Diffusion experiments in liquid Sn-Bi and Al-Ni systems with a stable density layering using the FOTON shear cell under 1g conditions

In this study we investigated the possibility of reliable diffusion experiments in liquid metals under 1g conditions, instead of expensive μg-experiments. To minimise buoyancy convection we used thick layer diffusion from a binary alloy into apure metal. This can provide a stable density layering, which we have shown to be an important factor for successful 1g-experiments. To avoid the segregation problem and to minimize free surfaces a shear cell was used, which was specially developed for the mission FOTON-M2 and was equipped with reservoirs providing pressure on the liquid samples. Thick layer diffusion experiments from SnBi2.5wt% into Sn and from AlNi3.5wt% into pure Al were performed at 300°Cfor 8h and at 730°Cfor 5h respectively. For each set-up four parallel experiments were performed at the same time. The concentration profiles were obtained by AAS (atom absorption spectroscopy) and the diffusion coefficients were evaluated by fitting with the thick layer solution. For the evaluation a correction method was used for the shear convection and the AAS averaging effect inside a cell. As a result, the obtained concentration curves agreed well with the fitting function. The diffusion coefficients D_Bi=2.35x10^{? 9}m²/s andD_Ni=3.81x10^{? 9}m²/s agreed well within the error range with the μg-reference data obtained in the FOTON-12 mission and reference data obtained in the 1g diffusion experiment in a magnetic field. The reproducibility of the diffusion coefficients among four parallel experiments was very good with a standard deviation among four capillaries smaller than 3.1% including the standard temperature deviation. From these results we conclude that buoyancy convection was practically absent and thus the applied method was very effective.     

Diffusion measurements on a liquid monotectic alloy PbGa using the shear cell technique under µg in the foton-M2 mission and under 1G

Diffusion experiments of a liquid monotectic alloy PbGa were performed under 1g-conditions at 623, 773 and 903K, and under µg-conditions at 903K. As a measurement tool the shear cell specially designed for Russian satellite Foton missions within the AGAT-furnace was used to have a homogenization time before the diffusion and to avoid the influence of decomposition into two liquid phases during cooling-down. The experiment type was diffusion from a thick layer of PbGa5at% into pure Pb. The liquid sample in the capillary was pressurized using a reservoir system in order to minimize Marangoni convection. In 1g-experiments the diffusion axis was arranged vertically and the sample was set such that the density increases monotonously parallel to the gravity vector in order to suppress buoyancy convection. Four experiments were performed simultaneously for each set-up. The µg-experiment was performed in the AGAT facility during the Foton-M2 mission (June 2005) where one capillary was assigned for the PbGa-Pb experiment (PbGa was containing an enriched Pb-isotope tracer). The concentration profiles were obtained by AAS (1g) and by ICP-MS (µg). The diffusion coefficients were evaluated by fitting with the thick layer solution. For the evaluation a correction method was used for the shear convection and the averaging effect inside each cell. As a result, the obtained concentration curves agreed well with the fitting function. The reproducibility of the diffusion coefficients among four parallel 1g-experiments was good with a standard deviation among four capillaries smaller than 5.5% including the standard temperature deviation. The diffusion coefficients agreed well between 1g- and µg-experiments. The temperature dependence of the diffusion coefficients could be fitted well to a power law with an exponent about 2.4. From these results we conclude that the 1g-experimental method in this study for diffusion measurement is effective also for monotectic systems.     

Surfactants with NH3H2O

To avoid rivulet flow in compact absorbers, which consist of compact heat exchanger plates, the surface tension of the ammonia-water solution has to be reduced by surfactants. The influence of these additives on the absorption of ammonia into water is investigated for two anionic tensides, two non-ionic tensides and the alcohol 1-octanol in a stagnant pool measuring cell. All four tensides had no influence on the absorption in the concentration range investigated (0.25–0.91 wt%), although they were able to reduce the surface tension of water down to 30mN m⁻¹. In order to increase the mass transfer, 1-octanol was used in three concentrations, which probably lead to a convection in the liquid layer, called Marangoni convection. The experiments have shown that the presence of surfactant islands is not necessary to induce Marangoni turbulence. The best result was achieved with 50 ppm 1-octanol dissolved in water.     

Flow-dependent platelet behaviour in blood—material interactions

Platelet activation and adhesion are important parameters characterizing blood compatibility of biomaterials. A platelet transport theory based on convection diffusion, which describes the influence of wall shear rate, platelet concentration, axial position, hematocrit and red cell size, was originally proposed by Turitto and Baumgartner and later expanded by Aarts. This theory was applied in an in vitro perfusion system for three different materials with wall shear rates between 100s^-1 and 4300 s^-1 in order to cover the regions of diffusion controlled, reaction controlled and intermediate platelet adherence. Platelet diffusivity and platelet vessel wall surface reactivity were determined for these cases and the constants m and n were calculated using the relation between platelet diffusivity and shear rate as expressed by the following power law function: D _w=m*y ⁿ _w.     

Solid state bonding of superplastic AA 7475

Abstract

Solid state (diffusion) bonds have been developed in AA 7475E sheet material using different regimes of bonding temperature, pressure, and time. The bonds produced have exhibited shear strengths in the range 30–150 MN m^?2 and have been found to be capable of withstanding peel during a slow high-temperature superplastic forming operation. However, the bonds formed under any one set of bonding conditions showed an extremely wide variation in both shear strength and shear fracture mode. The source of this wide variation has not yet been identified.

MST/601     

Improvements in the fatigue strength of Ti-6Al-4V through microstructure control

Fatigue deformation and stage I (shear mode) crack initiation in Ti-6Al-4V alloy test pieces have been studied using optical microscopy. Two types of stage I fatigue crack initiation were observed, (a) alongα/β interfaces and (b) transcrystalline initiation acrossα grains in partly transformed microstructures and acrossαβ interfaces in fully transformed microstructures. The α/β interface cracking occurred predominantly in the low stress regions of the test pieces. These observations suggested that a microstructure with a smallα grain size, to minimize the mean free slip path, and with minimum lengths ofα/β interface, would have a high fatigue strength. Such a microstructure, with anα grain size of < 10 μm, and spheroidal or near spheroidalβ particles, was produced by thermo-mechanical processing. The rotating cantilever fatigue strength of this microstructure, ± 670 MN m^?2 at 10^?7 cycles, compares with fatigue strengths in the range ± 480 to ± 590 MN m^?2 for commercial Ti-6Al-4V bars.     

Experimental study on time evolution of Marangoni flow instability in molten silicon bridge

Temperature oscillations due to Marangoni flow instability in molten silicon half zone bridges with various aspect ratios of As = 0.5–2.0 were measured using six thermocouples set azimuthally 60^° apart in the liquid bridge close to the cold rod. The Marangoni number was estimated to range from 3000 to 14000, based on the measured axial temperature difference. Fourier spectra of the temperature oscillations were broad and continuous; each peak was not clearly distinguished but rather appeared as a frequency band. Thus, the convection was estimated to be turbulent-like. The time evolution of the azimuthal wave number was observed by analyzing the time-dependence of the phase relationship of the temperature oscillation detected by the six thermocouples. Analyzing the mode appearance coefficient MAC as a function of the aspect ratio, the relationship between the azimuthal mode number m and the aspect ratio As was observed to be m ? As ≈ 2.4; the basic structure of flow instability is sustained even under high Marangoni number. The temperature oscillation data was decomposed into that for each frequency band by using wavelet analysis. The frequencies for the m = 1 and m = 3 modes were estimated to be 0.08 to 0.2 Hz and 0.01 to 0.2 Hz, respectively.     

Pool Boiling with Non-condensable Gas in Microgravity: Results of a Sounding Rocket Experiment

Pool boiling experiments in microgravity have been performed in the Sounding Rocket Maser 11. A heated plate of 1cm ² was located at the bottom of a small cylindrical tank partly filled with a refrigerant Novec HFE7000 pressurized with Nitrogen. Experiments were performed at different reservoir pressures and wall heat fluxes. The wall heat flux and wall temperature were simultaneously measured during the experiment and the behavior of the bubbles on the heater was filmed with a video camera through the transparent wall of the reservoir. The presence of Nitrogen dissolved inside the liquid led to a strong Marangoni convection around the bubble. The effect of Marangoni convection and evaporation on the wall heat transfer is analyzed in function of the relative values of the wall temperature and saturation temperature.     

Experimental investigation of Marangoni convection and vibration-induced crystal motion during protein crystal growth

The occurrence of Marangoni convection during cytochrome c’ crystal growth and vibration-induced motion of lysozyme crystals were investigated using a High Density Protein Crystal Growth (HDPCG) apparatus. Particle image velocimetry was used to visualize fluid motion, but no particle motion was observed, which suggests that under the experimental conditions used, Marangoni convection is not a significant cause of fluid and crystal motion. When horizontal vibrations of controlled amplitude and frequency were applied to the HDPCG apparatus, lysozyme crystals located on the liquid-vapour interface of the HDPCG cell made significant movements up to 0.5mm in amplitude and velocities reaching 0.06mm/s. These results from the Marangoni convection and horizontal vibration experiments suggest that protein crystal movements observed in past space experiments were most likely caused by g-jitter on the spacecraft rather than Marangoni convection.     

表面张力对流对 BaB2O4晶体生长界面边界层的作用

利用高温光学实时观察方法，实时地观察了BaB2O4（BBO）高温熔体的表面张力对流效应以及BBO单晶的旋转生长过程，计算了固液界面附近的浓度、温度以及动量边界层厚度δc，δT和δv，并研究了热毛细对流对边界层厚度的影响．结果发现，浓度边界层厚度远远小于温度以及动量边界层厚度，说明晶体生长过程中，质量扩散在界面输运过程中起着主导性作用，同时发现，边界层厚度随体系无量纲Marangoni数的增大而线性地减小．     

Measurement of the Diffusion Coefficient of Water in RP-3 and RP-5 Jet Fuels Using Digital Holography Interferometry

The diffusion coefficient of water in jet fuel was measured employing double-exposure digital holographic interferometry to clarify the diffusion process and make the aircraft fuel system safe. The experimental method and apparatus are introduced in detail, and the digital image processing program is coded in MATLAB according to the theory of the Fourier transform. At temperatures ranging from 278.15 K to 333.15 K in intervals of 5 K, the diffusion coefficient of water in RP-3 and RP-5 jet fuels ranges from 2.6967?×?10 ^?10 m²·s^?1 to 8.7332?×?10 ^?10 m²·s^?1 and from 2.3517?×?10 ^?10 m²·s^?1 to 8.0099?×?10^?10 m²·s^?1, respectively. The relationship between the measured diffusion coefficient and temperature can be well fitted by the Arrhenius law. The diffusion coefficient of water in RP-3 jet fuel is higher than that of water in RP-5 jet fuel at the same temperature. Furthermore, the viscosities of the two jet fuels were measured and found to be expressible in the form of the Arrhenius equation. The relationship among the diffusion coefficient, viscosity and temperature is analyzed according to the classic prediction model, namely the Stokes–Einstein correlation, and this correlation is further revised via experimental data to obtain a more accurate predication result.     

An experimental study of bubble behavior in a temperature gradient near a heated wall under low-gravity and microgravity conditions aboard NASA DC9 airplane

The behavior of a single bubble and a pair of bubbles under microgravity conditions has been investigated using the NASA-DC9 aircraft in order to understand the effects of various parameters and to control the bubble behavior in space. Silicone oil was used as the test liquid, and a nitrogen gas bubble was injected from the top wall under different experimental conditions. In an isothermal case, two different microgravity conditions were achieved by either fixing the experimental apparatus to the aircraft floor or freely floating the apparatus in the aircraft cabin. The bubble behavior was found to be clearly influenced by the quality of the microgravity environment, and variations of the bubble aspect ratio with the Bond number were presented. The results indicate that there is a critical Bond number of the order of 10⁻¹ which determines the bubble shape deformation. In the free-floating experiments, a temperature gradient was imposed on the liquid around the bubble near the heated top wall. Marangoni convection was expected to occur around the bubble and the bubble behavior was studied under various temperature gradients. The bubble aspect ratio was found to decrease with an increase in the Marangoni number. A theoretical model for the relation between the Marangoni flow around the bubble and the aspect ratio is proposed based on simple assumptions. Visualization of Marangoni convection around the bubbles using the photochromic dye activation method was successfully performed. The aspect ratios predicted by the model agreed with the experimental results reasonably well. Direct measurements of surface velocity are, however, necessary to further evaluate the validity of the model.     

Joining of particulate silicon carbide reinforced 2124 aluminiumalloy by diffusion bonding

Abstract

The diffusion bonding of 2124 aluminium alloy reinforced with particulate silicon carbide has been investigated. The shear strength of the bonded composite decreased with increasing volume fraction of particulate V_p owing to the reduced area of metal to metal contact across the joint, from 60 MN m^?2 for V_p=0·25 to 35 MN m^?2 for V_p=0·40. The use of an interlayer of 2124 alloy increased the area of metal to metal contact to give improved bond strength. Microstructural characterisation of the bonds showed that the magnesium present in the 2124 alloy was a crucial factor in forming a joint, since it disrupted the surface oxide to allow metal interdiffusion.

MST/3189     

Marangoni convection in a rectangular container

If the free liquid-gas interface of a liquid in a rectangular Container is subjected to a temperature gra-dient the shear stress on the free liquid surface being temperature dependent transmits by viscous traction a thermo-capillary convection into the bulk of the liquid. For constant temperature T ₁ at one wall and T ₂ at the other a steady Marangoni convection takes place while for time-oscillatory temperatures of the walls a time-dependent thermo-capillary convection appears, which will create wave patterns on the free liquid surface. They shall, depending on the forcing frequency of the temperature, exhibit resonance peaks. The velocity distribution, the response magnitude inside the Container, the forced free surface displacement and the influence of the Prandtl number have been investigated.     

Electron Field Emission Enhancement of Vertically Aligned Ultrananocrystalline Diamond‐Coated ZnO Core–Shell Heterostructured Nanorods

Enhanced electron field emission (EFE) behavior of a core–shell heterostructure, where ZnO nanorods (ZNRs) form the core and ultrananocrystalline diamond needles (UNCDNs) form the shell, is reported. EFE properties of ZNR‐UNCDN core–shell heterostructures show a high emission current density of 5.5 mA cm^?2 at an applied field of 4.25 V μm^?1, and a low turn‐on field of 2.08 V μm^?1 compared to the 1.67 mA cm^?2 emission current density (at an applied field of 28.7 V μm^?1) and 16.6 V μm^?1 turn‐on field for bare ZNRs. Such an enhancement in the field emission originates from the unique materials combination, resulting in good electron transport from ZNRs to UNCDNs and efficient field emission of electrons from the UNCDNs. The potential application of these materials is demonstrated by the plasma illumination measurements that lowering the threshold voltage by 160 V confirms the role of ZNR‐UNCDN core–shell heterostructures in the enhancement of electron emission.     

Heat transfer enhancement by Marangoni convection in the NH3–H2O absorption process

The objectives of this paper are to quantify the effect of Marangini convection on the absorption performance for the ammonia–water absorption process, and to visualize Marangoni convection that is induced by adding a heat transfer additive, n-octanol. A real-time single-wavelength holographic interferometer is used for the visualization using a He–Ne gas laser. The interface temperature is always the highest due to the absorption heat release near the interface. It was found that the thermal boundary layer (TBL) increased faster than the diffusion boundary layer (DBL), and the DBL thickness increased by adding the heat transfer additive. At 5 s after absorption started, the DBL thickness for 5 mass% NH₃ without and with the heat transfer additive was 3.0 and 4.5 mm, respectively. Marangoni convection was observed near the interface only in the cases with heat transfer additive. The Marangoni convection was very strong just after the absorption started and it weakened as time elapsed. It was concluded that the absorption performance could be improved by increasing the absorption driving potential (x_vb−x_vi) and by increasing the heat transfer additive concentration. The absorption heat transfer was enhanced as high as 3.0–4.6 times by adding the heat transfer additive that generated Marangoni convection.     

The effect of shear convection on diffusion measurements in liquid metals using the foton shear cell

The shear convection in the Foton shear cell was investigated quantitatively and interpreted as an additional diffusion-like mixing. Diffusion experiments were done under 1g and µg (Foton-M2 mission June 2005). Since the measured mean squarediffusiondepth (MSDD) is a linear function of time, the offset represents the additional mass transport (and the averaging effect due to the analysis by Atom Absorption Spectroscopy, AAS). The offset is 1–3% of the MSDD and can be used to eliminate this systematic error. The influence of the liquid’s viscosity on the shear convection is discussed, using a simple fluid dynamical model. The absence of buoyancy convection in 1g-experiments was shown by comparison with the µg-result.     

Marangoni convection in a rectangular container

If the free liquid-gas interface of a liquid in a rectangular Container is subjected to a temperature gra-dient the shear stress on the free liquid surface being temperature dependent transmits by viscous traction a thermo-capillary convection into the bulk of the liquid. For constant temperature T ₁ at one wall and T ₂ at the other a steady Marangoni convection takes place while for time-oscillatory temperatures of the walls a time-dependent thermo-capillary convection appears, which will create wave patterns on the free liquid surface. They shall, depending on the forcing frequency of the temperature, exhibit resonance peaks. The velocity distribution, the response magnitude inside the Container, the forced free surface displacement and the influence of the Prandtl number have been investigated.

     

Morphological and kinetic aspects of thermal oxidation of SiC whisker reinforced Al203

Abstract

The thermal oxidation susceptibility of a 30 vol.-%SiC whisker reinforced Al₂0₃ composite in air at 1300–1500°C has been investigated. It was found that the morphological changes associated with scale evolution lead to the formation of mullite and a glassy phase. In particular, the scale cross-section consisted of a porous external scale (white), and an internal black layer in contact with the unreacted matrix. The black scale region was relatively thin (0–6 μm), and contained partially oxidized SiC whiskers as well as glassy phases. In some .instances, the inner black scale/matrix interface exhibited intergranular cracking. The presence of these cracks was attributed to internal stresses induced by volume increases associated with the oxidation reaction. Additionally, the rates of scale thickening and of weight gain exhibited parabolic behaviour. At 1500°C parabolic rate constants K_p and K_m of 1·18 × 10^?14 m² S^?1 and 2·23 × 10^?9 kg² m^?4 S^?1, respectively werefound. From the rate constant data, activation energies of 577 and 542 kJ mol^?1 were determined on the basis of scale thickness and weight gain, respectively. Apparently, the oxidation rates in the black subzone were somewhat high to be rate limiting. Hence, the exhibited activation energies were ascribed to the diffusion of oxidant across the white scale region.

MST/2013     

Development of an advanced A-TIG (AA-TIG) welding method by control of Marangoni convection

A new type of tungsten inert gas (TIG) welding has been developed, in which an ultra-deep penetration is obtained. In order to control the Marangoni convection induced by the surface tension gradient on the molten pool, He gas containing a small amount of oxidizing gas was used. The effect of the concentration of O₂ and CO₂ in the shielding gas on the weld shape was studied for the bead-on-plate TIG welding of SUS304 stainless under He–O₂ and He–CO₂ mixed shielding gases. Because oxygen is a surface active element for stainless steel, the addition of oxygen to the molten pool can control the Marangoni convection from the outward to inward direction on the liquid pool surface. When the oxygen content in the liquid pool is over a critical value, around 70 ppm, the weld shape suddenly changes from a wide shallow shape to a deep narrow shape due to the change in the direction of the Marangoni convection. Also, for He-based shielding gas, a high welding current will strengthen both the inward Marangoni convection on the pool surface and the inward electromagnetic convection in the liquid pool. Accordingly, at a welding speed of 0.75 mm/s, the welding current of 160 A and the electrode gap of 1 mm under the He–0.4%O₂ shielding, the depth/width ratio reaches 1.8, which is much larger for Ar–O₂ shielding gas (0.7). The effects of the welding parameters, such as welding speed and welding current were also systematically investigated. In addition, a double shielding gas method has been developed to prevent any consumption of the tungsten electrode.