Orientational phase transition and the solvation force in a nematic liquid crystal confined between inhomogeneous substrates

A nematic liquid crystal confined between two identical flat solid substrates, with an alternating stripe pattern of planar and homeotropic anchoring, is studied in the framework of the Frank-Oseen theory. By means of numerical minimization of the free energy functional we study the effect of the sample thickness D on the location of the phase transition between a uniform alignment, either planar or homeotropic, and a distorted nematic texture. The solvation force f due to distortions of the nematic director is also studied. It is found that f is always attractive, and for D small compared to the periodicity of the surface structure it exhibits two distinct asymptotic behaviors: f ∼ - D ^-1/2 or f ∼ - D ^-1, depending on the relation between D and the extrapolation lengths. Received 12 November 2002 Published online: 16 April 2003     

Electric field generated solitons,disclinations and vortical flows in freely suspended nematic 8CB

We present the results of optical studies on the instabilities in a substrate-free nematic 8CB film, subject to an in-plane electric field. The initial director field involves a −1/2 strength disclination loop, separating the central pseudoisotropic zone from the splay-bend (SB) birefringent boundary. Three regimes of sample thickness are distinguishable on the basis of field-induced instabilities. Thick (∼75 μm) films display growth of SB zones (independently of disclination movement), wall-formation, and reversible transition between walls and disclinations. Moderately thick films, a few μm in the central part, exhibit distinctive undulations at the border of advancing SB layers. Submicron thin films, with smectic-like homeotropic central plateau, show spectacular isotropic vortex-pairs at either end of this plateau. Further, the end regions of the birefringent zone exhibit both electro-convective flows and reorientational effects. The latter are associated with the formation of open and closed walls, and loop-wall emission. The final high field instability involves jet-like flows at the two ends of the film.     

Accurate method for determining tilt bias angles in thin films of nematic liquid crystals

We have developed a new method for measuring tilt bias angles in spatially uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an exponentially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoelectric torque, are excited with an external periodic field. A periodic variation of the ellipticity of the light wave reflected from the interface is detected at both the first and second harmonics of the exciting electric field. When these two Fourier components of the electrooptic response are known, it is possible to calculate both the tilt bias angle θ₀ of the director and the dynamic deviation δ₀ of the tilt bias angle. The angles θ₀ and δ₀ measured by this method on the surface of an electrode (ITO) and on the surface of a ferroelectric film (a copolymer of vinylidene fluoride and trifluoroethylene), oriented in a corona discharge, were equal to θ ₀=5.1°, δθ=0.5° and θ₀=89°, δθ=0.06°, respectively. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 387–392 (25 March 1998)     

Fluctuations in liquid crystals in the presence of a flexoelectric effect

We examine director fluctuations in nematic liquid crystals in an electric field in the presence of a flexoelectric effect. For the planar and homeotropic orientations we calculate the correlation functions and analyze these functions near the Freedericksz transition and the flexoelectric instability threshold. For both geometries we calculate the angular dependence of the intensity of light scattered by director fluctuations and examine its behavior at electric field strengths close to critical. Zh. éksp. Teor. Fiz. 112, 1675–1693 (November 1997)     

Relaxation and shift of the precession frequency of the spin of a negative muon in n-type silicon

The residual polarization of negative muons in n-type silicon with impurity density (1.6±0.2) · 10¹³ cm⁻³ is investigated as a function of temperature in the range 10–300 K. The measurements are performed in an external magnetic field of 0.08 T oriented transversely to the spin of the muons. Relaxation of the muon spin and a shift of the precession frequency are observed at temperatures below 30 K. The relaxation rate at 30 K equals 0.25±0.08 μs⁻¹. The shift of the precession frequency at 20 K equals 7 · 10⁻³. Both the relaxation rate and the shift of the precession frequency increase as the temperature decreases. At temperatures below 30 K the relaxation rate is described well by the relation Λ=bT ^−q , where q=2.8±0.2. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 539–543 (10 April 1996)     

Polarisation resistance of the O2, Au/O2− and H2-H2O, Au/O2− electrode systems

Gold electrodes with known contact geometries were studied using impedance spectroscopy. From these data it was possible to determine the specific polarisation conductivity per unit length of three-phase boundary (TPB). The values were found to be (3÷22)×10⁻⁴ S·cm⁻¹ dependent on the electrode history in pure oxygen at 977 °C and 2×10⁻⁶ S·cm⁻¹ at 977 °C in “pure” hydrogen (P_O2=10⁻²⁰ atm at 1001 °C). The results are compared with previous data obtained for platinum electrodes.     

Surface driven transition in a nematic liquid crystal cell

Surface driven reorientation effects in a nematic liquid crystal cell caused by light-induced changes of the anchoring parameters were studied. Theoretical consideration of one-dimensional flat distributions of the director has shown that the director can undergo threshold reorientation between hybrid, homeotropic, and planar alignments as the anchoring energy varies continuously. The threshold reorientation takes place when the reference and light-induced easy axes are perpendicular. In the one-elastic-constant approximation the light-induced transition was found to be of second order as shown by a critical increase of the director thermal fluctuations in the vicinity of the transition point. These effects were experimentally studied in the cells containing 5CB liquid crystal aligned by the photosensitive azo-containing polymer layer. Zh. éksp. Teor. Fiz. 112, 2045–2055 (December 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Influence of the alignment surface on the characteristics of optically addressed liquid-crystal modulators

Two optically addressed transmission-type liquid-crystal modulators based on a nematic liquid crystal-a-Si: C: H photoconductor structure are studied. The modulators differ in the initial tilt angle of the director and in the barrier height at the interface. It is shown that these parameters influence the dependence of the diffraction efficiency on the feed conditions, recording pulse repetition rate, and spatial frequency of the recorded grating. The spatial resolution of the modulators at a level of half the diffraction efficiency is equal to 75 mm⁻¹. The maximal diffraction efficiency is high, 69%, owing to the asymmetric profile of the grating, which arises at a large initial tilt angle of the liquid crystal director.     

Polarisation conductivity of the O2,Pt/Zr(Y)O2 and H2-H2O, Pt/Zr(Y)O2 electrodes

Pt electrodes with defined contact geometries were studied by using impedance spectroscopy. The specific polarisation conductivity per unit length of the three-phase boundary was determined. It is found to be 1 × 10⁻⁴ S·cm⁻¹ at 977 °C in an atmosphere of “pure” hydrogen with an oxygen partial pressure of 10⁻²⁰ atm at 1000 °C. Investigations carried out in an atmosphere of pure oxygen revealed a pronounced dependence of the polarisation conductivity on the electrode history. The polarisation conductivity was found to be in a range of 2 × 10⁻⁴ to 6.5 × 10⁻⁴ S·cm⁻¹ at a temperature of 977 °C. It was possible to estimate the area of the electrolyte surface which takes part in the electrode reaction. The real exchange current density was determined.     

Anharmonic force constants of GaSb from the measured third order elastic constants at 300°K

The third-order elastic constants of single crystal GaSb are determined using ultrasonic pulse interferometer at 10 MHz. The constants at 300°K, in units of 10¹¹ N.m.⁻², are C_l11 = ™ 4 ·75 ± 0·06 C₁₄₄ = + 0·50 ± 0·25 C₁₁₃ = ™ 3 ·08 ± 0·02 C₁₆₆ = ™ 2·16 ± 0·13 C₁₂₃ = ™ 0 ·44 ± 0·29 C₄₅₆ = ™ 0·25 ± 0·15 These constants are used to evaluate the three anharmonic first and second neighbour force constants based on modified Keating’s model. The constants are (in units of 10¹¹ N.m⁻²)γ=− 2·406;δ=0·407;ε=−0·222.     

Filler-matrix thermal boundary resistance of diamond-copper composite with high thermal conductivity

A composite material with a high thermal conductivity is obtained by capillary infiltration of copper into a bed of diamond particles of 400 μm size, the particles having been pre-coated with tungsten. The measured thermal conductivity of the composite decreases from 910 to 480 W m⁻¹ K⁻¹ when the coating thickness is increased from 110 to 470 nm. Calculations of the filler/matrix thermal boundary resistance R and the thermal conductivity of the coating layer λ _i using differential effective medium, Lichtenecker’s and Hashin’s models give similar numerical values of R and λ _i ≈ 1.5 W m⁻¹ K⁻¹. The minimal thickness of the coating h ∼ 100 nm necessary for ensuring production of a composite while maximizing its thermal conductivity, is of the same order as the free path of the heat carriers in diamond (phonons) and in copper (electrons). The heat conductance of the diamond/tungsten carbide coating/copper interface when h is of this thickness is estimated as (0.8–1) × 10⁸ W m⁻² K⁻¹ and is at the upper level of values characteristic for perfect dielectric/metal boundaries.     

Anisotropic stokes drag and dynamic lift on cylindrical colloids in a nematic liquid crystal

We have measured the Stokes drag on magnetic nanowires suspended in the nematic liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The effective drag viscosity for wires moving perpendicular to the nematic director differs from that for motion parallel to the director by factors of 0.88 to 2.4, depending on the orientation of the wires and their surface anchoring. When the force on the wires is applied at an oblique angle to the director, the wires move at an angle to the force, demonstrating the existence of a lift force on particles moving in a nematic. This dynamic lift is significantly larger for wires with homeotropic anchoring than with longitudinal anchoring in the experiments, suggesting the lift force as a mechanism for sorting particles according to their surface properties.     

EPR of Fe3+ ion and symmetry of [AIF5·H2O]2? complex ion in (NH4)2AIF5·H2O single crystals

We used the spin-Hamiltonian method for the analysis of the electron paramagnetic resonance (EPR) spectrum of Fe³⁺ as a probe ion in (NH₄)₂AlF₅·H₂O single crystalline basic material. The theoretical expressions for the magnetic field (at which the fine structure transition lines appear) versus the angle between the magnetic field and the axis of symmetry of the magnetic complex are also given. These values were calculated by applying the perturbation theory to the second-order terms. From the experimental results (at 300 K and 9.21 GHz), the spin-Hamiltonian parameters were deduced:D=(668±10)·10⁻⁴ T,E=(−56±10)·10⁻⁴ T,a=(−54±10)·10⁻⁴ T,F=(30±10)·10⁻⁴ T. An isotropic superhyperfine structure was evidenced for the five fluorine ions. The obtained EPR data were used to determine the local symmetry of the Al³⁺ ion. A good agreement with X-ray diffraction measurements was found.     

Local surface segregations of implanted aluminum in an iron crystal with a low density of defects

The surface distribution of elements is studied by scanning a 3-MeV proton beam along the surface of a bcc-Fe sample implanted with aluminum ions in the dose interval (1–50) · 10¹⁶ cm⁻². Ring-shaped regions, up to 30 μm in diameter, with a high density of aluminum, which appear at implantation doses (5–20) · 10¹⁶ cm⁻², are observed. These regions appear as a result of radiation-stimulated segregation processes. A mechanism based on the existence of a low density of dislocations in the initial crystal is proposed to explain the implanted impurity segregation processes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 86–89 (10 January 1997)     

Calculation of thermal parameters of SiGe microbolometers

The thermal parameters of a SiGe microbolometer were calculated using numerical modeling. The calculated thermal conduction and thermal response time are in good agreement with the values found experimentally and range between 2·10⁻⁷ and 7·10⁻⁸ W/K and 1.5 and 4.5 ms, respectively. High sensitivity of microbolometer is achieved due to optimization of the thermal response time and thermal conduction by fitting the geometry of supporting heat-removing legs or by selection of a suitable material providing boundary thermal resistance higher than 8·10⁻³ cm²·K/W at the SiGe interface. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 44–50, December, 2007.     

Unsteady conjugate thermogravitational convection in a cylindrical region with local energy source

The mathematical modelling of unsteady regimes of natural convection in a closed cylindrical region with a heat-conducting shell of finite thickness was carried out in the presence of a local heat source under the conditions of convective heat exchange with the ambient medium. The mathematical model was constructed in dimensionless variables “stream function — vorticity vector — temperature” in the cylindrical coordinate system. The influence of the Rayleigh number, 10⁴ ≤ Ra ≤ 10⁶, of the unsteadiness factor 0 < τ < 300, of the thermal conductivity ratio λ _2,1 = 5.7·10⁻⁴, 4.3·10⁻², and the energy source sizes on both local characteristics (streamlines and temperature fields) and on the integral complex (the mean Nusselt number on typical boundaries) was analysed in detail. Thermohydrodynamic peculiarities due to the geometry of the object of research were established.     

Hole mobility and trapping in PVK films doped with CdSe/CdS and CdSe quantum dots

Experimental study of the hole mobility in polyvinylcarbazole (PVK) films doped with two kinds of nanocrystals, on bare core CdSe and core-shell CdSe/CdS quantum dots, with concentrations ranging from 3 · 10¹⁰ to 3 · 10¹⁵ cm⁻³, is presented. The quantum dots investigated were made using colloidal chemistry. The hole mobility was measured using the time-of-flight technique as a function of the applied electrical field in the range 10⁵–10⁶ V/cm and for temperatures from 20°C to 50°C. The transient curves, being featureless on a linear plot, show on a double logarithmic scale a sharp inflection point indicating a dispersive carrier drift process. The recovered values of the mobility are in the range 3 · 10⁻⁸–10⁻⁶ cm²·V⁻¹·s⁻¹ and their field and temperature dependences can be analyzed formally within the framework of the Gaussian disorder model proposed by B?ssler. The energetic disorder is, within the experimental accuracy, independent of the concentration and type of quantum dots for the CdSe quantum dots at all concentrations and for the CdS/CdSe quantum dots up to 10¹⁴ cm⁻³. The spatial disorder factors are very large (from 5.3 to 8.7) and do not depend in a systematic way upon the type and concentration of quantum dots (QDs). The experiments show that the apparent mobility does not change considerably with concentration, but it was found that the samples with CdSe/CdS quantum dots at concentrations from 10¹⁵ to 3 · 10¹⁵ cm⁻³ show a decreased photocurrent response. The dependence of the time-integrated transients (corresponding to the full charge value) upon the quantum-dot concentration has been determined. Differences in total photogenerated charge for pure and doped polymer films imply that the quantum dots of that type are the hole traps with capture times much more smaller than the transit time and with emission times a few orders longer than the transit time. CdSe quantum dots without a shell do not seem to exhibit the same properties as core shells and do not produce considerable changes in the charge transfer, even at a density of 10¹⁵ cm⁻³.     

Effects ofpH on the chromophore orientation in purple membrane

Summary Suspensions of purple-membrane fragments from Halobacterium halobium were oriented by a static electric field with intensity up to 25V/cm in the range ofpH from 4 to 9. It was found that the orientation of the transient dipole moment of the retinal chromophore for what concerns the permanent dipole moment of the membrane fragment undergoes a transition from (60±1)° atpH 5 to (71±1)° atpH 6. Moreover, the permanent dipole moment of the membrane fragments of 1.8·10⁻²³ C·m found at thepH values from 4 to 7 reduces to 1.3·10⁻²³ C·m atpH higher than 7.

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Riassunto Sospensioni di frammenti di membrana purpurea del Halobacterium halobium apH fra 4 e 9 furono orientati da un campo elettrico statico d’intensità fino a 25 V/cm. Fu trovato che l’orientazione del momento di transizione di dipolo del cromoforo, il retinale, rispetto al momento di dipolo permanente dei frammenti di membrana subisce una variazione da (60±1)°apH 5 a (71±1)° apH 6. Inoltre, il momento di dipolo permanente dei frammenti di membrana di 1.8·10⁻²³C·m trovato nell’intervallo dipH da 4 a 7 si riduce a 1.3·10⁻²³C·m per valori dipH delle sospensioni maggiori di 7.

     

Dissociation equilibrium of dinitrogen tetroxide in organic solvents: An electron paramagnetic resonance measurement

The dissociation equilibrium of N₂O₄−NO₂ has been measured in hexane, carbon tetrachloride and chloroform at different temperatures. The equilibrium constants at 298.15 K (25°C),K _m (molality basis), are 3.5·10⁻⁵ in hexane, 5.9·10⁻⁶ in carbon tetrachloride and 5.3·10⁻⁶ in chloroform. The EPR technique has been used to quantify the NO₂ radical. These data are compared with gas-phase and solution data of previous reports. The applicability of Hildebrand and Scatchard theory of solutions is also discussed and some thermodynamic properties are deduced, such as Henry’s N₂O₄ and NO₂ constants in different solvents.