Table of contents

Hydrogen-terminated diamond can exhibit a pronounced two-dimensional hole conductivity when it is exposed to appropriate adsorbates. This phenomenon was first encountered in the form of the so-called hydrogen-induced surface conductivity but is meanwhile understood as a novel kind of doping mechanism by which the surface adsorbates act like acceptors. Such surface acceptors can be solvated ionic species that undergo electrochemical reactions after accepting an electron from the diamond's valence band. But they can just as well be neutral molecular species with a sufficiently high electron affinity such as fullerenes or fluoro-fullerenes. This review summarizes the physical principles behind this unusual type of doping.

Ribbons of nanocomposite Pr₂(FeCo)₁₄B/α-(FeCo) with an additive of low melting point metals, such as Al, Zn, Sn or In, were prepared by melt spinning. It was found that the remanence could be obviously improved at the expense of coercivity by the addition of Sn. The remanence J_r about 1.30 T and maximum energy product (BH)_max about 20.5 MG Oe were obtained in Pr₉Fe₇₄Co₁₂B₅ doped with 0.5% Sn, while J_r and (BH)_max in the ribbons without the additive were about 1.17 T and 17.9 MG Oe, respectively. The additive Sn is located at the grain boundaries rather than in grain interior in the ribbons. The obvious improvement of the remanence originates from the refinement of the microstructure and the increase of soft phase content by the addition of Sn. The effect of additive Sn on the coercivity was also discussed. By increasing the content of the magnetically hard phase, the intrinsic coercivity _iH_c can be elevated. Thus, the optimum magnetic properties, such as J_r = 1.27 T, _iH_c = 5.7 kOe and (BH)_max = 22.5 MG Oe, were obtained in Pr_9.5Fe_73.57Co_11.93B₅ ribbons doped with 0.5% Sn.

The domain configuration of micron-sized permalloy ellipses was studied under the influence of an in-plane rotating magnetic field using magnetic force microscopy. The field amplitude was chosen such that when the field is applied parallel to the long axis of the ellipses they are saturated, but when the field is perpendicular to the long axis they exhibit multi-domain states. The rotation angle for nucleation and annihilation of domains was determined for different magnitudes of the applied magnetic field and for two different lateral sizes of ellipses, 6 µm × 2 µm and 3 µm × 1 µm. It was found that both nucleation and annihilation occur over a range of angles for both lateral sizes of ellipses. Saturated states are stable for a wider range of angles for larger values of the applied field.

The structural and magnetic properties of mixed compounds Tb_1−xY_xFe₁₁Mo with x = 0–1.0 have been investigated by means of x-ray diffraction and magnetic measurements. The compounds crystallize in the tetragonal ThMn₁₂-type structure with space group I4/mmm. The lattice parameters a, c and the unit-cell volume V vary slightly with the Y content, since the atomic radius of Y is close to that of Tb. The Curie temperature of the compounds increases for x ⩽ 0.1, then decreases with the increase in the Y concentration. Spin reorientation phenomena in the Tb_1−xY_xFe₁₁Mo compounds have been studied in detail. The room temperature easy-magnetization direction of Tb_1−xY_xFe₁₁Mo compounds changes from planar to the c-axial with increasing Y concentration. Variation of the anisotropy constants of the Fe-sublattice K_1(Fe) (T) with temperature is determined experimentally. By minimizing the magnetocrystalline anisotropy energy, the derived spin reorientation temperature for the Tb_1−xY_xFe₁₁Mo system shows reasonable agreement with the observations.

Well-oriented LaSr₂Mn₂O₇ thin film with layered perovskite structure was fabricated via the magnetron sputtering method on an a-axis oriented SrTiO₃ single crystal substrate. It exhibits large magnetoresistance and photoinduced decrease in resistance at low temperature. The study of the influence of the current, magnetic field and laser illumination on the charge-ordering (CO) state shows that the CO state at low temperature is sensitive to the external stimulus. The decrease in resistance is attributed to the relaxation of MnO₆ octahedra distortion under the laser illumination as well as the magnetic field.

La_0.7Sr_0.3Mn_1−xTi_xO₃ films (0 ⩽ x ⩽ 0.5) were fabricated using chemical solution deposition. It was found that with an increase in Ti content, the lattice constant was first increased due to the substitution of Mn⁴⁺ with Ti⁴⁺, and then it tended to decrease when x ⩾ 0.3 due to the substitution of Mn^{3 +} by Ti⁴⁺ with a larger ionic radius, as well as when excess oxygen was introduced. With the increase in x, the Curie temperature was decreased and broadened; however, it was found that all the films showed ferromagnetic properties at 300 K, which indicated that the excess oxygen played an important role. The transport properties showed that with an increase in Ti content the resistivity increased and the insulator–metal transition tended to a lower temperature due to the depression of the double exchange and the dilution of the Mn^{3 +}–O–Mn⁴⁺ network. Moreover, the temperature dependence of magnetoresistance (MR) results showed that with the Ti content increasing, the maximum MR increased and the transition was broadened suggesting that it is useful to tune the MR behaviour through Ti doping, and when x ⩽ 0.1 the derived films showed no obvious MR at low temperature; however, when x = 0.3, the film did show obvious MR at low temperature.

The structure, luminescence and etching kinetics for porous silicon stain-etched at different temperatures are studied. The results reveal that for temperatures below 10 °C and for short etching times, a novel anisotropic structure based on surface roughness preferentially oriented in the ⟨100⟩ direction is observed. At temperatures higher than 10 °C or large etching times, typical macropores and mesopores with non-preferential pore wall orientation are detected. The luminescence spectra of the samples with preferential surface roughness orientation are red-shifted with respect to the samples with typical isotropic orientation. The results are interpreted in terms of average etching rates and pore growth.

Optical absorption and emission spectra were performed for Dy³⁺ ions doped alkali tellurofluorophosphate glasses. The intensity parameters from the measured oscillator strengths were evaluated using the Judd–Ofelt (J–O) theory. The observed and calculated oscillator strengths (f) are found to be in reasonable agreement with one another. The evaluated J–O parameters are used to predict radiative transition probabilities (A_R), total radiative transition probabilities (A_T), radiative lifetimes (τ_R) and branching ratios (β_R) for various excited levels of Dy³⁺ doped alkali tellurofluorophosphate glasses. The predicted τ_R and β_R values of ⁴F_9/2 transition are compared with the experimentally measured values. The lifetime of the ⁴F_9/2 level was measured with a quantum efficiency of 80% for Dy³⁺: RTFP (R = Li, Na, K) glasses. Stimulated peak emission cross-sections (σ_e) for different emission transitions were estimated from emission spectra. From the magnitudes of the stimulated emission cross-sections, branching ratios, multiphonon relaxation rates (W_MP), the most potential laser transitions are identified and the utility of these glasses as laser active materials is discussed. A thermalization process between the ⁴F_9/2 and ⁴I_15/2 levels is also reported.

X-ray diffraction and differential scanning calorimetric studies were performed on bulk Sn–Sb–Se chalcogenide alloys, which were obtained by the conventional melt quenching technique. The addition of Sn reduces the crystalline nature of Sb₂₀Se₈₀ chalcogenide alloy and amorphous samples are obtained for a-Sn_xSb₂₀Se_80−x 8 ⩽ x ⩽ 18. The T_g and T_x increase with composition up to the chemical threshold with a sharp change in slope at x = 10, corresponding to the average coordination number Z = 2.40, where a Philip–Thorpe rigidity transition occurs. Thin films were obtained by the thermal evaporation method for dc conductivity and optical measurements. From the temperature dependence of dc conductivity measurements, the activation energy (ΔE) and the pre-exponential factor (σ₀) were calculated for each glassy alloy. An approximate linear dependence of ln σ₀ on ΔE is observed which proved the validity of the Meyer–Neldel rule in the investigated samples. It has been observed that the difference in dc activation energy (ΔE) is less than half of the optical band gap (E₀) indicating that the Fermi level is not located near the centre of the gap.

The electron drift velocity v_e, the density-normalized longitudinal diffusion coefficient ND_L, the density-normalized effective ionization coefficient (α − η)/N and the limiting field strength E/N_lim have been measured for the binary mixtures SF₆–air and SF₆–O₂ for SF₆ concentrations ranging from 1% to 50%. The overall reduced electric field strength E/N was varied between 1.2 and 500 Td (1 Townsend = 10⁻¹⁷ V cm²). A pulsed Townsend technique was used to perform these measurements, which turned out to be sensitive enough to distinguish relatively small differences in the electron drift velocities and longitudinal diffusion coefficients over the mixture range 1 to 50% SF₆ in SF₆–air and SF₆–O₂. On the other hand, the effective ionization coefficients show a well-defined variation with the SF₆ content in the mixture. The limiting value of E/N at which the effective ionization coefficient is zero, that is, the critical field strength, turned out to be smaller than that for the SF₆–N₂ mixture. However, the difference in behaviour between SF₆–air and SF₆–N₂ is small. To the best of our knowledge, we are not aware of any previously published measurements of these parameters for these mixtures.

Post-breakdown expansion of a submerged discharge was considered taking into account the curvature of the plasma boundary and the sound and shock wave fronts. Numerical calculations were performed for an inter-electrode gap of 10⁻⁵ m, electrical current and voltage of 10 A and 100 V, respectively and a water medium. With an initial temperature of T₀ = 2 × 10⁴ K, a subsonic solution was found. The maximum expansion velocity is 1200 m s⁻¹. The electrical current increases up to 9 A during ∼60 ns. With an initial temperature of T₀ = 3 × 10⁴ K, the plasma channel expansion starts as subsonic but after ∼5 ns it reaches the sonic velocity and transits it. The maximum velocity of the plasma boundary of about 2000 m s⁻¹ is reached after ∼17 ns, and a shock wave appears in the undisturbed water. At this stage, water intensively evaporates into the plasma cavity. The electrical current jumps at the sonic transition point up to 10 A. The calculated pressure maximum is about 8 × 10¹⁰ Pa.

We report measurements of the breakdown curves for low-pressure rf capacitive discharges in nitrogen, hydrogen, argon, oxygen and ammonia. The electron drift velocity in these gases was deduced, as a function of reduced electric field, from the low-pressure turning points of the breakdown curves. The equation for rf breakdown proposed by Kihara (1952 Rev. Mod. Phys.24 52) allows the position of both the turning point and the breakdown curve minimum to be calculated from the transport properties of each gas. Therefore we propose a new technique to determine the electron drift velocity from the position of the rf breakdown curve minima. We have determined the drift velocity in the range E/p = 52–1324 V cm⁻¹ Torr⁻¹ for nitrogen, E/p = 33–720 V cm⁻¹ Torr⁻¹ for argon, E/p = 32–713 V cm⁻¹ Torr⁻¹ for ammonia, E/p = 32–550 V cm⁻¹ Torr⁻¹ for hydrogen and E/p = 69–1673 V cm⁻¹ Torr⁻¹ for oxygen.

Simulations of the switching process of an SF₆-selfblast test circuit breaker have been performed and compared with experimental results by measuring voltage, current and pressure build-up. Arc radiation has been considered with the P1-radiation model. To include turbulence effects the shear stress transport model was applied. A comparison of the measured voltage, pressure and nozzle ablation in the test circuit breaker with the calculations is used to show the precision of the simulation tool.

Low-pressure inductive plasma was used to study SiO₂ and Si₃N₄ etching with NF₃/hydrocarbon chemistry. NF₃ and a hydrocarbon were used so that fluorine and carbon could be supplied from feed gases other than global warming fluorocarbons. Etch rates of SiO₂ are less than the Si₃N₄ etch rates over a wide range of conditions. With 50 W of wafer power, the SiO₂ etch rate was of the order of 25 nm min⁻¹ while the Si₃N₄ etch rate was of the order of 50 nm min⁻¹. Similarly to a fluorocarbon chemistry, the etch process yields a very thin carbon-based steady-state film whose characteristics were determined with ex situ x-ray photoelectron spectroscopy (XPS). From mass spectrometry and XPS, CH_xF, CF₂ and CF₃ were produced but in small concentrations compared with CH_x and CN. Comparisons of normalized F 1s spectra of nitride and oxide show that relative concentrations of CF₂ and CF₃ on SiO₂ are much lower than the concentrations on Si₃N₄. It appears that SiO₂ preferentially reacts with CF₂ and CF₃ only but not with C–C or CH_xF. Differences in the abilities of SiO₂ and Si₃N₄ to react with C–C structures contributed to higher etch rates of Si₃N₄.

A theoretical modelling of the plasma plume induced during welding of iron sheets with CO₂ laser is presented. The set of equations consists of the equations of conservation of mass, energy, momentum and the diffusion equation and is solved with the use of the commercially available program Fluent 6.1. The computations are made for a laser power of 1700 W and for two shielding gases—argon and helium. The results show a significant difference between these two cases. When helium is used as the shielding gas, the plasma is much smaller and burns only where the metal vapour is slightly diluted by helium. In the case when argon is the shielding gas, there are actually two plasmas: argon plasma and metal plasma. The flowfield shows that the velocity increases in the hot region but only part of the mass flux enters the plasma core. In the case when argon is used as the shielding gas, the total absorption of the laser radiation amounts to 18–33% of the laser power depending on argon and iron vapour velocities. In the case of helium the total absorption is much lower and amounts to ∼5% of the laser power.

A complex plasma disc (CPD) is a stable two-dimensional arrangement of n particles, each with mass m and charge q, confined by a parabolic well and interacting through a shielded Coulomb force (i.e. a Yukawa potential). It is shown that the breathing frequency of a CPD is accurately predicted by an analytical theory in which it is assumed that the breathing mode corresponds to a uniform expansion and contraction of the disc. Predictions of the theory are compared with exact eigenmode computations for n = 2–100 particles with Debye shielding parameters κ = 0.1–10. The breathing frequency is predicted exactly for n ⩽ 8 particles due to symmetry. For n > 8 particles, the error in the square of the breathing frequency is always less than 0.4% and increases approximately linearly with κ. The utility of this theory for analysing experimental data is illustrated by finding the average particle charge and Debye length for a disc with n = 40 particles.

The discharge behaviour of a dielectric barrier discharge (DBD) in low-pressure argon gas was investigated by experiments and modelling. The electrical characteristics and light emission dynamics of the discharge were measured and compared with the results of a two-dimensional fluid model. Our investigations showed that the discharge consisted of a single, diffuse discharge per voltage half-cycle. The breakdown phase of the low-pressure DBD (LPDBD) was investigated to be similar to the ignition phase of a low-pressure glow discharge without dielectrics, described by Townsend breakdown theory. The stable discharge phase of the LPDBD also showed a plasma structure with features similar to those of a classical glow discharge. The presence of the dielectric in the discharge gap led to the discharge quenching and thus the decay of the plasma. Additionally, the argon metastable density was monitored by measuring light emission from nitrogen impurities. A metastable density of about 5 × 10¹⁷ m⁻³ was present during the entire voltage cycle, with only a small (∼10%) increase during the discharge. Finally, a reduction of the applied voltage to the minimum required to sustain the discharge led to a further reduction of the role of the dielectric. The discharge was no longer quenched by the dielectrics only but also by a reduction of the applied voltage.

The natural law governing the interfacial contact on a microscopic scale of a nominally flat viscoelastic solid (VS) is studied from a perspective that differs from those of existing contact theories. Contact is viewed as a random process; its progress is tracked by treating the interface as a set of many small elements and partitioning them into subsets according to their contact status. The extent of contact, in the form of a map of pressure and dwell time, is obtained through non-deterministic considerations that require neither imposing linearity on viscoelastic behaviour nor input of surface topographic data. Calculated results for VS contact with stainless steels, and for VS self-contact are verified with published experimental results. The structure and implications of the present proposition are discussed; comparisons with other contact theories are also given.

This paper describes a theoretical study of adhesive friction at the contact between rough surfaces with small-scale surface asperities using a scale-dependent friction model and an elastic–plastic model of contact deformation that is based on accurate finite element analysis of an elastic–plastic single asperity contact. The single asperity friction model of Hurtado and Kim is integrated with the Johnson–Kendall–Roberts adhesion model to consider the adhesive friction behaviour of rough surfaces. The model considers a large range of interference values from fully elastic through elastic–plastic to fully plastic regimes of contacting asperities. Four non-dimensional parameters, namely, surface roughness parameter, friction regime parameter, adhesion parameter and plasticity index, are used to consider different conditions that arise as a result of varying load, surface and material parameters. Results are obtained for the coefficient of friction against applied load for various combinations of these parameters. The results show that the coefficient of friction depends strongly on applied load, surface roughness and adhesion. However, the friction coefficient is less sensitive to the plasticity index and the friction regime parameter. Adhesive friction can be reduced by modifying the surfaces so as to yield a high surface roughness parameter and low adhesion parameter.

The exchange biasing of Co_0.9Fe_0.1 film by the antiferromagnet Ir–Mn via a thin intermediate Co_0.6Fe_0.4 layer was investigated. It has been found that the exchange bias is drastically enhanced with the insertion of an ultra-thin Co_0.6Fe_0.4 layer at the interface between Co_0.9Fe_0.1 and Ir–Mn layers. For the Ir_0.25Mn_0.75(100 Å)/Co_0.9Fe_0.1(150 Å) bilayer, the pinning field at room temperature is raised from 92 to 206 Oe when a 4 Å Co_0.6Fe_0.4 layer is introduced at the interface, and further enhanced to 262 Oe with a 10 Å Co_0.6Fe_0.4 insertion layer. The corresponding unidirectional anisotropy increases from 0.18 to 0.4 erg cm⁻² and reaches a value of 0.54 erg cm⁻². The extra large unidirectional anisotropy is maintained as the Co_0.6Fe_0.4 thickness exceeds 10 Å. The high blocking temperature of the Ir–Mn/Co_0.9Fe_0.1 bilayer is unaffected by the insertion of the Co_0.6Fe_0.4 layer. The present results indicate that exchange bias is essentially determined by interfacial spins and, most importantly, the performance of the exchange bias system Ir–Mn/Co_0.9Fe_0.1, which is commonly used in spin valve giant magnetoresistance devices, can be greatly promoted by the insertion of an ultra-thin interfacial Co_0.6Fe_0.4 layer.

The structure of CaO–ZrO₂–SiO₂ glasses doped with V₂O₅ (0.1–2 mole%) was studied using density and molar volume measurements. It was observed that the molar volume increased while the density decreased with increasing V₂O₅ content. Magnetic susceptibility measurements showed the diamagnetic behaviour of this glass system. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume and fractional free volume have also been calculated from the positron annihilation lifetime data. The overall measured parameters from the structural and/or positron annihilation spectra indicated that the majority of the vanadium ions in the range 0–0.5 mole% of V₂O₅ are in the V⁵⁺ valence state which act as a glass former. On the other hand, the majority of the vanadium ions in the range 0.7–2 mole% of V₂O₅ are the V³⁺ and V⁴⁺ states acting as a glass modifier.

In the present work, starting from elemental bismuth, antimony and tellurium powders, p-type 25%Bi₂Te₃–75%Sb₂Te₃ thermoelectric materials with high density were prepared by mechanical alloying (MA) and plasma activated sintering (PAS). The single phase 25%Bi₂Te₃–75%Sb₂Te₃ alloys were obtained after MA for 12 h. The effect of sintering temperatures on microstructure and thermoelectric properties of the as-PASed samples was researched. Highly compact samples with relative density over 99% could be obtained when sintering temperature was over 653 K. A preferentially orientated microstructure with the (1 1 0) plane parallel to and the basal planes (0 0 l) perpendicular to the pressing direction was formed, and the orientation factors of the (0 0 l) planes changed from 0.11 to 0.12 at different sintering temperatures. The maximum power factor and figures of merit (Z) at room temperature were 3.10 × 10⁻³ W m⁻¹ K⁻² and 2.85 × 10⁻³ K⁻¹, respectively. The Vickers microhardness reached 112.7 Hv, which was twice that of the single crystal samples prepared by zone-melting.

Deformation twinning is confirmed upon large plastic deformation in nanocrystalline (nc) Ni by transmission electron microscopy examinations. New and compelling evidence has been obtained for several twinning mechanisms that operate in nc grains, with the grain boundary emission of partial dislocations determined as the most proficient. Twinning in nc Ni may be interpreted in terms of molecular dynamics simulation based on generalized planar fault energy curves.

Conducting polymers are suitable as electrode materials for high performance supercapacitors because of their high specific capacitance and high dc conductivity in the charged state. Ion beam (energy >1 MeV) irradiation of materials is a novel technique to modify their properties. Polyaniline conducting polymer thin films doped with HCl are synthesized electrochemically on indium tin oxide coated glass substrates and are irradiated with 160 MeV Ni¹²⁺ ions at different fluences, namely, 5 × 10¹⁰, 5 × 10¹¹ and 3 × 10¹² ions cm⁻². The dc conductivity measurements of the irradiated films showed up to 70% increase in conductivity, which may be due to the increase of carrier concentration in the polymer film as observed in UV-Vis spectroscopy and other effects like the cross linking of polymer chains, bond breaking and creation of defects sites. An x-ray diffractogram study shows that the degree of crystallinity of polyaniline increases upon swift heavy ion (SHI) irradiation with the increase in ion fluence. The capacitance of the irradiated films is lowered but that of the supercapacitors with irradiated films showed enhanced electrochemical stability compared with the devices with unirradiated films while characterized for a cycle life up to 10 000 cycles. This effect could possibly be ascribed to the stabilization of volatile surface groups upon SHI irradiation.

The first measurements of the thermal conductivity, thermal diffusivity and volumetric heat capacity of polycrystalline D-Er₂Si₂O₇ have been made simultaneously in the temperature range 77–300 K. Both the thermal conductivity and thermal diffusivity follow a modified Eucken's law in this temperature region. The heat capacity at constant pressure (C_P), determined from the volumetric heat capacity, agrees with the calculated one at room temperature.

The effect of uniaxial compressive pre-stress on the ferroelectric properties of commercial soft PZT ceramics is investigated. The ferroelectric properties under the uniaxial compressive pre-stress of the ceramics are observed at stress up to 24 MPa using a compressometer in conjunction with a modified Sawyer–Tower circuit. The results show that the ferroelectric characteristics, i.e. the area of the ferroelectric hysteresis (P–E) loops, the saturation polarization (P_sat), the remanent polarization (P_r), and the loop squareness (R_sq) decrease with increasing compressive pre-stress, while the coercive field (E_c) is virtually unaffected by the applied stress. The stress-induced domain wall motion suppression and non-180° ferroelectric domain switching processes are responsible for the changes observed. In addition, a significant decrease in these parameters after a full cycle of stress application has been observed and attributed to the stress-induced decrease in the switchable part of the spontaneous polarization at high stress. Furthermore, the permittivity calculated from the P–E loops is found to decrease with increasing applied pre-stress. This finding differs considerably from the results in the low-field experimental condition. Finally, this study clearly shows that the applied stress has a significant influence on the ferroelectric properties of soft PZT ceramics.