Table of contents

An in-depth study related to a new method of characterizing properties in thermoelectrics is proposed in this paper. This technique is appropriate for single or multi-layered thermoelectric devices. A modulated laser beam is used as a heater in order to generate a Seebeck electromotive force (EMF). The laser beam, line shaped, can be focused at any location along the sample surface, allowing spatially resolved measurements. Seebeck EMF measurements, associated with a versatile model based on the thermal quadrupoles method, allow determination of the sample Seebeck EMF profile and identifying of the sample thermal contact resistances, and should be useful for identification of devices and material thermoelectric properties.

Micrometric scale thermal diffusivity measurement by photoreflectance microscopy is now a well-known technique. Until now the experiments were performed essentially at room temperature. In this paper we explore temperature resolved tests. We describe the modifications introduced in the experimental set-up and the procedure followed to determine the temperature dependent diffusivity. Different analysis methods are described and first tests performed on known materials and a C/C composite is presented.

Heat transfer mechanisms at very small length scales are studied at the boundary between solid and superfluid helium-4 (He II) at temperatures of 1–2 K. We shall show that this configuration is interesting and may prove useful in future heat conduction analysis in microstructures and across film/substrate boundaries. We define two possible heat conduction regimes, namely a classical surface effect regime and a scattering effect regime for the solid/He II system. The distinction between these regimes depends upon the (l/λ) ratio, where the surface roughness extent is l at a given length scale and the phonon wavelength is λ. We analyse our results of heat conduction across solid–superfluid helium interfaces and we show that the conductance can be entirely explained by the scattering effect regime where diffuse phonon scattering from surface irregularities of nanometric scale lengths play a dominant role. We also provide evidence of the existence of a frequency dependence of the heat transmission coefficient and, we compare the heat conduction with that in microstructures.

Solid-phase crystallization (SPC) of amorphous silicon (a-Si) under an external force field is investigated experimentally and numerically. Experimental results show that the kinetics of crystallization can be greatly enhanced by applying induction fields without the heating problems of a-Si film and its substrate, since temperature rises during the crystallization process are negligibly small. To explore the underlying acceleration mechanisms for the SPC process under the external fields, molecular dynamics simulations are carried out using the Tersoff potential. The numerical amorphous structure is obtained by the liquid quenching method and is utilized to simulate the crystallization processes at various process temperatures with and without external force fields. While homogeneous crystallization of a-Si could not be achieved readily, it is shown that the heterogeneous crystallization can be significantly accelerated by external force fields. This enhancement is due to increased molecular jumping frequencies associated with the molecular potential energies being increased by external excitations, rather than due to thermal mechanisms dominant in conventional SPC processes.

Roughness effects on the heat transfer and pressure losses in microscale tubes and channels are investigated through a finite element CFD code. Surface roughness is explicitly modelled through a set of randomly generated peaks along the ideal smooth surface. Different peak shapes and distributions are considered; geometrical parameters are representative of tubes in the diameter range from 50 to 150 µm. The use of a sufficiently fine mesh allows the direct computation of tube performances under the assumption of incompressible, fully developed flow; a comparison with the predictions of simplified models is also presented. As a result, in the present computations a significant increase in Poiseuille number is detected for all the configurations considered, while the effect of roughness on the heat transfer rate is smaller and highly dependent on the tube shape.

Total internal reflection fluorescence (TIRF), a general term that embraces any spectroscopic or microscopic technique based on the evanescent field created by TIR of light, is further establishing itself as an important tool for studying near-surface phenomena. Impingement of a femtosecond-pulsed infrared beam on a reflecting interface creates the conditions for 'macroscopic' evanescent-field two-photon fluorescence excitation. The two-photon fluorescence excitation volume is confined by both the non-linearity of the multi-photon process and the spatial inhomogeneity of the evanescent field. The absence of scattered excitation resulting in a low background and the possibility of simultaneous multi-colour fluorescence excitation should make non-linear evanescent-field excitation particularly attractive for quantitative single-molecule observation and ultra-sensitive screening assays. In this topical review, we survey the requirements, present the current results and explore the potential of this novel non-linear microscopy.

The influence of Co doping on the hyperfine, structural, magnetic and transport properties of La(Fe_1−xCo_x)_11.44Al_1.56 (x = 0, 0.04, 0.08 and 0.12) compounds was investigated by Mössbauer spectroscopy, x-ray diffraction, magnetization and electric resistivity measurements. Stabilization of the NaZn₁₃ structure by Al atoms was also investigated in Co undoped samples and a relatively large amount of α -Fe segregated phase was observed for Al content lower than 0.10, which indicate a solubility limit for Al substitution. The Mössbauer spectra indicate that Fe atoms see a chemically disordered environment, which is related to the presence of substitutional Al and/or Co atoms. The electric field gradient value at the Fe sites of the compounds is not affected by the Al substitution, with a quadrupole splitting value of 0.44 mm s⁻¹. The compound magnetic ordering temperature varies from 195 to 360 K and can be set according to the Co content. An enhancement of the Fe effective magnetic moment with increasing Co doping has also been observed. The applied magnetic field induces antiferromagnetic (AFM) to ferromagnetic (FM) phase transition, with a field value depending on the Co concentration, and is about 5 T for x = 0 and about 0.5 T for x = 0.04. A second magnetic phase transition occurs on cooling to low temperatures from an AFM to a spin-glass-like (SGL) state for x = 0. A possibly reentrant SGL state may occur from the FM state for x = 0.04. For x > 0.04, the compounds behave as FM for measuring field of 0.04 T.

Thin films of Zn_xCd_1−xSe were electrodeposited from an acidic aqueous solution of selenite (SeO₂) and Cd-/Zn-sulphates. The film composition was determined by x-ray diffraction (XRD) and energy dispersive x-ray (SEM/EDX) spectroscopy. In addition, the structural and optical film properties were studied by micro-Raman spectroscopy at room temperature. The Raman spectra were fitted with the modified random element isodisplacement model including interactions to first and second neighbour atoms. It is shown that the dependence of the Raman phonon modes on the molar fraction x can be described in terms of an intermediate-mode behaviour. The results of both the XRD and Raman techniques are in good agreement for most of the prepared films. Deviations were observed for small fractions x due to the presence of an excessive CdSe phase. The dependence of the band-gap energy of the ternary system on the molar fraction is discussed in the light of photoelectrochemical measurements as well as considerations within the virtual crystal approximation.

The power of a focused laser beam with a Gaussian intensity profile attenuated by powder in coaxial laser cladding is investigated experimentally and theoretically, and its resolution model is developed. With some assumptions, it is concluded that the attenuation of laser power is an exponential function and is determined by the powder feed rate, particle moving speed, spraying angles and waist positions and diameters of the laser beam and powder flow, grain diameter and run of the laser beam through the powder flow. The attenuation of laser power increases with powder feed rate or run of laser beam through the powder flow. In the experiment presented, 300 W laser power from a focused Gaussian beam is attenuated by a coaxial powder flow. The experimental results agree well with the values calculated with the developed model.

The coupling of an excitonic state in a quantum disc (QD) with an exciton–polariton propagating in a quantum well (QW) waveguide, which was placed at a certain distance, was investigated. The modes and transmission of the exciton–polariton in the waveguide were analysed by solving Maxwell's equations numerically, including the non-local polarizability. A clear dip was found in the ground-state energy of the 1s heavy-hole exciton in the QD. It was found that the coupling strength depends on the disc/well size as well as the distance in between, which is attributed to the formation of horizontal modes in the QD. The numerical results were confirmed by experiments on a sample with a GaAs/AlGaAs QW structure.

Sulfur hexafluoride, SF₆, is successfully employed by the electric power industry for gas insulated equipment. However, it is a potent greenhouse gas and mixtures of SF₆ with more friendly environmental gases have recently been proposed. It was demonstrated that SF₆ mixed with N₂ and/or CO₂ could be a good substitute for some insulation applications such as gas-insulated transmission lines (GILs).

Considering the possible occurrence of corona discharges in GILs, the decomposition of these gas mixtures under such stress must be studied. In this paper, the chemical stability under negative corona of SF₆ and SF₆–N₂ (10 : 90) or (5 : 95) mixtures was investigated at 400 kPa on adding percentages of CO₂ up to 80%.

The corona discharges were generated at 23°C in a 340 cm³ experimental cell between a stainless steel point connected to a high voltage source and a plane of aluminium. The gaseous decomposition products were assayed by gas chromatography at the end of each run carried out over a range of transported charges covering 8–17 C.

The presence of CO₂ in SF₆ and SF₆–N₂ mixtures submitted to negative coronas leads to:

1. a considerable increase in the formation of the major compound group (SOF₄+SO₂F₂),

2. formation of similar large quantities of COF₂,

3. abundant production of CO with diluted SF₆ in nitrogen,

4. decreased formation of S₂F₁₀.

Carbonaceous vapour created by nitrogen and helium discharges in graphite hollow cathodes has been investigated to identify the mechanisms for CN formation. The characteristics of hollow cathode discharge have been exploited to provide the two-energy-regime electrons that initiate a complex process for the production of ions and metastable atoms and molecules of He and N₂. These introduce the sputtered carbon species C_x (x ≥ 1) from the graphite cathode. Dominant among the C_x species are C₁ and C₂ that are excited and participate in inter- and intra-species collisions. CN, therefore, is the direct by-product of reactions between sputtered C and N₂. From the optical emission spectra the level densities of nitrogen and helium were recorded during the cycling of discharge current i_dis of the graphite hollow cathode. The emission spectra are dominated by strong emission bands of CN (violet system Δν = −1, 0, +1) and N₂ (2nd positive system), along with the atomic lines of He and C. We have identified and suggested a mechanism for the formation of CN in nitrogen plasmas in graphite hollow cathodes.

A self-consistent nonlinear theory for the outer-slotted-coaxial-waveguide cyclotron autoresonance maser (CARM) amplifier is presented, which includes the characteristic equation of the wave, coupling equation of the wave with the relativistic electron beam and the simulation model. The influences of the magnetic field, the slot depth and width are investigated. The interesting characteristic of the device is that the mode competition can be efficiently suppressed by slotting the outer wall of the coaxial waveguide. A typical example is given by the theoretical design of a 137 GHz outer-slotted-coaxial-waveguide CARM amplifier by utilizing an electron beam with a voltage of 90 kV, current of 50 A, velocity pitch angle of 0.85 and a magnetic field of 43.0 kG. The nonlinear simulation predicts a power of 467.9 kW, an electronic efficiency of 10.4% and a saturated gain of 46.7 dB, if the electron beam has no velocity spread. However, the axial velocity spread deteriorates the device; for example, if the axial velocity spread is 2%, the peak power decreases to 332.4 kW with an efficiency of 7.4% and a saturated gain of 45.22 dB. Simulation shows that the efficiency of the outer-slotted-coaxial-waveguide CARM amplifier may be increased from 10.4% to 29.6% by tapering the magnetic field.

The Boltzmann transport equation is solved to determine the time-dependent electron velocity distribution function in various mixtures of methane and argon subjected to an external electric field, E. In the solution of the equation no expansion of the distribution function is made. The concentration of methane in the mixture is varied systematically. The distribution function is used to calculate several time-dependent electron swarm parameters in these gas mixtures. For each gas mixture a wide range of E/N values, from 0.1 to 1000 Td, is investigated. Steady-state values of various calculated swarm parameters (drift velocity, Townsend ionization coefficient and characteristic energy) agree quite well with the corresponding experimental values.

This paper proposes a coherent set of electron impact inelastic cross sections for argon, based on recent experimental measurements. The updated set is validated by comparing calculated swarm parameters and rate coefficients (obtained by solving the two-term approximation electron Boltzmann equation) with available experimental data. This validation procedure is usually adopted when the cross section set is to be later used in plasma discharge modelling. Simulation results for the electron drift velocity and characteristic energy are in very good agreement with experimental values of these quantities. Calculations, using cross section sets proposed by different authors, of the total (direct + cascade) excitation coefficients to the 4s and 4p states, and of the Townsend ionization coefficient, show that the present set ensures the best overall agreement with measured values. The agreement is particularly good for the excitation coefficient to metastable 4s'[1/2]₀ and the Townsend ionization coefficient, which are probably the most relevant electron macroscopic coefficients in the modelling of discharge plasmas.

The thickness shear-mode (TSM) sensor is extended for operating in a liquid environment for application in biological engineering. The interfacial effect between the sensor surface and the attached medium is one of the major controversies in modelling of the TSM resonator in a liquid environment. Several slip models have been proposed to describe the interfacial mechanism of a TSM resonator in a liquid environment. In this paper, the correlation between the spring–damper model and other slip models is presented. Three TSM sensors with different surface modifications were fabricated to verify the models. The results of the measured experimental frequency shifts are found to deviate from theoretical results evaluated using a non-slip model. By adjusting the attraction strength between the contacted interfaces, the results can be fitted by the spring–damping interface model.

The electrical conductance of a colloidal CdS suspension, self-assembled in a network like structure, is modified when water vapour is absorbed or desorbed on it. The electrical conductance has a power law dependence on the colloid volume fraction Φ.

High-permittivity dielectric core/shell structured Li_0.01Si_xNi_0.99−xO ceramics (abbreviated as LSNO) have been successfully prepared. Analyses of the ceramic microstructure and composition indicate that the dopants of SiO₂ are mainly dispersed into the grain boundaries and exist as the insulating Ni₂SiO₄ phase. All the LSNO ceramic samples exhibit high permittivity, and the permittivity reaches about 1.2 × 10⁴ for the LSNO-1 sample. The concentration of Si has a remarkable effect on the dielectric properties of the LSNO ceramics. The origin of the high permittivity observed in these LSNO ceramics is attributed to the Maxwell–Wagner polarization mechanism and a thermally activated mechanism.

The effect of samarium substitution on the structural, dielectric and piezoelectric properties of lead calcium titanate ceramics was investigated. Dense ceramics with nominal composition Pb_0.76−3x/2Sm_xCa_0.24Ti_0.98Mn_0.02O₃ (x = 0–0.08) were fabricated by a novel, simple, energy and time efficient, and environment friendly microwave sintering process. The samples were sintered at 1100°C for 30 min. All the compositions show single phase with tetragonal structure. Tetragonality decreases and relative density increases with increase in samarium substitution. The variation of the dielectric constant with temperature shows a ferroelectric phase transition and the transition temperature decreases with increase in the amount of samarium. Diffusivity increases with samarium substitution. All the compositions show well-defined ferroelectric behaviour. The electromechanical thickness coupling coefficient (k_t) and piezoelectric longitudinal coefficient (d₃₃) showed an increasing trend whereas the electromechanical planar coupling coefficient (k_p) and the magnitude of the piezoelectric transverse longitudinal coefficient (d₃₁) showed a decreasing trend with increase in samarium substitution level, which in turn results in an increase in electromechanical anisotropy.

Upconversion luminescence characteristics of a multi-rare-earth-doped 0.05K₂O–0.1ZnO–0.1BaO–0.2PbO–0.2GeO₂–0.35TeO₂ glass have been investigated under 976 nm excitation for potential use in full colour displays. Two luminescence ions, Er³⁺ and Tm³⁺, were used as blue, green and red emitters to control the colour of the emitted light. Sensitizer ions, Yb³⁺, were used to increase the upconversion emission intensity. With the substitution of Er³⁺ for Tm³⁺, the intensities of the blue, green and 657 nm upconversion emissions simultaneously increase.