, we have observed direct band gap decreases with increasing thickness. The refractive index is found to be decreases of the wavelength of incident photon.

In this article, we have reported the structural, optical and electrical properties of ZnPc thin films. The samples were prepared by thermal evaporation method. The thickness of the samples were measured by the quartz crystal monitor. The structural analysis were carried out by X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). The XRD studies and SEM studies of the ZnPc thin films are reveals that the films are amorphous in nature. The optical properties in the transmission mode have studied by spectrophotometer in the visible region. The possible transition in these films is found to be direct and allowed. AC conduction mechanism and dielectric properties in these films (Al-ZnPc-Al structure) were studied by the LCR meter for various frequencies (12 Hz to 100 KHz) at different temperatures. The field dependence behaviour on activation energy and possible conduction mechanism in the ZnPc films under dc field have also been discussed.

Nitrogen contained iron films were deposited on Si (100) substrate by using Filtered Cathodic Vacuum Arc (FCVA) technique under various N₂ pressure and at various substrate temperatures. Atomic force microscope, surface profile measuring system, x-ray diffraction, x-ray photoelectron spectroscopy and vibrating sample magnetization were used to characterize the structural and magnetic properties of the films, respectively. The films exhibit a smooth surface and possess a polycrystalline structure. It was found that although the as-deposited films were α″ phase free, within a certain nitrogen concentration, the saturation magnetizations were still higher than that of the bulk pure Fe. This indicates that Fe⁺ ion or Fe^dipole formation possessing higher magnetic momentum upon nitridation. The study inspires the development of a new technique for deposition of magnetic films.

We present a fabrication of polarity sensing superconducting magnetometer of Wheatstone bridge type. Superconducting sensor was field cooled in the magnetic field of a 0.15T permanent magnet by placing near the N or S pole. Two variable resistors were used to balance the bridge circuit. Passing a current along the superconductor in a transverse magnetic field, a voltage develops between the ends as the current varied. The magnetic field dependence of the induced output voltage of the HTS magnetometer has been tested in the the 10⁻⁵ to 10⁻⁴T range by changing the distance between the pole of a permanent magnet to the S pole of the superconducting magnetometer. The positive voltage increased, when the S pole of the magnet approached the N pole of the sensor.

No abstract received.

Bingham model is used to characterize the constitutive behavior of the magnetorheological (MR) fluids subject to an external magnetic field strength. Based on the momentum equation of continuum mechanics, the effect of applied magnetic field on shear flow within the gap between two cylinders is analyzed theoretically. The expression of the rotating speed is derived to provide the theoretical foundations in the analysis of fluid’s circular flow. The results indicate that with the increase of applied magnetic field strength the flow of the fluid changes slow, the yield surface gets close to the inner cylinder.

A Cylindrical-type sample cell has been designed and produced to investigate the dielectric properties in the frequency range from 10⁻² Hz to 10⁹ Hz of heavy oils using impedance analyzer. The characteristics of the measurement cell are optimized to give high sensitivity. High-sensitivity complex dielectric constant measurements are obtained by calibration with several known fluids. From complex dielectric spectra, we observed the dielectric consisting of two regions for measuring frequency: the low frequency region may be due to diffusion charge transport caused by impurities while the dielectric relaxation mechanism of high frequency region seems to be the non-interacting Debye type. In the high frequency region, it has been observed that the relaxation time was in lineal proportion to viscosity of the heavy oils.

AUTHOR INDEX