Powder stream characteristics in cold spray nozzles

The exponential increase of industrial demand in the past two decades has led scientists to the development of alternative technologies for the fast manufacturing of engineering components, aside from standard and time consuming techniques such as casting or forging.Cold Spray (CS) is a newly developed manufacturing technique, based upon the deposition of metal powder on a substrate due to high energy particle impacts. In this process, the powder is accelerated up to considerable speed in a converging-diverging nozzle, typically using air, nitrogen or helium as a carrier gas. Recent developments have demonstrated significant process capabilities, from the building of mold-free 3D shapes made of various metals, to low porosity and corrosion resistant titanium coatings.In CS, the particle stream characteristics during the acceleration process are important in relation to the final geometry of the coating. Experimental studies have shown the tendency of particles to spread over the nozzle acceleration channel, resulting in a wide exit stream and in the difficulty of producing narrow tracks.This paper presents an investigation on the powder stream characteristics in CS supersonic nozzles. The powder insertion location was varied within the carrier gas flow, along with the geometry of the powder injector, in order to identify their relation with particle trajectories. Computational Fluid Dynamics (CFD) results by Fluent v6.3.26 are presented, along with experimental observations. Different configurations were tested and modeled, giving deposited track geometries of copper and tin ranging from 1. mm to 8. mm in width on metal and polymer substrates. © 2011 Elsevier B.V.

Structure of igniting ethanol and n-heptane spray flames with and without swirl

This paper explores the ignition and subsequent evolution of spray flames in a bluff-body configuration with and without swirl. Ethanol and n-heptane are used to compare the effects of volatility. Ignition is performed by a laser spark. High speed imaging of OH *-chemiluminescence and OH-PLIF collected at 5kHz are used to investigate the behaviour of the flames during the first stages of ignition and the stable flame structure following ignition. Swirl induces a wider and shorter flame, precession, and multiple reaction zones, while the non-swirling flames have a simpler structure. The reaction fronts seem thinner with ethanol than with heptane. The dataset can be used for model validation. © 2012 Elsevier Inc.

Effect of laser heating temperature on coating characteristics of Stellite 6 deposited by cold spray

Laser-assisted cold spray (LCS) is a new coating and fabrication process which combines some advantages of CS: solid-state deposition, retain their initial composition and high build rate with the ability to deposit materials which are either difficult or impossible to deposit using cold spray alone. Stellite 6 powder is deposited on medium carbon steels by LCS using N 2 as carrier gas pressure. The topography, cross section thickness, structure of the coatings is examined by SEM, optical microscopy, EDX. The results show that thickness and fluctuation of coating are improved with increased deposition site temperature. Porosity of coating is affected by N 2 and deposition site temperature. In this paper, it presents optimal coating using N 2 at a pressure of 3 MPa and temperature of 450°C and deposition site temperature of 1100°C.

Electrical transport and magnetic properties of Mn3O4-La0.7Ca0.3MnO3 ceramic composites prepared by a one-step spray-drying technique

La0.7Ca0.3MnO3/Mn3O4 composites can be synthesized in one step by thermal treatment of a spray-dried precursor, instead of mixing pre-synthesized powders. Another advantage of this composite system is that a long sintering step can be used without leading to significant modification of the manganite composition. The percolation threshold is reached at ∼20 vol% of manganite phase. The 77 K low field magnetoresistance is enhanced to ∼11% at 0.15 T when the composition is close to the percolation threshold. © 2007 Elsevier Ltd. All rights reserved.

Low-field magnetoresistance in La0.7Ca0.3MnO 3 manganite compounds prepared by the spray drying technique

Calcium-substituted lanthanum manganite compounds were synthesized by the spray drying technique. This method - whose main advantages are versatility, high reproducibility and scalability - yields small grain materials of high homogeneity and displaying low-field magnetoresistance effects. We report about the physical and chemical characterizations of these samples in order to investigate the potential interest of spray drying for the production of materials for low-field magnetoresistance applications. We have studied the dependence of the low-field magnetoresistance on the temperature and duration of the thermal treatment applied to the pelletized powders. The issue of the shape anisotropy (demagnetisation effects) influence on the magnetoresistance properties has also been dealt with. © 2005 Springer Science + Business Media, Inc.

Investigation of the Young's modulus and thermal expansion of amorphous titania-doped tantala films

The current generation of advanced gravitational wave detectors utilize titania-doped tantala/silica multilayer stacks for their mirror coatings. The properties of the low-refractive-index silica are well known; however, in the absence of detailed direct measurements, the material parameters of Young's modulus and coefficient of thermal expansion (CTE) of the high refractive index material, titania-doped tantala, have been assumed to be equal to values measured for pure tantala coatings. In order to ascertain the true values necessary for thermal noise calculations, we have undertaken measurements of Young's modulus and CTE through the use of nanoindentation and thermal-bending measurements. The measurements were designed to assess the effects of titania doping concentration and post-deposition heat-treatment on the measured values in order to evaluate the possibility of optimizing material parameters to further improve thermal noise in the detector. Young's modulus measurements on pure tantala and 25% and 55% titania-doped tantala show a wide range of values, from 132 to 177 GPa, dependent on both titania concentration and heat-treatment. Measurements of CTE give values of (3.9 +/- 0.1) x 10^-6 K^-1 and (4.9 +/- 0.3) x 10^-6 K^-1 for 25% and 55% titania-doped tantala, respectively, without dependence on post-deposition heat-treatment.

Thermal analysis of microstructural evolution of Al2O3 surface modified layers

Solidification behavior and microstructural evolution of surface modified layers in plasma cladding technique are studied via numerical simulations. Both the coupling effect of temperature and solid volume fraction are considered in the proposed thermal analytical model, by which the transient temperature distributions are calculated and the shape of melting pool is determined. Furthermore, we perform microscopic thermal analysis on the nucleation and growth behaviors of ceramic hardening phases and dendrites, as well as the kinetics of related two-phase flow systems. By comparing with experimental observations, the evolution mechanisms of the morphology of Al2O3 ceramic hardening layer are explained. Based on the above results, a relationship among the scanning velocity of plasma stream, dendritic growth rate and the advancing speed of solid/liquid interface is found, and an energy criterion is proposed for predicting the pushing/engulfing transition of ceramic particles by grain growth fronts. (C) 2009 Elsevier B.V. All rights reserved.

Syntheses of opened hollow clay microspheres through a spray-drying approach and their derivative clay/carbon nanotubes composites

Opened hollow microspheres of organoclays were prepared via spray drying the suspension of modified Na+-montmorillonite (Na+-MMT) with alkylsulfonate. The microstructure and thermal properties of these opened hollow spheres were characterized by means of wide-angle X-ray diffraction, field emission scanning electron microscopy, and thermogravimetric analysis. The results showed that the organoclays had larger interlayer spacing compared with pure Na+-MMT and higher thermal stability relative to the alkylsufonate.

Thermal-shock resistance of LnMgAl(11)O(19) (Ln = La, Nd, Sm, Gd) with magnetoplumbite structure

Lanthanide hexaaluminates including LaMgAl11O19, NdMgAl11O19, SmMgAl11O19 and GdMgAl11O19 were synthesized via Sol-Gel method. Due to the anisotropic crystal growth, these oxides crystallize in the form of platelets and the platelet thickness increases with the decrease of rare-earth ionic radius. It was observed that the thermal-shock resistances of LaMgAl11O19, NdMgAl11O19 and SmMgAl11O19 oxides were superior to 8YSZ as proved by water quenching tests. In addition, the thinner the platelet. the more interstices are retained in the sintered specimen, and the better thermal-shock resistance the oxide has. Based on SEM images, it can be seen that the SmMgAl11O19 sample exhibits a mixture of the intergranular and transgranular fracture after thermal cycling failure.

Perfectly Hydrophobic Silicone Nanofiber Coatings: Preparation from Methyltrialkoxysilanes and Use as Water-Collecting Substrate

Perfectly hydrophobic (PHO) coatings consisting of silicone nanofibers have been obtained via a solution process using methyltrialkoxysilanes as precursors. On the basis of thermal gravimetry and differential thermal analysis (TG-DTA) and Fourier transform infrared spectroscopy (FTIR) results, the formula of the nanofibers was tentatively given and a possible growth mechanism of the nanofibers was proposed. Because of the low affinity between the coatings and the small water droplet, when using these coatings as substrate for collecting water vapor, the harvesting efficiency could be enhanced as compared with those from bare glass substrate for more than 50% under 25 degrees C and 60-90% relative humidity. By removing the surface methyl group by heat treatment or ultraviolet (UV) irradiation, the as-prepared perfectly hydrophobic surface can be converted into a superhydrophilic surface.

On improving the phase stability and thermal expansion coefficients of lanthanum cerium oxide solid solutions

The phase stability of lanthanum cerium oxide (La2Ce2O7), which is stable up to 1400 degrees C, and the thermal expansion coefficient of La2Ce2O7 doped with Ta2O5 or WO3 were studied. The thermal expansion coefficient of La2Ce2O7 below 400 degrees C was increased by adding more CeO2 or doping with either Ta2O5 or WO3.

Synthesis and characterization of spherical Sr2CeO4 phosphors by spray pyrolysis for field emission displays

A blue emitting Sr2CeO4 phosphor with a one-dimensional structure has been prepared by a two-step spray pyrolysis (SP) method, starting from the aqueous solutions of metal nitrates with citric acid and polyethylene glycol (PEG) as additives. The material is ultimately designed for field emission displays (FEDs). X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), field emission scanning electron microscope pictures (FE-SEM) as well as photoluminescence (PL) and cathodoluminescence (CL) spectroscopy and lifetime measurements have been employed to characterize the samples. The morphology, PL and low voltage CL properties of Sr2CeO4 phosphors as-prepared using the SP method have been investigated by changing the concentration of the precursor solution, concentration of PEG, annealing temperature, acceleration voltage and filament current. The obtained Sr2CeO4 phosphor particles are spherical and of submicron size, 0.5-2 mu m. The emission spectrum of the phosphors shows a broad band with maximum at 467 nm (lifetime = 37.4 mu s; CIE chromaticity coordinates: x = 0.15 and y = 0.21), presumably due to a ligand-to-metal charge-transfer transition.

The thermal cycling behavior of Lanthanum-Cerium Oxide thermal barrier coating prepared by EB-PVD

Bulk material and coatings of Lanthanum-Cerium Oxide (La2Ce2O7) with a fluorite structure were studied as a candidate material for thermal barrier coating (TBC). It has been showed that such material has the properties of low thermal conductivity about four times lower than YSZ, the difference in the thermal expansion coefficient between La2Ce2O7 and bond coat is smaller than that of YSZ in TBC systems, high phase stability between room temperature and 1673 K, about 300 K higher than that of the YSZ. The coating prepared by electron beam physical vapor deposition (EB-PVD) showed that it has good thermal cycling behavior, implying that Such material can be a promising thermal barrier coating material. The deviation of coating composition from ingot can be overcome by the addition of excess La2O3 during ingot preparation and/or by adjusting the process parameters.

Effect of Rapid Thermal Annealing on the Structure and Magnetic Properties of Chemical Vapour Deposition Cobalt Layers

Future read heads in hard disc storage require high conformal coatings of metal magnetic layers over high aspect ratio profiles. This paper describes pioneering work on the use of MOCVD for the deposition of cobalt layers. While pure cobalt layers could be deposited at 400C their magnetic properties are poor. It was found that the magnetic properties of the layers could be significantly enhanced with an optimised rapid thermal anneal. This work was sponsored by Seagate Technology and led to a follow up PhD studentship on the co-deposition of cobalt and iron by MOCVD.