Table of contents

A systematical study is presented on the impact of disorder and inhomogeneity on the high reflection range (HRR) in a one-dimensional binary dielectric multilayer reflector for various incident angles. It is found that the introduction of disorder can expand the HRR for both transverse electric (TE) and transverse magnetic (TM) waves, irrespective of the incident angle. The HRR is found to become wider as the degree of disorder increases, until the effect becomes saturated. On the other hand, for the TE wave only, the introduction of inhomogeneity may broaden the width of the HRR regardless of the incident angle. While for the TM wave, the effect of inhomogeneity depends on the angle of incidence, and it can expand the HRR only for the case of a small incident angle. The phenomenon is expected to find significant applications in the optimal design of broadband binary dielectric omnidirectional reflectors.

Using specular x­ray reflectivity, we have examined the vertical structure of charge coupled polyelectrolyte (PDADMAC)-lipid (DPPA) films on the water surface and on a silicon substrate, prepared by means of the Langmuir-Blodgett technique. The complicated structure of these films, characterized by electron density variations on the sub­nanometre scale and low density contrast between sub­layers, causes failure of the standard box model fitting method - it yields poor matching to data and/or ambiguous density profiles. We show that this problem can be overcome utilizing a different, model­free fitting method, which yields perfect fits and less ambiguous electron density profiles for all investigated films. The ambiguity of results can be further reduced when the sets of possible density profiles obtained for several similar samples are compared to each other. Discussing the model­free fitting method in detail along with general questions of reflectivity data evaluation, we aim to give practical instructions for the structure analysis of thin organic films.

The obtained density profiles, being in good agreement with the structure model based on previous diffraction experiments, reveal some new details: The DPPA's phosphate head groups penetrate inside the PDADMAC helix rather than being bound onto its surface. The structure remains almost unchanged after transferring one layer onto Si wafer, but subsequently transferred layers exhibit decreasing order.

The dc conductivity of MoO₃-TeO₂ glasses prepared by melt quenching has been investigated in the temperature range 323-473 K. Seebeck coefficient measurements indicate the glasses to be n-type. The conductivity increases with increase of MoO₃ content and at 418 K varies from about 6×10^-6 to 1×10^-8 S cm^-1. The conduction is attributed to non-adiabatic hopping of small polarons due to the different valence states, Mo⁵⁺ and Mo⁶⁺, the existence of which was confirmed from x-ray photoelectron spectroscopy analysis of the glasses. Fourier transform infrared spectroscopic studies show the presence of MoO₆, MoO₄, TeO₃ and TeO₄ structural units in the glasses.

We have derived the expressions for the 12 third-order elastic constants for a tetragonal system by the method of homogeneous deformation which are used to determine the third-order elastic constants of TiAl. These third-order elastic constants are used to determine the low-temperature limit of the volume thermal expansion of TiAl. The value of C₃₃₃ was calculated to be a high value. The high value of the third-order elastic constant C₃₃₃ shows a marked anisotropy in the crystal. The low-temperature limit is found to be negative, indicating a negative volume thermal expansion down to zero kelvin.

The effects of titanium content on the properties of silica-titania films by the sol-gel method using γ-glycidoxypropyltrimethoxysilane (GLYMO), tetraethoxysilane (TEOS) and tetrapropylorthotitanate (TPOT) as the starting materials have been investigated. Atomic force microscopy (AFM), ellipsometry, thermal gravimetric analysis (TGA), UV-visible spectroscopy (UV-VIS), Fourier transform infrared (FT-IR) spectroscopy, x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy have been used to investigate the optical and structural properties of the films. These results showed that crack-free and high transparency (in the visible and near infrared range) silica-titania films with a thickness of 0.75 µm could be obtained by a single spin-coating process after a heat treatment at 500 °C. The hardness and Young's modulus of the films were also characterized by a nanoindenter and showed a linear dependence on the titanium content. Hardness as high as 9.0 GPa was achieved in a high titanium content film at a heat treatment temperature of 800 °C.

H⁺ and He⁺ were implanted into single crystals in different orders (H⁺ first or He⁺ first). Subsequently, the samples were annealed at different temperatures from 200 °C to 450 °C for 1 h. Cross sectional transmission electron microscopy, Rutherford backscattering spectrometry and channelling, elastic recoil detection were employed to characterize the defects and the distribution of H and He in the samples. Furthermore, the positron traps introduced by ion implantation and annealing were characterized by slow positron annihilation spectroscopy. Both orders in the coimplantation of H and He have the ability to decreases the total implantation dose after annealing. No bubbles or voids but cracks and platelets, were observed by cross sectional transmission electron microscopy. The different implantation orders affect the density of interstitial atoms and positron traps.

Calorimetric measurements on the amorphous alloy (CoFe)₈₉(MnMoSiB)₁₁ (Vitrovac 6150) were carried out. On the basis of the Johnson-Mehl-Avrami equation, the isothermal crystallization kinetics have been studied using differential scanning calorimetry. Isothermal measurements were carried out at annealing temperatures ranging from 685 to 697 K. The activation energy for crystallization, E, and the Avrami exponent n have been calculated.

The most widely used material for active transducer applications is the ferroelectric ceramic PZT. Composites made of ferroelectric ceramics combined with polymers have several advantages over pure ceramics. Diphasic composites of PZT (and La/Ca modified PZT) combined with various polymers such as PVDF, PVC, PVA, epoxy resin and co-polymers have been widely studied and reported in the literature.

In this paper we present the preparation and study of PZT-PVDF composites of 0-3 connectivity. The composites are prepared by two methods, namely (i) the solvent-cast method and (ii) the hot-press method. The piezoelectric strain coefficient (d₃₃) is studied as a function of volume fraction of PZT in PVDF. The dielectric constant (k) is measured as a function of temperature and the piezoelectric voltage coefficient (g₃₃) of the composite is determined.

The experimental values of the dielectric constant, d₃₃ and g₃₃ of the composites are compared with the earlier reported values and the theoretical values proposed by Furukawa et al (Furukawa T, Ishida K and Fukada E 1979 J. Appl. Sci.50 4904) and Bhimasankaram et al (Bhimasankaram T, Suryanarayana S V and Prasad G P 1998 Curr. Sci.4 967).

The composite with 0.5 PZT volume fraction prepared by the hot-press method is found to possess a reasonably high d₃₃ value and exhibit good flexibility and stability with time.

The optically stimulated luminescence (OSL) from quartz relates to not only the trapped electrons associated with the OSL signals, but also the luminescence centres involved. In this paper we report pulse annealing and isothermal annealing experiments on a 17 ka old sedimentary quartz to study thermal stability of electrons in OSL traps and the trapped holes associated with the non-luminescence centres (R centres). The lifetimes obtained are in the order of magnitude of 10⁹ and 10⁴ years at 20 °C, for electrons in the stable OSL traps and the holes associated with sensitivity change, respectively. The results indicate that the OSL sensitivity change is dominated by the transferring of holes from one type of R centre to luminescence centres (L centres), and is different from the 110 °C TL peak, which appears to involve more than one type of R hole centre.

The photoemission properties of metallic photocathodes are studied by a time-resolved single-photon laser technique in the femtosecond regime. Experimental results obtained for different metals (Au, Cu, W, Al, Fe) are interpreted by a theoretical model, taking into account electron-electron and electron-phonon dynamics during the illumination by ultrashort, high-intensity ultraviolet laser pulses. This model points out the physical process of ultrashort photoemission from metals and the correlation of microscopic transient effects with macroscopic steady-state characteristics.

ZnS_xSe_1-x (0<x<1) nanocrystalline films were deposited onto quartz substrates by a dc magnetron sputtering technique. The optical properties of the films were studied at room temperature (~300 K). The optical absorption was found to be dominated by the combined effects of optical losses due to absorption and scattering. The absorption spectra indicated a blue shift, the extent of which depended upon the crystallite size of the film. The electrical conductivity of the films deposited onto the quartz substrate was measured in the temperature range of 150-300 K. It was observed that the low-temperature conductivity could be explained by hopping of the charge carriers in the Coulomb gap, while at elevated temperature Mott's hopping was the predominant mode of conduction in these films. A distinct cross-over from Efros-Shklovskii to Mott's hopping was observed for all the films.

Pr₈Fe₈₈B₄ ribbons prepared by melt spinning and subsequent annealing have been investigated by x-ray diffraction, thermomagnetic analysis, an atomic force microscope and a superconducting quantum interference device magnetometer. The results show that the annealed ribbons consist of the Pr₂Fe₁₄B and α-Fe phases. The mean grain size of the samples increases with the annealing temperature and the annealing time. The result of magnetic measurement shows that the exchange interaction between the hard and soft phases in the sample with a smaller mean grain size is larger than that in the sample with a larger mean grain size. The particle exchange interaction in Pr₈Fe₈₈B₄ nanocomposite magnets was calculated using the mean field Stoner-Wohlfarth model. We found that the result calculated from mean field Stoner-Wohlfarth model is in agreement with the conclusion gained from microstructure observation and magnetic measurement.

The nonlinear behaviour of a system of blocked single-domain ferroparticles within a liquid matrix rotating in a transverse magnetic field that is small in comparison with the effective field of particle magnetic anisotropy is investigated. The ideal model of a rigid dipole, usually used for such systems, is extended to include small deflections of the particle magnetic moment from the anisotropy axis (the model of a quasi-rigid dipole). It is found that at frequencies exceeding some critical value the small orientational freedom of the particle magnetic moment can drastically modify the system orientational behaviour. The effect is described of a giant slowing down of the magnetic relaxation occurring in a rotating suspension of quasi-rigid particles after it had been magnetized to saturation in the rotation direction.

Nanocrystalline Sm₂Fe_14.5Cu_0.5Ga₂C_x (x = 1 and 2) and Sm₂Fe₁₅Ga(Si)₂C ribbons prepared by melt spinning and subsequent annealing have been investigated by x-ray diffraction, thermomagnetic analysis, a superconducting quantum interference device magnetometer and an extracting sample magnetometer. The results show that the annealed ribbons consist of the Th₂Zn₁₇-type phase and a very small amount of α-Fe. The result, calculated from the high external field part of the demagnetization curve, shows that H_eff (the effective field)-H (the external field) is linear with y (the ratio of the magnetization in the external magnetic field H and the saturation magnetization). This result is in agreement with the conclusion gained from a mean field Stoner-Wohlfarth model.

We have used the transmission electron microscope to study how the magnetization reversal mechanism of thin NiFe layers exchange-biased by IrMn and FeMn varies over a wide temperature range. The reversal behaviour was qualitatively similar for layers biased by both types of antiferromagnet. At room temperature and below the most striking feature was the scale of the domain structures observed. Very high density domain structures with micron (or sub-micron) wall separations developed. By contrast at elevated temperatures, the reversal mechanism simplified. This is consistent with there being a strong local variation of the pinning strength between the NiFe and the antiferromagnetic layer. The overall temperature variation of the pinning changes much more rapidly than the magnetic properties of an isolated NiFe layer over a similar temperature range.

³Current address: Sun Microsystems, Springfield, Linlithgow, West Lothian, EH49, UK.

⁴Current address: Veeco Instruments, 80 Las Colinas Lane, San Jose, CA 95119, USA.

Electromagnetic fields of the simplest time-dependent sources (current loop, electric dipole, toroidal solenoid, etc), their interactions with an external electromagnetic field and between themselves are found. They are applied to the analysis of the Lorentz and Feld-Tai lemmas (or reciprocity-like theorems) having numerous applications in electrodynamics, optics, radiophysics, electronics, etc. It is demonstrated that these lemmas are valid for more general time dependences of the electromagnetic field sources than it was suggested up to now. It is shown also that the validity of reciprocity-like theorems is intimately related to the equality of electromagnetic action and reaction: both of them are fulfilled or violated under the same conditions. Conditions are stated under which reciprocity-like theorems can be violated. A concrete example of their violation is presented.

Soft x-ray emission from plasmas produced by ablation from a magnesium target employing a ruby laser is studied using a grazing incidence spectrograph in the spectral region 3-12 nm. Emission intensities are investigated for different ionic lines as a function of position, time after the maximum of the laser pulse and laser irradiance. A gated pinhole camera is employed for studying the evolution of the plasma at early stages. The propagation distances and velocities are measured. Our results show that the line intensities of all ionic lines increase with laser irradiance until a saturation plateau is observed at high irradiance levels. The increase in ionization and intensity saturation with varying laser irradiance seems to suggest strong interaction of the laser pulse with the dense plasma formed near the target within the pulse duration.

Plasma spraying of metallic particles in the ambient atmosphere is accompanied by their oxidation. The oxides formed on the particle surfaces are often unstable at room temperature. Fast cooling of the particles may conserve these oxides, leaving thus the particles heterogeneous and consisting of at least two materials - the oxide layer on the surface and the metal in the centre. The times required to cool the particles below the melting points of the two materials are estimated for slow and fast cooling in gas and liquid nitrogen, respectively. The estimates are carried out numerically by solving a one-dimensional heat transfer equation for solidifying spherical iron and \mbox{Fe-Cr} alloy particles having either a thin (1 µm) or a thick (10 µm) oxide layer on their surface. It has been found that for the typical particle size of 120 µm in diameter, the cooling rates, under the assumed conditions, are 10⁵-10⁶ K s^-1 and it takes 10-1 ms to completely solidify the initially liquid particles. The temperature histories of the particles without any oxide layer and with a thin oxide layer are almost identical.

Dielectric barrier discharges (DBDs) are being investigated to remediate NO_x from atmospheric gas streams. The interaction of species generated during previous pulses of repetitively pulsed devices, such as DBDs, make their operation fundamentally different from single pulse operation. To investigate these effects, the reaction chemistry of multiple pulses during NO_x remediation was modelled. Reactions between radicals produced during a pulse with the products from previous pulses produce significantly different end products. For example, approximately 10 ppm of methyl nitrate (CH₃ONO₂) was obtained by multiple pulsing whereas less than 1 ppm of it was produced with single discharge formats. Multiple pulsing has also been found to be more efficient for NO_x remediation. W-values for NO_x remediation decreased from 240 eV for a single pulse (58 J l^-1) to 185 eV when the same energy was distributed over 20 pulses.

³Author to whom correspondence should be addressed.

Dc positive point-to-plane discharges in the 1-100 Torr pressure range display a positive slope of dV/dI in the low-current part of the voltage-current characteristic curves. Whereas the visual aspect is a tiny luminous region close to the point (anodic glow) the gap remains dark. The anodic glow is still present at higher currents, and is superimposed in the `classical glow' regime with whole gap illumination.

In nitrogen, and also in dry air, the current in the dark discharge regime is characterized by a low dc component upon which oscillations are superimposed. These oscillations have been carefully studied, varying parameters such as pressure, applied voltage, gap length and the tip curvature radius of the stressed electrode.

From the analysis of the current (amplitude, frequency and waveform) and of the optical signals coming from precise places of the interelectrode gap (inside or outside the anodic glow), it can be deduced that these oscillations should not be attributed only to the classical ion drift mechanism which dominates the high-pressure corona glow. A mechanism involving double-layer formation and fluctuation is proposed to explain the discharge behaviour in this low-current domain where the applied voltage is not sufficient to cause the transition to a classical glow discharge.

Breakdown sequence in SF₆ of an electrode arrangement which represents the contacts of a circuit-breaker is investigated at pressures up to 8 bar under positive impulse voltages. The Schlieren frames point out the role played by the cathode as a source emitting electrons by field effect emission. Photomultiplier records demonstrated, at P_d = 2 bar, that the discharge's formative time strongly decreases: from 1 µs at pressure lower than P_d, down to 150 ns at pressure higher than P_d. A change in the discharge regime is associated with this transition. An experimental investigation, using photomultipliers and Schlieren devices, allows us to describe both breakdown sequences. At low pressures, there is a critical volume at the anode. Nevertheless, it is a negative discharge which is responsible for the breakdown although the geometrical electrical field at the cathode boundary is about 60 kV cm^-1. At high pressures, there is no critical volume. However, a positive discharge develops across the gap. Physical considerations taking into account the electron energy are proposed to explain the mechanisms involved in both types of discharge.

In this paper, we will report NO_x removal via reduction processes using two types of combined system of pulse corona discharge and catalysts: the single-stage plasma-driven catalyst (PDC) system, and the two-stage plasma-enhanced selective catalytic reduction (PE-SCR) system. Several catalysts, such as γ-alumina catalysts, mechanically mixed catalysts of γ-alumina with BaTiO₃ or TiO₂, and Co-ZSM-5 were tested. In the PDC system, which is directly activated by the discharge plasma, it was found that the use of additives was necessary to achieve NO_x removal by reduction. Removal rates of NO and NO_x were linearly increased as the molar ratio of additive to NO_x increased. The dependence of NO and NO_x removal on the gas hourly space velocity (GHSV) at a fixed specific input energy (SIE) indicates that plasma-induced surface reaction on the catalyst plays an important role in the PDC system. It was found that the optimal GHSV of the PDC system with the γ-alumina catalyst was smaller than 6000 h^-1. Mechanical mixing of γ-alumina with BaTiO₃ or TiO₂ did not enhance NO and NO_x removal and γ-alumina alone was found to be the most suitable catalyst. The dielectric constant of the catalyst only influenced the plasma intensity, not the NO_x removal. In the PE-SCR system, plasma-treated NO_x (mostly NO₂) was reduced effectively with NH₃ over the Co-ZSM-5 catalyst at a relatively low temperature of 150 °C. Under optimal conditions the energy cost and energy yield were 25 eV/molecule and 21 g-N (kWh)^-1, respectively.

The results of numerical simulations of a positive streamer development in air in a weak and uniform electric field are presented. Streamer dynamics is considered in an electrode gap of 33 mm in length and the configuration was `protrusion on a plate-plate'. This particular configuration was chosen in order to perform direct comparison between simulated results and experimental data. The electrostatic field in such a system decreases rapidly with increase of the distance from the protrusion (anode) and the region with a weak and uniform background field covers ~30 mm of the gap. The parameters of the propagating streamer are studied at six different values of the background field strength: 0.24; 0.30; 0.345; 0.37; 0.43 and 0.50 MV m^-1. Stable streamer development (with constant velocity) takes place in a field of 0.5 MV m^-1 (the stability field) but the streamer is able to cross the gap in a background field of 0.3 MV m^-1. These values are in excellent agreement with experimental data. During the stable streamer propagation, the electron density and plasma conductivity in the discharge channel and the electric field at its front remain constant. In a background field lower than 0.5 MV m^-1, the discharge front velocity and the electric field at the front decrease linearly with an increase of streamer length. The discharge propagation in the stability field is associated with an increase of electrostatic energy at the streamer front but it decreases if the streamer develops in a weaker electric field. This behaviour is accompanied by a constant Joule dissipation at 0.5 MV m^-1 and decreasing energy losses at the streamer front in a weaker background electric field.

We investigate the possibility of using the electric-field-dependent laser-induced fluorescence scheme X¹Σ(v'',J'')→B¹Π(ν',J')→X¹Σ(v,J = J',J'±1) to create an image (or movie) of the electric field surrounding an object. Specifically we find that when NaRb is probed in this way, vector correlations between the excitation laser polarization, fluorescence radiation polarization, and electric field vector are sensitive to the magnitude of the electric field, even when the wavelength of the fluorescence (and hence final state J) is not resolved.

Under a given temperature, the theoretically calculated spectral distributions of long wavelength photon emission of different size ultra-fine silver in CsO semiconductors with and without external voltage are shown and discussed. A specific expression for the photoemission of a field-assisted Ag-O-Cs photocathode is obtained. According to theoretical calculations, the dependence of the photon emission upon applied bias voltage and the size of Ag, which ranged from 1.8 to 36.9 nm, have been predicted. It is thus beneficial to design a field-assisted photocathode and choose optimum operation conditions.

A simulation program has been set up, which calculates the x-ray profiles of x-ray line scans taken across features of arbitrary structural and compositional geometries embedded in a matrix. The simulated scan intensities are compared to experimental scan intensities, obtained in an analytical scanning transmission electron microscope. The program is applied to calculate x-ray profiles of InAs quantum dots embedded in GaAs and to SiGe quantum dots embedded in Si in a thin foil in cross-sectional [011] projection. With the help of simulations of the indium L line scan intensities for the case of the InAs dots it is demonstrated how sensitively the x-ray profiles depend on the position of the dot in the thin foil. For the case of the SiGe dots, a full quantitative composition analysis is carried out by adjusting the model geometry and model composition distribution until the best match with the experimental germanium K line scan intensity profile is achieved.

We describe wave propagation from a partially coherent x-ray source taking into account the free-space wave propagation and the Bragg diffraction on a collimating crystal. We find conditions when the free-space propagation becomes irrelevant and the coherence length is given by the extinction length of the corresponding Bragg reflection. We show that a wide wavelength range can be retained if the collimating crystal is illuminated by a convergent beam. The increase of the coherence length by Bragg diffraction is demonstrated in a laboratory experiment on periodic surface gratings with different periods.

A chemical heat transformer uses a chemically reacting system for the internal cycle. Chemical conversions in a chemical heat transformer produce entropy due to irreversibilities, which are taken into account in our derivation of a new, more realistic value for the thermal efficiency of this heat transformer. We studied the example of the endothermic dehydrogenation of 2-propanol, yielding acetone and hydrogen, versus the exothermic hydrogenation of acetone yielding 2-propanol. Different from other papers we allow the dehydrogenation temperature T_m to be higher than the boiling point T_b of the 2-propanol and we allow the isolation of 2-propanol from the mixture to be incomplete. The internal entropy production in the dehydrogenation and hydrogenation reactor is calculated as a function of the hydrogenation temperature T_h with three different values of T_h-T_m and several incomplete separation values. The thermal efficiency is much lower than Carnot's efficiency for low-temperature lifts because of the high irreversibilities that are already present for these temperatures and because of the low efficiency of the heat engine. A higher, but still low, efficiency is obtained for T_m>T_b and for the incomplete separation of 2-propanol from the mixture.