The characteristics of microplasma discharge propagation over the titanium surface covered with a thin oxide film

The propagation and structure of a microplasma discharge initiated in vacuum by a pulsed plasma flow with a density of 1013 cm–3 on the surface of a titanium sample covered with a thin continuous dielectric titanium oxide film with a shickness of 2–6 nm were studied experimentally when the electric current of the discharge changes from 50 A to 400 A. It was found that the microplasma discharge glow visually at the macroscale has a branched structure of the dendrite type, which at the microscale consists of a large number of brightly glowing “point” formations – cathode spots localized on the metal surface. The resulting erosion structure on the titanium surface is visually “identical” to the structure of the discharge glow and consists of a large number of separate non-overlapping microcraters with characteristic sizes from 0.1–3 μm, which are formed at the sites of localization of cathode spots at distances of up to 20 μm from each other. It was found that the propagation of a single microplasma discharge over the titanium surface covered with a thin oxide film a thickness of 2–6 nm occurs at an average velocity of 15–70 m/s when the amplitude of the discharge electric current changes in the range of 50–400 A. In this case, the microplasma discharge propagation on the microscale has a “jumping” character: the plasma of “motionless” burning cathode spots, during their lifetime 1 μs, initiates the excitation of new microdischarges, which create new cathode spots at localization distances of 1–20 μm from the primary cathode spots. This process repeated many times during a microplasma dis- charge pulse with a duration from 0.1 ms to 20 ms.

NUMERICAL SIMULATION OF THERMAL CONDITION OF A LOW CURRENT ELECTRIC CONTACT

Electrical contacts may include various sub-systems or wiring harness connected via detachable connectors which depend on physical contacts for the electrical connectivity. Electrical contacts range from high, medium to low current depending on their usage. However, in the real-life condition, electrical contacts characteristics, especially at the interface, undergoes a gradual change which can be due to corrosion, temperature variation, aging, strained harnesses, discontinuities induced by vibration etc. These changes introduce additional parasitic circuits in the system. Moreover, in some cases where the contact resistance increases due to electrical loses, the local temperature may increase, thereby accelerating contact degradation. This paper presents a numerical analysis on the variation of temperature of a simple low current contact model having a thin oxide film layer at the interface which serves as the ageing factor using finite element method (FEM).

Physicochemical characteristics of solid particles of contaminants in the coolant of automobile and tractor engines

The article describes the results of a study of the physicochemical characteristics of solid particles of contaminants present in the coolant of automobile and tractor engines. The data on the fractional, physical and chemical composition of solid particles of contamination are given. It was established that the generalized reason for the appearance of contaminants of various nature in liquid cooling systems of engines is the physicochemical interaction of the coolant (antifreeze) with different elements and dissimilar materials of the cooling system. The use of absolutely pure coolant in the cooling systems of automobile and tractor engines is practically unrealistic, since there will always be operating conditions that contribute to the formation of contamination. A number of chemical elements (in an amount from 1 to 47% of each element) were found in the composition of solid particles of coolant contaminants: iron Fe, silicon Si, aluminum Al, lead Pb, tin Sn, zinc Zn, calcium Ca, magnesium Mg, copper Cu. In addition, at a level of less than 1.0% (wt.), Such chemical elements as potassium K, sodium Na, titanium Ti, phosphorus P, sulfur S, chromium Cr, molyb-denum Mo, chlorine Cl, iridium Ir, nickel Ni, manganese Mn, etc. were found. The most dangerous contaminants are particles of iron Fe and silicon Si, contained in the coolant in an amount of up to 47 and 37%, respectively, and possessing significant hardness and angularity. The abrasive proper-ties of Fe and Si particles create the danger of removing a thin oxide film on the inner surface of the walls of the cooling radiator channels, leading to their premature destruction. In this regard, it is concluded that high-performance engine coolant filters should be used in automobiles and tractors to remove these contaminants from the flow.

Evolution of Non-metallic Inclusions Through Processing in Ti-V Microalloyed 316L and Al-V Microalloyed 17-4PH Stainless Steels for Hipping Applications

Powders produced by air-melted gas atomization (AMGA) and vacuum induction gas atomization (VIGA) from Ti-V microalloyed 316L and Al-V microalloyed 17-4PH stainless steels along with their feedstock material and Hot Isostatically Pressed (HIP’d) products have been examined. Inclusion characteristics and development through process along with changes in grain size have been characterized. The main findings are that a thin oxide film forms on the powder surface, thicker for the 316L powder than the 17-4PH powder as indicated by XPS analysis of selected powder precursors, and large inclusions (predominantly oxides) are also observed on the 316L powder. This results in a high number of inclusions, including more complex two-phase inclusions, on the prior particle boundaries in the HIP’d material. Grain growth occurs during HIPping of the 316L powders with some evidence of inclusions locally pinning boundaries. In the vacuum-melted powder, smaller Ti-rich inclusions are present which give more grain boundary pinning than in the air-melted powder where Ti was lost from the material during melting. Consideration has also been made to determine the variation of Ti and V microalloying elements and residual Cu through processing. It was found that Ti was lost during air melting but partly retained after vacuum melting leading to the presence of fine and complex Ti-containing precipitates which provided grain boundary pinning during HIPping and heat treatment. V was retained in the melt by the use of both AMGA and VIGA processes, and therefore available for precipitation during HIPping. Residual Cu was retained during both air and vacuum melting and was associated with Mn S and Mn O S inclusions overwhelmingly outweighing that of Mn O inclusions in the two HIP’d 316L samples.

Combining scanning tunneling microscope (STM) imaging and local manipulation to probe the high dose oxidation structure of the Si(111)-7×7 surface

Understanding the atomistic formation of oxide layers on semiconductors is important for thin film fabrication, scaling down conventional devices and for the integration of emerging research materials. Here, the initial oxidation of Si(111) is studied using the scanning tunneling microscope. Prior to the complete saturation of the silicon surface with oxygen, we are able to probe the atomic nature of the oxide layer formation. We establish the threshold for local manipulation of inserted oxygen sites to be +3.8 V. Only by combining imaging with local atomic manipulation are we able to determine whether inserted oxygen exists beneath surface-bonded oxygen sites and differentiate between sites that have one and more than one oxygen atom inserted beneath the surface. Prior to the creation of the thin oxide film we observe a flip in the manipulation rates of inserted oxygen sites consistent with more oxygen inserting beneath the silicon surface.

Characterization of Oxide Film of Implantable Metals by Electrochemical Impedance Spectroscopy

The oxide film resistance (RP) and capacitance (CCPE) diagrams of implantable metals (commercially pure Ti, four types of Ti alloys, Co–28Cr–6Mo alloy, and stainless steel) were investigated by electrochemical impedance spectroscopy (EIS). The thin oxide film formed on each implantable metal surface was observed in situ by field-emission transmission electron microscopy (FE-TEM). The Ti–15Zr–4Nb–1Ta and Ti–15Zr–4Nb–4Ta alloys had higher oxygen concentrations in the oxide films than the Ti–6Al–4V alloy. The thickness (d) of the TiO2 oxide films increased from approximately 3.5 to 7 nm with increasing anodic polarization potential from the open-circuit potential to a maximum of 0.5 V vs. a saturated calomel electrode (SCE) in 0.9% NaCl and Eagle’s minimum essential medium. RP for the Ti–15Zr–4Nb–1Ta and Ti–15Zr–4Nb–4Ta alloys was proportional to d obtained by FE-TEM. CCPE was proportional to 1/d. RP tended to decrease with increasing CCPE. RP was large (maximum: 13 MΩ·cm2) and CCPE was small (minimum: 12 μF·cm−2·sn−1, n = 0.94) for the Ti–15Zr–4Nb–(0 to 4)Ta alloys. The relative dielectric constant (εr) and resistivity (kOX) of the oxide films formed on these alloys were 136 and 2.4 × 106–1.8 × 107 (MΩ·cm), respectively. The Ta-free Ti–15Zr–4Nb alloy is expected to be employed as an implantable material for long-term use.