Dielectric properties of 4′-n-alkyl-4-cyanobiphenyls in their nematic phases

The principal dielectric constants of the pentyl to octyl derivatives of 4′-n-alkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases. All the compounds exhibit a strong positive dielectric anisotropy due to the presence of a large dipole moment along the major molecular axis. The principal dielectric constantsε _‖ andε _⊥ as well as the anisotropy Δε decrease with increasing alkyl chain length. The experimental value of (ε_‖+2ε_⊥) decreases with decreasing temperature, and is throughout less than the extrapolated isotropic value, in conformity with the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar.     

Impact of ethylene carbonate on electrical properties of PVA/(NH4)2SO4/H2SO4 proton-conductive membrane

A new proton-conductive membrane (PCM) based on poly (vinyl alcohol) and ammonium sulfate (NH₄)₂SO₄ complexed with sulfuric acid and plasticized with ethylene carbonate (EC) at different weight percent were prepared by casting technique. The structural properties of these electrolyte films were examined by XRD studies. The XRD patterns of all the prepared polymer electrolytes reveal the amorphous nature of the films. ac conductivity and dielectric spectra of the electrolyte were studied with changing EC content from weight 0.00 to 0.75 g. A maximum conductivity of 7.3 × 10⁻⁵ S cm⁻¹ has been achieved at ambient temperature for PCM containing 0.25 g of ethylene carbonate. The electrical conductivity σ, dielectric constant ε′ and dielectric loss ε″ of PCM in frequency range (100 Hz to 100 KHz), and temperature range (300–400 K) were carried out. Measurement of transference number was carried out to investigate the nature of charge transport in these polymer electrolyte films using Wagner’s polarization technique. Transport number data showed that the charge transport in these polymer electrolyte systems was predominantly due to ions. The electrolyte with the highest electrical conductivity was used in the fabrication of a solid-state electrochemical cell with the configuration (Mg/PCM/PbO₂). Various cell parameters ldensity, and current density were determined. The fabricated cells gave capacity of 650 μAh and have an internal resistance of 11.6 kΩ.     

Iron based carbon nanocomposites for electromagnetic wave absorber with wide bandwidth in GHz range

The electromagnetic wave absorption properties of resin compacts containing 40 vol. % composite powders of α-Fe/C(a), and Fe₃C/C(a) were characterized in a frequency range of 0.05–26.5 GHz, according to a conventional reflection/transmission technique. The real part (ε_r ^′) and the imaginary part (ε_r ^′′) of relative permittivity were constantly low in the 2–14 GHz (ε_r ^′= ∼12.4 and ε_r ^′′= ∼0.6) for α-Fe/C(a) resin composites, and in the 1–26.5 GHz (ε_r ^′= ∼9.6 and ε_r ^′′= ∼0.8) for Fe₃C/C(a) ones. The imaginary part (μ_r ^′′) of relative permeability exhibited wide peaks in the 1–9 GHz range for α-Fe/C(a), and in the 2–26.5 GHz range for Fe₃C/C(a) owing to their different magnetocrystalline anisotropy field values. Consequently, the resin compacts with 40 vol. % α-Fe/C(a), and Fe₃C/C(a) powders provided good electromagnetic wave absorption performances (reflection loss <-20 dB) in ranges of 4.3–8.2 GHz, and 9–26.5 GHz over absorber thicknesses of 1.8–3.3 mm, and 1.0–2.4 mm, respectively. PACS 76.50.+g; 61.46.+w; 75.50.Bb; 75.30.Gw; 75.20.En     

On the harmonic analysis of non-linear dielectric effects

We investigate the nonlinear dielectric effects in a polar viscous liquid, propylene carbonate, by analyzing the voltage and current traces obtained for a sinusoidal electric field at a frequency of 1 kHz and field amplitudes as high as 162 kV/cm. The main source of non-linear behavior results from the energy absorbed from the field and is understood quantitatively. However, there is a qualitative difference in the behavior of the field induced change, Δε^′′(E), and the third harmonic amplitude of the current, I_3ω. Although both Δε^′′(E) and I_3ω are considered reliable measures of non-linear behavior, we show here that the third harmonic signal reflects only those non-linear responses that are instantaneous on the time scale of the test frequency.     

Electrical and optical properties of vacuum deposited MnPc thin films

Sandwich and planar structures were fabricated using manganese phthalocyanine (MnPc) as active layer and gold (Au) as electrodes by thermal evaporation method. The permittivity ε of MnPc was determined from the dependence of capacitance on film thickness. J-V characteristics of Au/MnPc/Au structure at room temperature were performed. Thermally generated hole concentration p₀, hole mobility μ_p, total trap concentration N_t and depth of the trap level were estimated. The activation energies of MnPc films were determined from the Arrhenius plots of ln σ versus 1000/T. The absorption and reflectance spectra of MnPc thin film deposited at room temperature were recorded in the spectral range 300–900 nm. The optical band gap of MnPc thin film was determined from the α² versus hν graph. The optical constants n and k were found. The real and imaginary parts of the optical dielectric constant ε₁ and ε₂ were calculated.     

Relaxation phenomena of polar non-polar liquid mixtures under low and high frequency electric field

Simultaneous calculation of the dipole moment μ_j and the relaxation time τ_j of a certain number of non-spherical rigid aliphatic polar liquid molecules (j) in non-polar solvents (i) under 9.8 GHz electric field is possible from real ε′_ij and imaginary ε″_ij parts of the complex relative permittivity ε*_ij. The low frequency and infinite frequency permittivities ε_0ij and ε_∞ij measured by Purohitet al [1,2] and Srivastava and Srivastava [3] at 25, 35 and 30°C respectively are used to obtain static μ_s. The ratio of the individual slopes of imaginary σ″_ij and real σ′_ij parts of high frequency (hf) complex conductivity σ*_ij with weight fractionsw _jatw _j → 0 and the slopes of σ″_ij— σ′_ij curves for differentw _js [4] are employed to obtain τ_js. The former method is better in comparison to the existing one as it eliminates polar-polar interaction. The hf μ_js in Coulomb metre (C m) when compared with static and reported μs indicate that μ_s s favour the monomer formations which combine to form dimers in the hf electric field. The comparison among μs shows that a part of the molecule is rotating under X-band electric field [5]. The theoretical μ_theos from available bond angles and bond moments of the substituent polar groups attached to the parent molecules differ from the measured μ_js and μs to establish the possible existence of mesomeric, inductive and electromeric effects in polar liquid molecules.     

Effect of illumination on a dielectric nonlinearity in Cr-doped SBN-75 single crystal

The dielectric nonlinearity in an SBN-75 single crystal doped with 0.01 at % Cr was investigated in the vicinity of a diffuse phase transition by studying the reverse dielectric dependences ε′(E ₌). The influence of illumination and exposure (aging) on the behavior of ε′(E ₌) was demonstrated. A reduction in the dielectric memory effect after sample illumination was detected.     

Space-charge relaxation and electrical conduction in?K0.5Na0.5NbO3 at high temperatures

Sodium potassium niobate K_0.5Na_0.5NbO₃(KNN) ceramic was synthesized by a solid-state technique. The X-ray diffraction of the sample at room temperature showed a monoclinic phase. The real part (ε′) and imaginary part (ε″) of dielectric permittivity of the sample were measured in a frequency range from 40 Hz to 1 MHz and in a temperature range from 350 to 850 K. The ε′ deviated from Curie–Weiss law above 702 K, due to additional dielectric contributions resulting from universal dielectric response and thermally activated space charges at high temperatures. This anomaly arose from a Debye dielectric dispersion that slowed down following an Arrhenius law. We have established a link between the dielectric relaxation and the conductivity.     

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (T _C*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for f_NRM, δε^NRM and distribution parameter α_NRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theT _C*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.     

Absolute absorber quantification in turbid media at small source–detector separations

The aim of our studies was to develop a method to determine the absorption coefficient of a turbid medium in a reflection geometry with small source–detector separations. Therefore, the time-integrated microscopic Beer–Lambert law (MBL) was modified in order to obtain the absolute absorption coefficient from mean time of flight and dc-intensity measurements. The new technique was evaluated using turbid phantoms having varying scattering (μ_s ^′ between 0.2 and 2.2 mm^-1) and absorbing properties (μ_a between 0.04 and 0.14 mm^-1), comparable to many biological tissues at various source–detector separations between 3 and 11 mm. The measurements were performed at a wavelength of 1064 nm. We found that this new method was able to determine the absolute absorption coefficient of the selected phantoms with a standard error of less than 0.005 mm^-1 over the range of optical properties investigated. Received: 23 November 2001 / Revised version: 28 February 2002 / Published online: 14 May 2002     

Electrical conductivity and dielectric constant of A-type Nd2O3

The measurements of electrical conductivity (σ) from 300 to 1200 K and dielectric constant (ε′) from 4·2 to 1200 K of A-type Nd₂O₂ pellets are reported here. Electrical conductivity (σ) data can be explained in terms of impurity. The dielectric constant (ε′) increases slowly up to 500 K as is expected for ionic solids. The increase ofε′ becomes much faster above 500 K, which is attributed to space charge polarization of thermally generated charge carriers.     

Direct patterning of gold oxide thin films by focused ion-beam irradiation

For direct writing of electrically conducting connections and areas into insulating gold oxide thin films a scanning Ar⁺ laser beam and a 30 keV Ga⁺ focused ion beam (FIB) have been used. The gold oxide films are prepared by magnetron sputtering under argon/oxygen plasma. The patterning of larger areas (dimension 10–100 μm) has been carried out with the laser beam by local heating of the selected area above the decomposition temperature of AuO_x (130–150 °C). For smaller dimensions (100 nm to 10 μm) the FIB irradiation could be used. With both complementary methods a reduction of the sheet resistance by 6–7 orders of magnitude has been achieved in the irradiated regions (e.g. with FIB irradiation from 1.5×10⁷ Ω/□ to approximately 6 Ω/□). The energy-dispersive X-ray analysis (EDX) show a considerably reduced oxygen content in the irradiated areas, and scanning electron microscopy (SEM), as well as atomic force microscopy (AFM) investigations, indicate that the FIB patterning in the low-dose region (10¹⁴ Ga⁺/cm²) is combined with a volume reduction, which is caused by oxygen escape rather than by sputtering. Received: 30 May 2000 / Accepted: 31 May 2000 / Published online: 13 July 2000     

Experimental investigation of muon-catalyzed t + t fusion

The muon-catalyzed fusion (μCF) process in tritium was studied by the μCF collaboration on the muon beam of the JINR Phasotron. The measurements were carried out with a liquid tritium target at the temperature 22 K and density approximately 1.25 of the liquid hydrogen density (LHD). Parameters of the μCF cycle were determined: the ttμ muonic molecule formation rate λ _ttμ = 2.84(0.32) μs⁻¹, the ttμ fusion reaction rate λ _f = 15.6(2.0) μs⁻¹, and the probability of muon sticking to helium ω _tt = 13.9(1.5)%. The results agree with those obtained earlier by other groups, but better accuracy was achieved due to our unique experimental method. The article is published in the original.     

EPR investigations of Cu(II) complexes with 5′-CMP and 5′-GMP

Electron paramagnetic resonance investigation of the Cu(II) complexes with guanosine-5′-monophosphate (5′-GMP) and cytidine-5′-monophosphate (5′-CMP) shows the affinity of the Cu(II) ion to interact with the base and the phosphate group. The different modes of the coordination of the metal ion at the nucleotide and the water molecules lead to octahedral species, distorted by dynamical Jahn-Teller effect (g₀ = 2.106) for the Cu(II)-5′-CMP complex and rhombically distorted (g₁ = 2.358, g₂ = 2.126, g₃ = 2.068) or tetragonally distorted (g_∥^′ = 2.299, g_⊥^′ = 2.126) for the Cu(II)-5′ -GMP complex. The compound with 5′-CMP presents also a more stable in time square-planar species (g_∥ = 2.265, A_∥ = 162 G, g_⊥ = 2.076). The local symmetry changes in aqueous solution by coordination of water molecules.     

Dielectric behaviour of ketone-amine binary mixtures at microwave frequencies

Values of dielectric constant (ε′) and loss factor (ε″) have been experimentally determined for binary liquid mixtures of ethyl methyl ketone+ethylenediamine and methyl isobutyl ketone+ethylenediamine at 9.44 GHz microwave frequencies at 30°C. The values ofε′ andε″ have been used to evaluate the molar polarization, apparent polarization and the excess permittivities. Excess refractive index, viscosity and activation energy of viscous flow have also been estimated. These parameters have been used to explain the formation of 1:1 complexes for both the systems.     

Dielectric relaxation behaviour of liquid crystals with opposite orientation of -COO ester group in molecular core

Dielectric measurements were performed for two smectogens, being structurally analogous with the opposite space orientation of the-COO ester group in the molecular core, using a HP4192A impedance analyser. Gold coated electrode cells of different thicknesses were used. Temperature dependences of the relaxation frequency fR and the inverse of the dielectric strength Δε′_┴ ⁻¹ obtained by fitting experimental values of perpendicular components, i.e., the real ε′_┴ and the imaginary ε″_┴ parts of the complex dielectric permittivity to the Cole-Cole equation as well as the investigation of modification of relaxation processes under bias were determined for the SmC_*, SmA_*, and N_* phases of studied compounds. One can conclude, on the basis of the above results, that dielectric relaxation processes observed by us in the studied compounds are similar to those of the soft and Goldstone mode typically observed by others in the SmC_*, SmA_*, and N_* phases. It is concluded from acomparison of their properties with other related compounds that the link between the biphenyl moiety and-COO ester group is closely related to the stability of smectic phases.     

Dielectric response of makrofol-KG polycarbonate irradiated with 145 MeV Ne6+ and 100 MeV Si8+ ions

The passage of heavy ions in a track detector polymeric material produces lattice deformations. These deformations may be in the form of latent tracks or may vanish by self annealing in time. Heavy ion irradiation produces modifications in polymers in their relevant electrical, chemical and optical properties in the form of rearrangement of bonding, cross-linking, chain scission, formation of carbon rich clusters and changes in dielectric properties etc. Modification depends on the ion, its energy and fluence and the polymeric material. In the present work, a study of the dielectric response of pristine and heavy ion irradiated Makrofol-KG polycarbonate is carried out. 40 μm thick Makrofol-KG polycarbonate films were irradiated to various fluences with Si⁸⁺ ions of 100 MeV energy from Pelletron at Inter University Accelerator Centre (IUAC), New Delhi and Ne⁶⁺ ions of 145 MeV from Variable Energy Cyclotron Centre, Kolkata. On irradiation with heavy ions dielectric constant (ɛ′) decreases with frequency where ɛ′ increases with fluence for both the ions. Variation of loss factor (tan δ) with frequency for pristine and irradiated with Si ions reveals that tan δ increases as the frequency increases. Tan δ also increases with fluence. While Ne irradiated samples tan δ shows slight variation with frequency as well as with fluence. Tan δ has positive values indicating the dominance of inductive behavior.      

The effect of preparation technique on the optical parameters of biological tissue

The absorption coefficient μ_a, the scattering coefficient μ_s, and the scattering anisotropy factor g of porcine liver were studied in vitro using the integrating sphere technique and inverse Monte Carlo simulation in the wavelength range 450 to 700 nm. A reference preparation technique was developed using a dermatome providing specimens of 200 to 800 μm thickness without pre-freezing the tissue. The optical parameters as measured applying the reference preparation were compared to those measured after cryo-homogenisation. We found significant deviations of the scattering coefficient and the anisotropy factor which were compensated when the reduced scattering coefficient μ_s ^′ was calculated. We also compared the effects of freezing reference specimens at -20 °C and at 77 K without homogenisation. For both freezing protocols noticeable deviations were found in all three optical parameters as well as in μ_s ^′. The impact of tissue storage at 4 °C was measured in the range 4 to 48 h post mortem and showed a clear reduction of μ_a and a significant increase of μ_s even after 24 h of storage. Short-time storage of the specimens in saline solution reduced all three optical parameters significantly. In conclusion, the tissue preparation must be controlled in order to provide in vitro optical parameters that sufficiently mimic the in vivo situation. Received: 29 June 1999 / Revised version: 1 October 1999 / Published online: 3 November 1999     

Measurement of helium-3 and deuterium stopping power ratio for negative muons

The measurement method and results measuring of the stopping power ratio of helium-3 and deuterium atoms for muons slowed down in the D/³He mixture are presented. Measurements were performed at four values of pure ³He gas target densities, ϕ_He = 0.0337, 0.0355, 0.0359, 0.0363 (normalized to the liquid hydrogen density) and at a density 0.0585 of the D/³He mixture. The experiment was carried out at PSI muon beam μE4 with the momentum Pμ= 34.0 MeV/c. The measured value of the mean stopping ratio S$_{3^He/D}$ is 1.66±0.04.     

Dielectric and conductivity relaxations in PVAc based polymer electrolytes

The polymer electrolytes composed of a blend of poly (vinyl acetate) (PVAc) and poly (methylmethacrylate) (PMMA) as a host polymer and LiClO₄ as a salt are prepared by a solution casting technique. The formation of blend polymer- salt complex has been confirmed by FT-IR spectral studies. The conductivity- temperature plots are found to follow an Arrhenius nature. Arrhenius plot shows the decrease in activation energy with the increase in salt concentration. The dielectric behaviour of the sample is analysed using dielectric permittivity (ε′), dielectric loss (ε″) and electric modulus (M″) of the samples. The impedance cole- cole plot shows the high frequency semi- circle is due to the bulk effect of the material and the depression in the semicircle shows the non-Debye nature of the material. The bulk conductivity is found to vary between 2.5×10⁻⁵ Scm⁻¹ to 1.7×10⁻³ Scm⁻¹ with the increase of salt concentration of blend polymer samples. The migration energy derived from the dissipation factor is almost equal to the activation energy calculated from conductivity. The modulus spectrum of the samples shows the non-Debye behaviour of the polymer electrolyte films. The low frequency dispersion of the dielectric constant implies the space charge effects arising from the electrodes. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.