Study on BMT Millimeter Wave Dielectric Resonators

The microwave dielectric properties and microstructures of Ba(Mg_1/3Ta_2/3)O₃ (BMT) ceramics were investigated. In order to reduce the evaporation of MgO at high temperature, burying in MgO powders sintered BMT ceramics. The relationship between the composition and Q value of BMT ceramics was discussed. Dielectric resonators using in millimeter wave band have been designed and fabricated. Their attenuations at central resonation frequency (_f) are less than 35dB. Temperature coefficients of resonant frequency are adjustable and less than 2 ppm/°C.     

Dielectric measurements of new antiferroelectric liquid crystals

Three new pure antiferroelectric materials, synthesized at the Military University of Technology (MUT), were investigated by dielectric means. Clear relaxation modes were detected, i.e., Soft, Goldstone, low frequency in-phase (P_L) as well high frequency anti-phase (P_H) modes for paraelectric Sm A*phase, ferroelectric Sm C* phase and antiferroelectric Sm C_A* phase, respectively. Additionally, a high relaxation mode in Sm C_A* was found. The relaxation frequencies f_R of the observed modes were calculated from Cole-Cole diagrams.     

Grain Boundaries in Barium Strontium Titanate Thin Films: Structure, Chemistry and Influence on Electronic Properties

In this paper, we investigate the role of grain boundaries in polycrystalline (Ba _x Sr_1–x )Ti_1+y O_3+z films, grown by metal organic vapor deposition, in the accommodation of nonstoichiometry, as well as their role in the strong composition dependence of the electric and dielectric behavior observed in these films. High-spatial resolution electron energy-loss spectroscopy is used for the analysis of composition and structural changes at grain boundaries, as a function of film composition. The existence of amorphous, titanium rich, TiO₂-like phases at the grain boundaries of films with large amounts of excess Ti (y 0.08) may explain the non-monotonic resistance degradation behavior of the films as a function of Ti content. However, we show that a grain boundary phase model fails to explain the strong composition dependence of the dielectric behavior. Electron energy-loss spectra indicate a distortion of the Ti–O octahedra in the grain interiors in samples with increasing Ti excess. The decrease of the dielectric constant with increasing amounts of excess Ti is therefore more likely due to Ti accommodation in the grain interiors.     

Polarity Estimate of the Hydrophobic Binding Sites in Erythroid Spectrin: A Study by Pyrene Fluorescence

The apparent dielectric constant, , of the hydrophobic pyrene binding sites in erythroid spectrin and human serum albumin (HSA) were estimated using the linear relationship [Turro, N.J., Kuo, P.L., Somasundaran, P. and Wong, K. (1986). J. Phys. Chem. 90, 288–291] between the ratio of the first (373 nm) and the third (384 nm) vibronic peak intensities (l₁/l₃) and the dielectric constant of the bulk medium. Binding of the hydrophobic fluorescent probe, pyrene, to erythroid spectrin and HSA was determined from concentration dependent change in the ratio l₁/l₃ from the emission spectra. Pyrene binds to spectrin (K_app = 6.2 × 10⁶ M^–1) with a higher affinity than that of HSA (K_app = 3.7 × 10⁴ M^–1) and the binding in both cases are saturable. The for spectrin and HSA was estimated to be 7 ± 2.1 and 5.4 ± 1.6 respectively. A case study with spectrin, covalently labeled with pyrene maleimide, have been presented for aging of pyrene-labelled spectrin showing the potential of the use of vibrational peak ratios (l₁/l₃) in the study of polarity of microenvironments in the neighborhood of cysteine residues of a protein. Large changes in the pyrene spectral components indicated conformational changes in the cysteine microenvironment of the protein upon storage at 4°C.     

Effect of ortho‐methylation on a bis(4‐cyanatocumyl)benezene low‐dielectric resin

A synthesis procedure for and characterization of a tetra‐ortho‐methyl substituted three‐ringed cyanate ester monomer, 4,4′‐[1,3‐phenylenebis(1‐methylethylidene)]bis[2,6‐dimethyl]phenylcyanato, and the corresponding cured resin are described with comparative characterization of the resin analogue without the ortho‐methyl substitutents. The othro‐methylation lowered the dielectric from 2.64 to 2.59 at 1 GHz and from 2.64 to 2.48 GHz at 15 GHz, whereas the tan δ was not significantly affected. Other physical properties also affected included a 40 °C decrease in the glass‐transition temperature, an increase in the thermal‐expansion coefficients, and a reduction in thermogravimetric stability. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 60–67, 2003     

Phase transitions in the hydroquinone hydrogen sulfide clathrate compound

Heat capacities of [C₆H₄(OH)₂]₃·(H₂S) _x were measured between 1 and 15 K. Heat capacity peaks were found at (7.56±0.09) K, (7.61±0.05) K, and (7.65±0.07) K for the compounds withx=0.92, 0.95, and 0.96. A weak anomaly was observed around 6.75 K for the compound withx=0.85. The temperatures of these anomalies are unusually low among the phase transitions of molecular crystals. The decrease of the transition temperature from that of crystalline H₂S (=103.52 K) is a clear indication of the effect of enclathration on the molecular interaction. A comparison of the rotational heat capacity of the trapped hydrogen sulfide molecules with that of crystalline hydrogen sulfide shows that the trapped hydrogen sulfide molecules have a large rotational freedom at low temperatures (13 K). This agrees with the results from far infrared spectroscopic data. The dielectric constant of the clathrate compound obeyed the Curie-Weiss law above 30 K and no significant dielectric loss was found over the whole temperature range. These results showed that the trapped hydrogen sulfide molecules execute free rotation or are orientationally disordered above 20 K.Dedicated to Professor H. M. Powell.     

Dielectric waveguide modulators at 95 GHz using LiNbO3

We describe electro-optic modulators at 95 GHz using LiNbO₃ in an H-guide configuration. We have observed 70% amplitude modulation at l KHz in a bridge configuration, corresponding to an induced phase shift of ±29°. The prospects for improved device performance will be discussed.Supported in part by Subcontract #67-471915 with California Institute of Technology under Prime Contract N00014-79-C-0839 with Office of Naval Research.     

AC conductivity and dielectric properties of sulphate glasses

AC conductivity and dielectric properties of sulphate glasses have been studied as a function of temperature, frequency and variation in interalkali concentration. AC conductivity at frequencies well beyond the dielectric loss peaks seems to arise from local motion of alkali ions within the neighbouring potential wells. Activation energies for AC conductivity were found to be very much lower than those for DC conductivity. Further, AC conductivity seems to be independent of interalkali variation, whereas ε′ and tan δ show a mild degree of mixed alkali effect. The observations made here have been explained on the basis of a structural model earlier proposed by us for these glasses. Communication No. 167 from Solid State and Structural Chemistry Unit.     

Analysis of a shielded TE011 mode composite dielectric resonator for stable frequency reference

Analysis of a TE₀₁₁ mode composite sapphire-rutile dielectric resonator has been carried out to study the temperature variation of resonance frequency, close to the Cs atomic clock hyperfine frequency of 9.192 GHz. The complementary behavior of dielectric permittivity with temperature of the composite has been exploited to obtain the desired turning point in the resonant frequency. The frequency of the composite structure is found to be independent of the shield diameter beyond four times the puck diameter.     

Electrical transport properties of CuWO4

The temperature dependence of the electrical conductivity, thermoelectric power and dielectric constant of the antiferromagnetic CuWO₄ have been studied in the temperature range 300–1000 K. The conductivity results can be summarised by the equations σ_I=6.31 × 10⁻³ exp (−0.29 eV/kT) ohm⁻¹ cm⁻¹ in the temperature range 300–600 K and σ_II=3.16 × 10⁵ exp (−1.48 eV/kT) ohm⁻¹ cm⁻¹ between 600 K and 1000 K. The thermoelectric power can be expressed byθ=[− 1.25 (10³/T) + 3.9] mV/K. Initially dielectric constant increases slowly but for high temperatures its increase is fast.