Lasers based on electrodynamically dispersed media

We demonstrate the possibility of creating a suspension of active-media particles in a discharge tube by using an electrodynamic dispersing system. An electric discharge in an electrodynamically dispersed system of 30-μm Cu particles was studied. The velocity of Cu (30 μm), Al (30 μm), and W (6-μm flakes) particles was measured at atmospheric pressure using a laser Doppler velocimeter. The velocities were found to be in the 0.1−5-m/s range. The electric field strength required to levitate Cu, Al, and W particles was studied as a function of buffer gas (air) pressure in the range from 2 × 10⁻² Torr to 1 atm. It is shown that powders can be suspended with the help of electrodynamic dispersing system at air pressure below 0.1 Torr or above 100 Torr.     

Field electron emission of carbon-based nanocone films

Diamond nanocone, graphitic nanocone, and mixed diamond and graphitic nanocone films have been synthesized through plasma enhanced hot filament chemical vapor deposition (HFCVD). The field emission properties of these films have been experimentally investigated. The studies have revealed that all three kinds of nanocone films have excellent field electron emission (FEE) properties including low turn-on electric field and large emission current at low electric field. Compared with the diamond nanocone films (emission current of 86 μA at 26 V/μm with the turn-on field of 10 V/μm), the graphitic nanocone films exhibit higher FEE current of 1.8×10² μA at 13 V/μm and a lower turn-on filed of 4 V/μm. The mixed diamond and graphitic nanocone films have been found to posses FEE properties similar to graphitic nanocone films (emission current of 1.7×10² μA at 20 V/μm with the turn-on field of 5 V/μm), but have much better FEE stability than the graphitic nanocone films. PACS 81.07.Bc; 81.05.Uw; 79.70.+q     

Coplanar Phase Shifters Based on Ferroelectric Thin Films

Barium strontium titanate (Ba_0.6Sr_0.4TiO₃, short for BST) thin films were prepared by RF-magnetron sputtering. The dielectric tunability, loss tangent, remanent polarization (2Pr) and coercive electric field (Ec) of the BST films are respectively 29.5%, 0.013, 2.29 μC/cm² and 22.27 kV/cm at 1 kHz and 20 V. The designed coplanar waveguide (CPW) phase shifter consists of 56 same sections. It is shown that the fabricated lines of electrodes are smooth and the widths at the tip of bottom electrodes are about 5 μm. At the central frequency of 28 GHz and a bias voltage of 20 V, the figure of merit is approximately 13°/dB.     

Structure and composition of tungsten silicide thermal-field microprotrusions

The thermal-field microprotrusions that grow on the surface of a tungsten tip coated with silicon when the tip is heated in an electric field are investigated by a suite of field emission methods, including electron field emission, ion desorption microscopy, and the atomic-probe method. For Si coatings more than a few monolayers thick, microprotrusions are observed to grow in the field desorption regime when the tip is heated to temperatures T=1100–1200 K in an electric field with initial intensity F=5.7–8.6×10⁷ V/cm. The field at which they evaporate is 1.2–1.8×10⁸ V/cm. The set of moving spots (i.e., microprotrusions) forms rings whose collapse signals the dissolution of the thermal-field growths on the developed faces. The most interesting structures are the sharp microprotrusions that grow on the central facet of a {110} tungsten tip under certain conditions. Atomic-probe analysis of their composition reveals that they consist of tungsten trisilicide WSi₃ with a monolayer surface skin whose composition is close to WSi₂. The intense growth of these formations on an initially smooth closepacked {110} face of tungsten is evidence that reconstruction of the latter takes place under the influence of the strong field and the interaction with silicon. Zh. Tekh. Fiz. 67, 102–109 (September 1997)     

Field evaporation of a Hf-Mo alloy

The field evaporation of a Hf-Mo alloy (15 wt. % Hf) is investigated using a time-of-flight atom probe. A moderately heated tip detects an impurity of Hf and Mo oxides on the surface. Thermofield microprotrusions grown at T=1440–1850 K in an electric field (retarding to electrons) of intensity E=(3.2–5)×10⁷ V/cm are analyzed at room temperature and above. Zh. Tekh. Fiz. 68, 69–73 (March 1998)     

Investigation of the electromagnetic wave scattering dynamics during microwave streamer evolution

A self-consistent electrodynamic model based on analytic relations, which makes it possible to describe the electromagnetic wave scattering from a thin (compared to the wavelength) plasma channel, is proposed. The results of experimental investigation of the dynamics of the signal scattered by a single channel in air in the pressure range P ₀ = 10⁴−1.4 × 10⁴ Pa are presented. The results are analyzed in terms of the electrodynamic model.     

Energy dissipation in fracture of bulk metallic glasses via inherent competition between local softening and quasi-cleavage

Compression, tension and high-velocity plate impact experiments were performed on a typical tough Zr_41.2Ti_13.8Cu₁₀Ni_12.5Be_22.5 (Vit 1) bulk metallic glass (BMG) over a wide range of strain rates from ～10^?4 to 10⁶ s^?1. Surprisingly, fine dimples and periodic corrugations on a nanoscale were also observed on dynamic mode I fracture surfaces of this tough Vit 1. Taking a broad overview of the fracture patterning of specimens, we proposed a criterion to assess whether the fracture of BMGs is essentially brittle or plastic. If the curvature radius of the crack tip is greater than the critical wavelength of meniscus instability [F. Spaepen, Acta Metall. 23 615 (1975); A.S. Argon and M. Salama, Mater. Sci. Eng. 23 219 (1976)], microscale vein patterns and nanoscale dimples appear on crack surfaces. However, in the opposite case, the local quasi-cleavage/separation through local atomic clusters with local softening in the background ahead of the crack tip dominates, producing nanoscale periodic corrugations. At the atomic cluster level, energy dissipation in fracture of BMGs is, therefore, determined by two competing elementary processes, viz. conventional shear transformation zones (STZs) and envisioned tension transformation zones (TTZs) ahead of the crack tip. Finally, the mechanism for the formation of nanoscale periodic corrugation is quantitatively discussed by applying the present energy dissipation mechanism.     

A method for studying catalytic reactions based on chemiemission of electrons

It was found that, if the energy released in the formation of product molecules in a heterogeneous reaction was smaller than the work function of the surface, the current density of the chemiemission of electrons from the surface of any semiconductor satisfied the condition j ≈ Bexp(βE), where B and β are coefficients and E is the electric field in the semiconductor surface plane. A mathematical model describing the transfer of hot metal electrons excited in a catalytic reaction through the metal-gas interphase boundary was studied. The parameters of the system at which a study of the distribution of catalytic centers over the surface of a metal or semiconductor by the scanning tip method on the basis of the chemiemission of electrons stimulated by an electric field was possible (resolution δr ∼ 10⁻⁸–10⁻⁷ m) were determined. Theoretical results corresponded to the experimental data obtained using weak electric fields (0 < E < 5 × 10⁶ V/m) for the heterogeneous recombination of hydrogen atoms on the surface of calcium, titanium, and n-type silicon.     

Crack extension force and rate of mechanical work of fracture in linear dielectrics and piezoelectrics

The physical significance of the crack extension force produced by mechanical loads and electric fields in linear dielectric and piezoelectric materials is examined using simple thermodynamic arguments. General expressions are derived for the crack extension force in dielectrics and piezoelectrics, under mechanical and electrical loads, in terms of the measurable parameters elastic compliance and electric capacitance . It is shown that the crack extension force produced by an electric field on an impermeable crack is always negative and it is argued that under combined electromechanical loads the total crack extension force in a piezoelectric cannot be separated into a mechanical component and an electrical component. Expressions for the crack extension force in terms of mechanical and electrical intensity factors are also given. Their derivation from available solutions for the electromechanical field at the crack tip (in a transversely isotropic material of crystal class 6 mm) is presented in detail to emphasize the physical significance of the coefficients that appear in the final expressions. In the light of these results, existing experimental observations that appear to be inconsistent with theoretical expectations are re-examined. The suggestion that the crack extension force is not a valid parameter to characterize the fracture behaviour of ferroelectrics is justified on physical grounds. Its importance is discussed and the rate of mechanical work of fracture is proposed as a more suitable parameter for those cases where the electric field does not produce dielectric rupture, nor degradation of the material at the crack tip.     

Mechanism of field emission from carbon materials

Field emission in diamond and graphite-like polycrystalline films is investigated experimentally. It is shown that the emission efficiency increases as the nondiamond carbon phase increases; for graphite-like films the threshold electric field is less than 1.5 V/μm, and at 4 V/μm the emission current reaches 1 mA/cm², while the density of emission centers exceeds 10⁶ cm⁻². A general mechanism explaining the phenomenon of electron field emission from materials containing graphite-like carbon is proposed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 56–60 (10 July 1998)     

Growth by laser ablation of Ti-based oxide films with different valency states

x La_2/3+yTiO_3-δ perovskite (with δ≤0.5) were deposited by the laser ablation technique from Li_0.33La_0.56TiO₃ targets. Their growth onto MgO substrateswas studied as a function of the oxygen pressure. For films grown in vacuum (10^-6 mbar), a La_0.63TiO_2.5 composition was obtained, meaning that Ti³⁺ alone is present in the films, while Li ions are not incorporated under these conditions. This material shows good electric conductivity (ρ=500 mΩ cm). By contrast, insulating films with a Li_0.1La_0.70TiO₃ composition corresponding to the Ti⁴⁺ species were obtained at high oxygen pressures (>0.05 mbar). For all conditions, textured films were grown with different orientations depending on the temperature and the oxygen pressure. Received: 10 September 1997/Accepted: 24 November 1997     

Electric-field-enhanced metal-induced crystallization of hydrogenated amorphous silicon at room temperature

A novel simple method of crystallization of hydrogenated amorphous silicon (a-Si:H) thin films is described. Namely, we studied a metal-induced crystallization enhanced by a dc electric field in sandwich p⁺–i–n⁺structures. The samples were fabricated from wide-bandgap a-Si:H with high hydrogen content (13–51 at. % H). Macroscopic islands of a-Si:H (up to ∼1 mm in diameter) in the region between upper (CrNi) and lower (ITO) contacts crystallize instantaneously when a sufficiently high dc electric field (≳10⁵ V cm^-1) is applied. The crystallization sets in at room temperature and ambient atmosphere and is spatially selective. A proposed microscopic mechanism of such an easy macroscopic crystallization consists in easy diffusion of Ni and/or Ni silicides (representing nucleation sites) through a dense network of voids in hydrogen-rich a-Si:H. Received: 30 November 2000 / Accepted: 3 May 2001 / Published online: 27 June 2001     

一次人工触发闪电上行正先导的传输特征

利用山东一次人工触发闪电的高速摄像和30 m, 60 m和480 m的同步电场测量, 清晰显示了人工触发闪电上行正先导的传输特征.上行正先导头部光强相对较强, 二维发展速度变化波动较大并呈现明显不规则性,表明上行正先导发展具有明显的梯级特征. 在先导开始阶段从340 m到705 m高度之间上行正先导平均发展速度为9.8×10⁴ m/s,起始速度是 3.8×10⁴ m/s,局部速度总体上随高度呈现增加趋势.电场变化在近距离产生有规律的梯级状 变化,记录到的28个梯级相邻梯级间隔变化从14 μs 到 39 μs,几何平均值为25.1 μs. 估计的先导梯级长度分布在0.9 m到3.7 m,几何平均值为1.7 m.先导电场变化由慢的正向梯级状变化和 脉冲变化组成,结合光学和电场变化测量结果,得出正先导头部通道发生弯曲可使其电场变化的梯级特征减弱 或消失;正负先导梯级形成机制可能类似,均由其先导头部前端的双向流光发展而来.     

Evolution of the concentration of point defects at the tip of a crack

The evolution of the distribution of interstitial impurity atoms in the plastic zone around the tip of a tension crack is analyzed. The transport of point defects is determined by: 1) the hydrostatic component of the elastic stress at the crack tip, created by the superposition of the elastic fields of the crack and dislocations; 2) the elastic field of moving dislocations (“sweeping out” of interstitial impurity atoms); 3) the dislocation-driven transport of point defects present in the dislocation cores. The contributions of each mechanism of transport of point defects to the crack tip are calculated over the entire time from the start of loading of a sample containing a crack until an equilibrium distribution of plastic deformation is established after the cessation of loading. Numerical calculations are carried out for interstitial hydrogen atoms dissolved in an α-Fe crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1580–1585 (September 1997)     

Hf-sputtered indium oxide films doped with tin

Indium oxide films doped with tin (ITO-films) have been hf-sputtered from an 80 at-%In₂O₃/20 at-%SnO₂ target onto glass substrates. The sputter atmosphere contained mainly argon (10⁻²Torr) with addition of oxygen (0≦p _{O 2}≦2·10⁻²Torr). The sputtered films aren-conductors. The conductivity and density of charge carriers depend on the oxygen content of the sputter gas. They could be varied by two orders of magnitude. In air or in oxygen atmosphere the films oxidize at the surface and for a certain depth beneath the surface, thus decreasing the conductivity. The Hall mobility of the sputtered films is smaller (≈10 cm²V⁻¹ s⁻¹) than one observes at ITO films produced by CVD sparaying or other methods. The conductivity of as sputtered films approached maximum values of about 1000Ώ⁻¹cm⁻¹.     

Influence of samarium on optical absorption in thin films of the solid electrolyte RbAg4I5

The fundamental optical absorption of films of the solid electrolyte RbAg₄I₅ electrolyte films decreases (by approximately 25%) after vacuum evaporation of Sm films onto them, and a broad strong-absorption band with a maximum at 2.4 eV appears within the bad gap. The films bleach after 5–10 days in dry air. The observed phenomena are attributed to a high concentration (∼3×10²⁰ cm⁻³) of point defects, including F-centers, in nonstoichiometric RbAg₄I₅:Sm, and also to the oxidation of Sm. In colored films the ionic conductivity is σ⋍0.9σ ₀, and in bleached films it is close to the initial value σ ₀. Fiz. Tverd. Tela (St. Petersburg) 39, 1544–1547 (September 1997)     

Preparation and properties of alumina composites modified by electric field-induced alignment of carbon nanotubes

Catalytic chemical vapor deposition (CVD) grown multi-walled carbon nanotubes (MWNTs) are treated with HF and deionized water and are then placed into alumina ceramics for improvement of both electrical conductivity and mechanical properties. In particular, an alternating current (ac) electric field is applied during the coagulation of the alumina slurries to induce the formation of aligned MWNT networks in the alumina matrix. The coagulated alumina matrix composite bases filled with 2 wt. % ac electric field-induced aligned MWNTs, are then sintered by hot pressing. The electrical conductivities of the prepared composites in directions both parallel and perpendicular to the MWNTs alignment, reach values of 6.2×10^-2 S m^-1 and 6.8×10^-9 S m^-1, respectively, compared with that of 4.5×10^-15 S m^-1 for pristine alumina ceramics. The fracture toughness and flexing strengths of the prepared composites in the two directions are 4.66±0.66 MPa m^0.5, 390±70 MPa, and 3.65±0.46 MPa m^0.5, 191±5 MPa, respectively, compared with 3.78±0.66 MPa m^0.5 and 302±50 MPa for pristine alumina, 4.09±0.15 MPa m^0.5 and 334±60 MPa for alumina filled with 2 wt. % MWNTs prepared without the effect of an electric field, respectively. The results indicate that the electric field leads to anisotropic behaviour. The properties of the composites along the direction of the MWNTs alignment are much improved with the addition of a small amount of CVD grown MWNTs. PACS 61.46.Fg; 61.66.Fn     

Structure and electrical characterization of amorphous ErSiO films deposited by rf magnetron sputtering on Si (001)

Amorphous ErSiO films have been fabricated on p-type Si (001) substrates using rf magnetron sputtering technique. X-ray diffraction, high-resolution transmission electron microscopy, and atomic force microscopy were employed to investigate the samples. It is found that ErSiO film exhibits a flat surface, a sharp interface and superior electrical properties after post-deposition annealing in O₂ ambience for 30 min at 450°C. The effective dielectric constant of the film is measured to be 14.2, and the effective oxide thickness reaches 1.9 nm, with a low leakage current density of 1.1×10⁻⁴ A/cm² at an electric field of 1 MV cm⁻¹ after annealing at 450°C. The obtained characteristics make the amorphous ErSiO films a promising substitute for SiO₂ as a high-k gate dielectric.     

Electrical and optical properties of reactive dc magnetron sputtered silver-doped indium oxide thin films: role of oxygen

Silver-doped indium oxide thin films have been prepared on glass and quartz substrates at room temperature (300 K) by a reactive dc magnetron sputtering technique using an alloy target of pure indium and silver (80:20 at. %). During sputtering, the oxygen flow rates are varied in the range 0.00–2.86 sccm keeping the magnetron power constant at 40 W. The resistivity of these films is in the range 10⁰–10^-3 Ω cm and they show a negative temperature coefficient of resistivity. The films exhibit p-type conductivity at an oxygen flow rate of 1.71 sccm. The work function of these silver–indium oxide films has been measured by a Kelvin probe technique. The refractive index of the films (at 632.8 nm) varies in the range 1.13–1.20. Silver doping in indium oxide narrows the band gap of indium oxide (3.75 eV). PACS 73.30.+y; 81.15.Cd; 78.20.Ci; 73.61.Le     

Thin TiO2 grown by metal–organic chemical vapor deposition on (NH4)2S x -treated InP

The electrical characteristics of thin TiO₂ films prepared by metal–organic chemical vapor deposition grown on a p-type InP substrate were studied. For a TiO₂ film of 4.7 nm on InP without and with ammonium sulfide treatment, the leakage currents are 8.8×10⁻² and 1.1×10⁻⁴ A/cm² at +2 V bias and 1.6×10⁻¹ and 8.3×10⁻⁴ A/cm² at −2 V bias. The lower leakage currents of TiO₂ with ammonium sulfide treatment arise from the improvement of interface quality. The dielectric constant and effective oxide charge number density are 33 and 2.5×10¹³ cm², respectively. The lowest mid-gap interface state density is around 7.6×10¹¹ cm⁻² eV⁻¹. The equivalent oxide thickness is 0.52 nm. The breakdown electric field increases with decreasing thickness in the range of 2.5 to 7.6 nm and reaches 9.3 MV/cm at 2.5 nm.