Interaction of reactive gases with platinum aerosol particles at room temperature: effects on morphology and surface properties

Nanoparticles produced in technical aerosol processes exhibit often dendritic structures, composed of primary particles. Surprisingly, a small but consistent discrepancy was observed between the results of common aggregation models and in situ measurements of structural parameters, such as fractal dimension or mass-mobility exponent. A phenomenon which has received little attention so far is the interaction of agglomerates with admixed gases, which might be responsible for this discrepancy. In this work, we present an analytical series, which underlines the agglomerate morphology depending on the reducing or oxidizing nature of a carrier gas for platinum particles. When hydrogen is added to openly structured particles, as investigated by tandem differential mobility analysis (DMA) and transmission electron microscopy (TEM) analysis, Pt particles compact already at room temperature, resulting in an increased fractal dimension. Aerosol Photoemission Spectroscopy (APES) was also able to demonstrate the interaction of a gas with a nanoscaled platinum surface, resulting in a changed sintering behavior for reducing and oxidizing atmospheres in comparison to nitrogen. The main message of this work is about the structural change of particles exposed to a new environment after complete particle formation. We suspect significant implications for the interpretation of agglomerate formation, as many aerosol processes involve reactive gases or slightly contaminated gases in terms of trace amounts of unintended species

Influence of spray drying parameters on the formation of β-phase poly(vinylidene fluoride)

A simple one-step spraying method to produce poly(vinylidene fluoride) (PVDF) in the desired conformation is presented. The content of the piezoelectric β-phase is measured at different spray drying conditions and during electrospray. The influence of a strong electrical field and charges on the droplet are investigated separately from the electrospray setup with a pneumatic atomizer. For this purpose, the electric field is integrated into a pneumatic atomization process by a plate capacitor and the charge of the droplets by corona discharge. To investigate the drying properties, the drying temperature and the flow rate of dry air are examined. The presented process offers the possibility to deposit PVDF films or to produce PVDF powders, in their piezoelectric β- and γ-phases or in the nonpolar α-phase

Influence of the impaction angle on the triboelectric charging of aerosol nanoparticles

A low pressure impactor is used to measure triboelectric charging behavior of metallic nanoparticles. Ag nanoparticles, produced by spark discharge, were impacted onto Pt sputtered targets. The influence of the impaction angle and impaction velocity on the triboelectric charging was investigated. While for perpendicular impaction the charge transfer behavior of previous work was confirmed, the oblique impaction revealed new phenomena. Additional charge transfer was observable, which increases with obliqueness. The possibility of mass transfer between particle and target due to the high-energy collisions was also investigated. SEM characterization and Auger spectroscopy indicate mass transfer from the particle to the target surface

Forced triboelectrification of fine powders in particle wall collisions

Triboelectric separation as an inexpensive and environmentally friendly technique could contribute to material-specific sorting. However, the application as a widespread method is limited due to the complexity of the process. In particle wall collisions, various parameters like collision energy and angle, work function of the contact partners, humidity, surface roughness, etc. influence the particle charging in a hardly predictable way. This study investigates the possibilities of forced triboelectric particle charging by applying an electrical potential to the metal contact partner (copper/steel pipe). The variations included different pipe lengths (0.5 m–3 m), particle materials, and particle sizes for limestone. A distinction is made between the net charge of the particles and the positive, negative, and neutral mass fractions. The work functions of the investigated materials vary from about 3.2 eV to >8.5 eV for glass, limestone, artificial slag, and lithium aluminate particles. With the applied high-voltage potential, the particle net charge can be shifted linearly. For limestone, it is shown that the neutral fraction is highest at the Point of Zero Net Charge (PZNC). This observation may identify an approach for the material selective separation of one target component from a multi-material mixture

Limitation in the performance of fine powder separation in a turbo air classifier

The deflector wheel classifier is a widely used device for the separation of fine powders in different industrial applications. The primary objective of the separation process is to achieve high-quality separation of fine powders characterized by a narrow particle size distribution and high separation sharpness. Theoretically, the reduction in the cut size is accomplished by decreasing the gas flow rate or increasing the rotational speed of the classifier, which amplifies the centrifugal forces compared to the drag forces exerted on the particles. This behavior is, indeed, observed in many cases, but it cannot be extrapolated arbitrarily. At their performance limit, classifiers may, against expectation, show an increase in cut size and, in addition, a reduction in the sharpness of the separation process. The limitation in the reduction in the cut size and in the improvement in the separation sharpness arises due to an imbalance between the operating rotational speed and flow rate, which results in a non-uniform flow field in the classifier. If the balance conditions are fulfilled, an optimum separation with a high separation sharpness can be achieved. In this work, CFD simulations validated by some experimental results are employed to represent this limitation, which is obtained by varying the operating parameters using different material densities with particles ranging from one to ten microns

Investigation of particle formation of electrospray flame pyrolysis using a scanning mobility particle sizer

Die elektrohydrodynamische Zerstäubung von flüssigen Präkursoren gekoppelt mit der Flammensprühpyrolyse erlaubt die Synthese von verschiedenartigen, metalloxidischen Nanopartikel. Ein Aufbau bestehend aus einem definiert verfahrbaren Brenner und einer Probennahme in der Flamme eröffnet dabei die Möglichkeit der Erfassung von Partikelgrößenverteilungen entlang der Flammenhöhe. Die Bildung von Eisenoxid- und Siliziumdioxid-Partikel in einer Flamme konnte so mithilfe eines 1nm-SMPS-Aufbaus (Scanning Mobility Particle Sizer) auch in den Frühstadien der Partikelbildung erfasst werden. Hierbei zeigte sich, dass auch für einfache Präkursorlösungen bei hinreichend kleinen Tropfen hochwertige, homogene Nanopulver erzeugt werden können.Electrospray of liquid precursor coupled to flame spray pyrolysis allows for the synthesis of many different metal oxide nanoparticles. A setup consisting of a moveable burner and a static sampling system gives the possibility of characterizing particle size distributions across and along the flame. The particle formation of iron oxide and silica particles in a flame spray was investigated even for early stages using a 1nm scanning mobility particle sizer. Thereby it was found that even for simple precursor solutions high quality nanopowders could be produced as long as the droplet size is sufficiently small

Spray-dried Ni catalysts with tailored properties for CO2 methanation

A catalyst production method that enables the independent tailoring of the structural properties of the catalyst, such as pore size, metal particle size, metal loading or surface area, allows to increase the efficiency of a catalytic process. Such tailoring can help to make the valorization of CO2 into synthetic fuels on Ni catalysts competitive to conventional fossil fuel production. In this work, a new spray-drying method was used to produce Ni catalysts supported on SiO2 and Al2O3 nanoparticles with tunable properties. The influence of the primary particle size of the support, different metal loadings, and heat treatments were applied to investigate the potential to tailor the properties of catalysts. The catalysts were examined with physical and chemical characterization methods, including X-ray diffraction, temperature-programmed reduction, and chemisorption. A temperature-scanning technique was applied to screen the catalysts for CO2 methanation. With the spray-drying method presented here, well-organized porous spherical nanoparticles of highly dispersed NiO nanoparticles supported on silica with tunable properties were produced and characterized. Moreover, the pore size, metal particle size, and metal loading can be controlled independently, which allows to produce catalyst particles with the desired properties. Ni/SiO2 catalysts with surface areas of up to 40 m2 g−1 with Ni crystals in the range of 4 nm were produced, which exhibited a high activity for the CO2 methanation

Spray-dried sol-gel glass-ceramic powders based on the tunable thermal expansion of quartz and keatite solid solutions

Lithium aluminosilicate glass-ceramic powders were synthesized by the heat treatment of spray-dried sol-gel glassy nanobeads, obtaining quartz solid solution (Qss) and keatite solid solution (Kss) crystals. Their composition ranged between 75 mol% SiO2 and pure silica along the spodumene join. The metastable crystals displayed tunable coefficients of thermal expansion ranging from +30 × 10−6 to −2.7 × 10−6 K−1 at room temperature, as obtained from their crystallographic characterization. The solid solution boundaries of Kss could be extended to 85 mol% SiO2. Concurrently, X-ray diffraction measurements performed in situ at high temperature and at cryogenic conditions confirmed the known linear shift of the high-low quartz inversion temperature upon increasing Al+Li doping. The obtained results qualify aerosol synthesis as a very versatile method for the production of glass-ceramic powders in the LAS system

Gas Phase Reaction of Silane with Water at Different Temperatures and Supported by Plasma

The interaction of silane and water is discussed controversially in literature: some authors suggest monosilane and water react kinetically and sufficiently fast enough to remove water, while others state the reaction occurs only at elevated temperatures. This question is of technological interest for the removal of unavoidable water residues in Ar gases. Thermodynamic calculations show that virtually complete removal of water is expected with superstoichiometric silane addition. However, mass spectrometric and infrared spectroscopic experiments give evidence that the addition of monosilane to such an Ar gas at room temperature is unable to remove residual water, which disagrees with some current hypotheses in the literature. This holds even for very high SiH4 concentrations up to 2 vol.-%. Silane reacts with water above temperatures of 555 °C, initiated by the thermal decomposition of silane. A cold dielectric barrier discharge-plasma used for silane and water dissociation enhances reactivity similar to elevated temperatures. Fourier-transformed infrared spectroscopy points toward silanol generation at temperatures between 400 and 550 °C, while quadrupole mass spectrometry indicates the creation of SiOH+, SiHOH+, SiH2OH+, and SiH3OH+. Cold plasmas generate smaller amounts of SiOH+, SiHOH+, and SiH2OH+ compared to elevated temperatures. Reaction products are hydrogen and nanoscaled particles of non-stoichiometric silicon oxides. The silicon-oxide particles produced differ in elemental composition and shape depending on the prevailing water content during decomposition: SiOx generated with residual water appears with relatively smooth surfaces, while the addition of water supports the formation of significantly rougher particle surfaces. Higher initial water contents correlate with higher oxygen contents of the particles

Spray-dried TiO2(B)-containing photocatalytic glass-ceramic nanobeads

Glass-ceramic nanospheres of molar composition 0.83 SiO2 · 0.17 TiO2 are produced by the sol-gel spray-drying method followed by controlled heat treatments up to 1200 °C. TiO 2 (B) and anatase nanocrystals are precipitated in the glassy matrix: the latter phase gradually predominates with increasing ceramization temperature and time, in parallel to an overall increase in crystal sizes. The nanospheres exhibit evident photocatalytic activity under UV-A irradiation, especially at annealing stages involving a comparatively higher amount of TiO 2(B) and smaller crystals. The occurrence of TiO2(B) in this simplified binary glass-ceramic material underlines the key role of this phase in the dynamics of crystallizing TiO 2-bearing silicate melts