Laser based powder bed fusion is a promising manufacturing method that can be used for the fabrication of hard magnets such as NdFeB with nearly any given geometrical shape. However, the weak ...performance, e.g., low coercivity, of the 3D-printed magnets currently hinder their application. In this work, we demonstrated a proof-of-concept of powder bed additive manufacturing of heavy rare earth free NdFeB magnets with technologically attractive coercivity values. The 3D-printed NdFeB magnets exhibit the highest (up-to-date for the additively manufactured magnets without heavy rare earth metals) coercivity values reaching μ0Hc = 1.6 T. The magnets were synthesized using a mixture of the NdFeB-based and the low-melting eutectic alloy powders. The essential function of the eutectic alloy, along with binding of the NdFeB-based magnetic particles, is the significant improvement of their coercivity by the in-situ grain boundary (GB) infiltration. The fundamental understanding of the magnetization reversal processes in these 3D-printed magnets leads to the conclusion that the excellent performance of the additively manufactured hard magnets can be achieved through the delicate control of the intergrain exchange interaction between the grains of the Nd2Fe14B phase.
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The cooling rate in molten metal gas atomization is the key determining factor for the microstructure of metal powders. Mathematical expressions for cooling rates often include the melt droplet ...diameter and a pre-exponential factor describing the materials and gas properties. A new mathematical cooling rate correlation for rapidly solidified melt droplets is proposed based on heat flow considerations during gas atomization. The model approach takes process conditions such as gas-to-melt mass flow ratio and the initial gas temperature into account. The mathematical formulation was experimentally developed using secondary dendrite arm spacing method. For this purpose, a Cu-6wt pct Sn alloy was atomized with close-coupled (CCA) and free-fall atomization (FFA). A novel approach was made to predict the pre-exponential factor that allows the transferability to other materials. Our correlation for the cooling rate and the pre-exponential factor was validated by experimental data from the literature. The novel correlation type is valid for two different atomizing systems (FFA and CCA), suggesting that it may be applicable to entirely different gas atomization systems.
Modern powder metallurgical processes such as additive manufacturing or metal injection molding require metal powders with specific properties, which are commonly produced by gas atomization ...processes. Modeling of cooling and solidification of molten metal droplets requires the knowledge of the droplet and particle motion and heat transfer. While there are correlations for drag and heat transfer of spheres at isothermal conditions or temperature differences between droplet and gas smaller than 200 K, knowledge for temperature differences as high as 1000 K is limited. In this work, we first critically review common correlations for the drag coefficient and Nusselt number of spheres and develop a computational model to solve the non-isothermal flow for such conditions. After validation, new correlations are benchmarked against computational experiments. While we could confirm a good agreement for the Nusselt number correlations, we develop a novel temperature-dependent correction for the drag coefficient in the laminar regime. This correlation is based on data for 1 < Re < 130 for two different ideal gases.
The microexplosion and nanoparticle formation of single isolated burning droplets using titanium tetraisopropoxide (TTIP) as nanoparticle precursor are investigated experimentally. Spherical along ...with fine agglomerated titanium dioxide (TiO2) nanoparticles are obtained from the single droplet combustion of TTIP dissolved in pure xylene, pure ethanol and the mixture of xylene and ethanol. Strong, global, and continuous microexplosions are observed during the combustion of TTIP/xylene droplet and TTIP/xylene–ethanol droplet. We proposed a hypothesis of droplet microexplosions as follows: hydrolysis of TTIP at or in the droplet surface leads to formation of TiO2 nanoparticles, which together with the low volatility component near the droplet surface creates an impermeable shell for the high volatility component. The high volatility component in the interior is superheated to induce heterogeneous nucleation of bubbles beneath the shell, which increases the pressure inside the shell until microexplosion occurs. These insights are very important mechanisms for the well-established flame spray pyrolysis (FSP) process.
Nanomaterials hold great promise for medical, technological and economical benefits. Knowledge concerning the toxicological properties of these novel materials is typically lacking. At the same time, ...it is becoming evident that some nanomaterials could have a toxic potential in humans and the environment. Animal based systems lack the needed capacity to cope with the abundance of novel nanomaterials being produced, and thus we have to employ in vitro methods with high throughput to manage the rush logistically and use high content readouts wherever needed in order to gain more depth of information. Towards this end, high throughput screening (HTS) and high content screening (HCS) approaches can be used to speed up the safety analysis on a scale that commensurate with the rate of expansion of new materials and new properties. The insights gained from HTS/HCS should aid in our understanding of the tenets of nanomaterial hazard at biological level as well as assist the development of safe-by-design approaches. This review aims to provide a comprehensive introduction to the HTS/HCS methodology employed for safety assessment of engineered nanomaterials (ENMs), including data analysis and prediction of potentially hazardous material properties. Given the current pace of nanomaterial development, HTS/HCS is a potentially effective means of keeping up with the rapid progress in this field--we have literally no time to lose.
The flame spray pyrolysis (FSP) process was systematically investigated using an external-mixing gas-assisted atomizer supported by six premixed methane–oxygen flameletes. The effect of oxidant and ...precursor fuel composition on the size of FSP-made silica primary particles (8–
40
nm
) was studied using as precursor hexamethyldisiloxane (HMDSO) dissolved in ethanol, iso-octane or methanol. As oxidant air and pure oxygen were used, that served also as droplet dispersion gases. Droplet size distributions were measured by laser diffraction, while droplet lifetimes were calculated using a spray combustion model to explain for the first time the difference in flame structure and especially product powder characteristics when air or oxygen was used as oxidant/dispersion gas. The spray flame temperature was measured by Fourier transform infrared (FTIR) emission/transmission (E/T) spectroscopy. The liquid solvent (fuel), especially its enthalpy content, was an important parameter in FSP as it affected the total net heating value of the spray flame. It is shown for the first time also how the specific surface area of the FSP-made particles can be systematically controlled through the oxidant flow rate and precursor/fuel composition.
Flame spray pyrolysis (FSP) was used to make pure and Pt-doped tin dioxide nanoparticles in one-step. The aerosol generated by the dry FSP method was directly, in situ thermophoretically deposited ...onto interdigitated Pt-electrodes to form a porous, thick film of controlled thickness within the active sensor area. Tin oxide grain size (10nm) and a high film porosity (98%) were preserved for all film thicknesses from 9 to 40D*mm using different deposition times. The dependence of the film thickness on deposition time was theoretically estimated to enable precise control of the deposition process. Platinum doping did not affect the SnO2 grain size, crystallinity, or the porous film structure. These sensors exhibited high carbon monoxide (CO) sensor signals (8 for 50ppm CO in dry air at 350DDGC), good reproducibility, high analytical sensitivity and a remarkably low detection limit (1ppm CO in dry air at 350DDGC). The in situ platinum doping enhanced the overall sensor performance. Increasing the film thickness increased the sensor resistance and can be used to tune sensor performance.
•Co/Al2O3 Fischer–Tropsch (FT) catalysts were prepared by one-step single (SFSP) and double flame spray pyrolysis (DFSP).•SFSP catalysts were only FT active using uneconically high cobalt ...loadings.•DFSP catalysts were FT active after adjusting the intersection distance (x) of the two flame reactors.•With increasing x, CoOx and Al2O3 nanoparticles grow independently in each flame resulting in higher Co reducibility.•Good adhesion of both oxides after intersection stabilizes Co-particles against sintering in the FT reaction.
The controlled deposition of a catalytically active material on a support, resulting in defined particle sizes and shapes, as well as control over the specific surface area of the active material and the support are usually limited when applying conventional catalyst preparation techniques. Flame spray pyrolysis offers the potential to overcome this limitation and is tested for the preparation of Fischer–Tropsch catalysts. Conventional single flame spray pyrolysis and, for the first time, double flame spray pyrolysis are compared for the synthesis of alumina supported cobalt catalysts. In the latter process the metal oxide precursors are combusted individually in two opposing nozzles. The key parameter for defining the final material composition is the intersection distance of the flames, which was systematically varied. The Fischer–Tropsch performance of the Co-based catalysts was studied in a fixed bed reactor at 230°C and 20bar. The catalytic results are discussed on the basis of structural characterization of the different catalysts by XRD, BET, TPR, UV–vis and TEM/EF-TEM. While catalysts made by single flame spray pyrolysis were inactive in the Fischer–Tropsch reaction regardless of whether cobalt was subsequently mixed with alumina or the supported catalyst was directly prepared in the flame reactor, the double flame sprayed catalysts showed good catalytic activity. Depending on the intersection distance of the two flames, the formation of cobalt oxide and alumina occurred separately in each flame reactor. In spite of the independent particle growth in the two flames, the double flame reactor geometry lead to good adhesion of the two oxides resulting in good stabilization of cobalt nanoparticles on the alumina support during the Fischer–Tropsch reaction.
Processing soft ferromagnetic glass-forming alloys through gas atomization and consolidation is the most effective technique to produce bulk samples. The commercial viability of these materials ...depends on commercial purity feedstock. However, crystallization in commercial purity feedstock is several orders of magnitude faster than in high purity materials. The production of amorphous powders with commercial purity requires high cooling rates, which can only be achieved by extending the common process window in conventional gas atomization. The development of novel cooling strategies during molten metal gas atomization on two model alloys ({(Fe0.6Co0.4)0.75B0.2Si0.05}96Nb4 and Fe76B10Si9P5) is reported. Hydrogen inducement during liquid quenching significantly improved the glass-forming ability and soft magnetic properties of {(Fe0.6Co0.4)0.75B0.2Si0.05}96Nb4 powders. Spark plasma sintering experiments verified that amorphous rings could be produced regardless of the cooling strategies used. While the saturation magnetization was almost unaffected by consolidation, the coercivity increased slightly and permeability decreased significantly. The magnetic properties of the final bulk samples were independent of feedstock quality. The developed cooling strategies provide a great opportunity for the commercialization of soft ferromagnetic glass-forming alloys with commercial purity.
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