Target Surface Modification Research Articles

Gas flow effect on the surface modification of aluminum and silver targets irradiated by a nanosecond laser

Gas flow effect on the surface modification of aluminum and silver targets irradiated by a nanosecond laser

Opportunities in Jet-Impingement Cooling for Gas-Turbine Engines

Impingement heat transfer is considered one of the most effective cooling technologies that yield high localized convective heat transfer coefficients. This paper studies different configurable parameters involved in jet impingement cooling such as, exit orifice shape, crossflow regulation, target surface modification, spent air reuse, impingement channel modification, jet pulsation, and other techniques to understand which of them are critical and how these heat-transfer-enhancement concepts work. The aim of this paper is to excite the thermal sciences community of this efficient cooling technique and instill some thoughts for future innovations. New orifice shapes are becoming feasible due to innovative 3D printing technologies. However, the orifice shape variations show that it is hard to beat a sharp-edged round orifice in heat transfer coefficient, but it comes with a higher pressure drop across the orifice. Any attempt to streamline the hole shape indicated a drop in the Nusselt number, thus giving the designer some control over thermal budgeting of a component. Reduction in crossflow has been attempted with channel modifications. The use of high-porosity conductive foam in the impingement space has shown marked improvement in heat transfer performance. A list of possible research topics based on this discussion is provided in the conclusion.

Effect of laser irradiance on the surface morphology and laser induced plasma parameters of zinc

AbstractThe effect of laser-irradiance on the surface morphology and laser induced breakdown spectroscopy of zinc has been investigated by employing Nd:YAG laser (wavelength λ = 1064 nm, pulse duration t ~ 10 ns, and repetition rate = 10 Hz) under ambient environment of argon at a pressure of 20 Torr. For this purpose, zinc targets were exposed to various laser irradiances ranging from 13 GW/cm2 to 100 GW/cm2. Scanning electron microscope analysis has been performed to analyze the surface modification of irradiated zinc targets. Scanning electron microscope analysis revealed the formation of various kinds of structures such as ripples, cones, cavities, and wave like ridges at the center and peripheral regions of ablated zinc. In the central ablated region with increasing laser irradiance, the growth of distinct and well defined ripples is observed. Further increase in irradiance makes the appearance of these ripples diffusive and narrow. In order to correlate the plasma parameters with the surface modification, laser induced breakdown spectroscopy analysis has also been performed. The electron temperature and number density of zinc plasma have been evaluated at various laser irradiances. For both plasma parameters, an increasing trend up to a certain value of laser irradiance is observed which is due to enhanced energy deposition. Afterword a decreasing trend is achieved which is attributed to the shielding effect. With further increase in irradiance a saturation stage comes and almost no change in plasma parameters is observed. This saturation is explainable on the basis of the formation of a self-regulating regime near the target surface. A strong correlation between surface modification and plasma parameters is established.

Effect of laser fluence on surface, structural and mechanical properties of Zr after irradiation in the ambient environment of oxygen

The laser irradiation effects on surface, structural and mechanical properties of zirconium (Zr) have been investigated. For this purpose, Zr samples were irradiated with Excimer (KrF) laser (λ ≈ 248 nm, τ ≈ 18 ns, repetition rate ≈ 30 Hz). The irradiation was performed under the ambient environment of oxygen gas at filling pressure of 20 torr by varying laser fluences ranging from 3.8 to 5.1 cm-2. The surface and structural modification of irradiated targets was investigated by scanning electron microscope (SEM) and X-ray diffractometer (XRD). In order to explore the mechanical properties of irradiated Zr, the tensile testing and Vickers micro hardness testing techniques were employed. SEM analysis reveals the grain growth on the irradiated Zr surfaces for all fluences. However, the largest sized grains are grown for the lowest fluence of 3.8 J cm−2. With increasing fluence from 4.3 to 5.1 J cm−2, the compactness and density of grains increase whereas their size decreases. XRD analysis reveals the appearance of new phases of ZrO2 and Zr3O. The variation in the peak intensity is observed to be anomalous whereas decreasing trend in the crystallite size and residual stresses has been observed with increasing fluence. Micro hardness analysis reveals the increasing trend in surface hardness with increasing fluence. The tensile testing exhibits the increasing trend of yield stress (YS), decreasing trend of percentage elongation and anomalous behaviour of ultimate tensile strength with increasing fluence.

Surface modification of copper using high intensity, 1015 W/cm2, femtosecond laser in vacuum

A study of copper target surface modification with high intensity (1015W/cm2, fluence of ∼300J/cm2) Ti:sapphire laser, operating at 800nm wavelength and pulse duration of 60fs is presented. The Cu surface variations were studied in vacuum ambience. The copper target specific surface changes and phenomena observed are: (i) creation of craters (with extra-low diameters); (ii) formation of periodic surface structures especially expressed at the reduced intensity; (iii) presence of the “clean” surface in the irradiated zone (mainly absence of the oxide(s)), and (iv) occurrence of plasma in front the surface, emitting predominantly X-ray radiation (soft and hard). It can be concluded from this study that the reported laser intensities can effectively be applied for copper surface modification. The appearance of plasma in front of the target, irradiating in the X-ray/UV region, offers additional effect of surface purification facilitating contaminant-free conditions which is highly important for specific applications, e.g. in microelectronics. Generally, femtosecond laser surface modification of copper is non-contact and very rapid compared to traditional modification methods.

Surface Modification of Metallic Targets with Ultrashort Laser Pulses

Interaction of pulsed femtosecond Ti:sapphire laser (160 fs), operating at high repetition rate (75 MHz) at 800 nm, with nickel-based superalloy Inconel 600 and tungsten–titanium (WTi) target was studied. The WTi target was in form of thin film deposited on silicon substrate. Low laser fluence of maximum 50 mJ/cm had modified the target surface during irradiation/exposure time of seconds or minutes. The radiation absorbed from the laser beam generates at the surface a series of effects, such as direct material vaporization, formation of clusters, etc. Morphological features of the targets can be summarized as: (a) intensive removal of material and crater appearance; (b) creation of nanostructures; (c) microcracking, etc. Ablation of Inconel 600 surface is effective, resulting in formation of holes with small diameter (≤ 10 μm) and relatively large depth (≤ 50 μm). In case of WTi target/thin film, the surrounding rim is not so expressed, and crater depths are lower. It can be concluded that the average laser power of the order of watts, pulse energies of the order of nanojoules and high repetition rates (MHz range) can successfully modify metallic materials.

Modeling of the target surface modification by reactive ion implantation during magnetron sputtering

The erosion rate of an aluminum target bombarded with Ar+ and O2+ ions was simulated using TRIDYN. An abrupt change of the erosion rate is noticed at a critical mole fraction of O2+ ions. This target behavior can also be described by a simple analytical model showing that the abrupt change finds its origin in an avalanche induced by reaction ion implantation. Indeed, by the reduction of the average sputter yield by compound formation, more reactive ions become implanted into the target as the number of implanted ions depends inversely on the average sputter yield. As such, an avalanche situation can develop which finally results in a completely oxidized target. It is also shown that the critical mole fraction depends linearly on the sputter yield of the target material and that target poisoning induced by ion implantation can result in a hysteresis behavior for the target condition.

Target voltage behaviour during DC sputtering of silicon in an argon/nitrogen mixture

On addition of nitrogen to an argon plasma, the silicon target voltage changes. By following the target voltage during the addition and removal of the reactive gas, the influence of the plasma condition was separated from the influence of the target condition on the target voltage. In this way, we were able to investigate the target surface modification during reactive sputtering. The target surface modification seems to be induced by reactive ion implantation. The target surface modification is not stable and N 2 desorption from a silicon target after reactive sputtering is noticed.

Influence of oxygen addition on the target voltage duringreactive sputtering of aluminium

The voltage of an aluminium target changes duringmagnetron sputtering when oxygen is added to the argon plasma. The variation in target voltage has been ascribed to a targetsurface modification, which alters the ion-induced secondaryelectron emission (ISEE) coefficient. As most models assumethat the target surface modification was induced by chemisorptionof oxygen on the aluminium target, we have measured theinfluence of chemisorption on the target voltage. At lowoxygen exposure an absolute target voltage increase as afunction of the exposure was noticed. Extending the oxidationperiod resulted in an absolute target voltage decrease if theexposure was increased. Comparing these results with themeasurements performed regarding reactive sputtering, we came tothe conclusion that chemisorption cannot explain the targetsurface modification during reactive sputtering. Indeed,experiments concerning the stability of the target surfacemodification induced by reactive sputtering clearly indicatedthat it is not stable. This shows that the target surface modification during reactivesputtering is not the formation of a stable surface compound bya chemical reaction between oxygen molecules and the aluminiumsurface, as noticed during chemisorption. The chemical reactionbetween implanted reactive gas atoms and the target atoms formsthe basic idea of the presented approach to describe the targetsurface changes.

Plasma optical emission studies of high- Tc superconducting and buffer thin film physical vapour deposition

The non-intrusive technique of optical emission spectroscopy was used to identify the plasma species active in two different physical vapour deposition techniques: sputtering and laser ablation, when applied for high T c superconducting (HTSC) and buffer thin film deposition. Peak identification of the plasma optical emission spectra taken for targets of YBa 2Cu 3O 7 − x (YBCO), BiSrCaCu 2O 7 − x (BiSCCO 1112), Bi 2Sr 3Ca 3Cu 4O 7 −x( BiSCCO 2334), SrTiO 3, BaTiO 3, Y 2O 3-stabilized ZrO 2 (YSZ) and CeO 2 was made from published values and measured results for the constituting elements (Cu) and elemental oxides (Y 2O 3, BaCO 3, CuO). Aiming at determination of an appropriate criterion for in situ assessment of ambient oxygen sufficiency, the plasma optical emission was related to the deposited HTSC film characteristics. During YBCO RF single target off-axis sputtering and pulsed laser ablation the gas-phase preoxidation, witnessed by YO-optical emission, could be used as a qualitative criterion ensuring high quality film deposition. During dc magnetron sputtering with two-opposedtargets, the oxygen-to-argon peak actinometric ratio was used as a criterion instead. SEM investigation of the ablation-induced surface modification of targets of HTSC and buffer materials revealed specific topography for different target structure and density, which can influence the deposited film thickness uniformity. An extensive table is presented of the plasma emission lines and band heads for YBCO compiled from this work and that of other authors.

Growth mechanism of YMnO3 film as a new candidate for nonvolatile memory devices

We have proposed ReMnO3 (Re: rare earth) thin films for nonvolatile memory devices. We examine the growth mechanism of YMnO3 films on (0001)ZnO:Al/(0001) sapphire substrate using rf magnetron sputtering and pulsed laser deposition methods with oxide compound target. We have succeeded in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001) sapphire substrate, and polycrystalline films on (111)Pt/(111)MgO. For an optimal structure, the film needed much less oxygen from the gas phase compared to other oxide films. The composition (Y/Mn ratio) of the YMnO3 films changed drastically by varying the partial oxygen pressure in the sputtering gas. In addition the Y/Mn ratio slinfled with sputter time due to target surface modification, probably caused by Y segregation. An extremely small amount of oxygen is required to form the YMnO3 crystal. This was confirmed by pulsed laser deposition experiments.

New plasma source ion-implantation technique for inner surface modification of materials

Plasma source ion-implantation (PSII) is a non-line-of-sight ion implantation technique for surface modification of materials, but the present technique is only suitable for outer surface modification of targets, not for inner surface modification of targets. In this paper we present a new technique which applies to inner surface implantation. Preliminary experiment results show that this new technique: (1) increases plasma density and uniformity obviously inside the target inner surface; (2) generates a plasma sheath between the inner surface of the target and plasma so as to implant the inner surface of the target effectively; (3) achieves a dose uniformity acceptable for industrial application.

New method of tubular material inner surface modification by plasma source ion implantation

Plasma source ion implantation is a non-line-of-sight ion implantation technique for surface modification of materials, but that technique is only suitable for outer surface modification of hollow targets, not for inner surface modification. In this article we present a new method that applies to inner surface implantation. Preliminary experimental results show that this new method increases plasma density and uniformity inside the tubular target with the inner surface, generates a plasma sheath between the inner surface of the target and the plasma to implant the inner surface of the target effectively, and achieves a dose uniformity that is acceptable for industrial applications.