Surface Chemistry Of Powders Research Articles

Controlling surface chemistry in cold sintering to advance battery materials

Cold sintering is a recently introduced densification method that is of interest due to the lower energy used in the process, the ability to densify metastable materials, the ability to integrate materials to develop unique composites, and the ability to synthesis materials that can be more easily recycled. Collectively, these advantages make cold sintering a promising approach for processing of battery components, and co-sintering into all solid-state batteries. To obtain high electrochemical performance with cold sintering, detailed control of the surface chemistry of powders is needed, to avoid or minimize the impact of carbonate formation in grain boundaries, and to limit concentrations of secondary phases that are a result of incongruent dissolution processes. In this paper, we outline the importance of surface chemical reactions of powders in cold sintering, and the mitigating processes that can be adopted to control these reactions and obtain high performance and unique opportunities for Li and Na-oxide secondary batteries. We focus on a series of examples that demonstrate how the control of low temperature densification (cold sintering) can address the environmental sensitivity of many battery materials, and highlight the issues faced and open questions. These examples include cold sintering of air-sensitive solid electrolytes, re-processing of crushed solid electrolytes, surface passivation of air-sensitive active materials for processing in air, and fabrication of solid-solid macroscopic interfaces for solid-state batteries.

Influence of powder surface chemistry on the defect formation in AlSi10Mg alloy processed via laser powder bed fusion additive manufacturing

In laser powder bed fusion(LPBF) additive manufacturing(AM) of Al alloys, surface chemistry and dissolved gas composition(oxygen/oxides/hydroxides, hydrogen and moisture) of AM powders is a major influencing factor for porosity formation. Therefore, in the present investigation, this aspect is addressed using two sets of virgin AM AlSi10Mg powders, namely, Hi-Ox (2770 ppm oxygen) and Lo-Ox (660 ppm oxygen), to assess the printability under various processing parameters and defect formation in printed parts. Comprehensive and in-depth metallurgical analysis of both powders utilizing multiple characterisation techniques established the presence of various types of oxides/hydroxides with different complex chemistries on the powders’ surface. Enhanced spherical gas porosity and oxide inclusions were witnessed in Hi-Ox print coupons in contrast to the least defects in Lo-Ox coupons rationalized due to surface chemistry difference between the virgin powders. Consequently, the mechanical properties of the Hi-Ox print coupons, even with minimum defect density, are significantly inferior to Lo-Ox specimens. These observations suggest a competing influence of surface chemistry/dissolved gas composition and laser parameters for the processing of both powders. Overall, by correlating the microstructures with processing conditions and oxide concentration, defects morphology, the probable melt pool formation mechanism in LPBF of AlSi10Mg alloy (for high and low oxygen concentration) is elucidated.

Insights into surface chemistry induced powder layer characteristic evolutions in additive manufacturing

Compared with large amount of work on the powder layer characteristics in powder-bed-based additive manufacturing (AM) under different operating conditions and powder properties, the significant effects of powder surface chemistry on the quality of the powder layer were less investigated. To further enhance people's insights in this aspect so as to obtain superior powder layer for subsequent process, Ti-6Al-4 V powders with different surface chemistries (e.g., the as received powder without any treatment, the moistened powder with surface H2O molecule, the exsiccated powder with grown nano-scaled oxides film) were firstly prepared, where the surface chemistry state of each powder was characterized and corresponding flowability and spreadability were measured in physical experiments. On this basis, systematic numerical investigations on the evolution of different surface chemistry induced powder layer characteristics during spreading were conducted. The results demonstrate that the surface moistened powder would deteriorate powder layer quality due to the strong cohesion effect between particles. However, the increase of the surface roughness on exsiccated powder could facilitate a greater deposition rate compared to that of the as-received powder during spreading, and therefore result in a powder layer with more pronounced apparent density. The mechanism lies in that high friction from the rough surface film could restrict the scattered motion behavior of sticky particles caused by the breakdown of cluster structure during the blade movement, and thus facilitate the most prominent deposition rate of this kind of particles during spreading. Nevertheless, the simultaneous increase of interactions within exsiccated powder also diminishes the boundary effect with blade and substrate, resulting in abnormal elevation of powder layer thickness, enlargement of internal pore volume, and depressed packing fraction correspondingly. Consequently, the powder with grown surface film is not ideal for printing, and the chemical reactions on powder that causes intensive film roughness should be prevented in powder storage and treatment process.

Degradation of AlSi10Mg powder during laser based powder bed fusion processing

Knowledge concerning powder degradation during additive manufacturing (AM) processing is essential to improve the reusability of the powder in AM and hence maximize feedstock powder reuse and economy of the process. AlSi10Mg powder degradation in Concept Laser XLINE 2000R machine over the total period of 30 months was analyzed in order to understand the extent and mechanism affecting powder aging. Thereby, detailed analysis of the powder morphology, microstructure and surface chemistry was performed by SEM, TEM and XPS. The results show an increase in volume fraction of heavily oxidized spatter particles up to 3% in 30 months. XPS analysis of the powder surface chemistry indicates that powder particles are covered by uniform oxide layer, formed by Mg- and Al-based oxides, average thickness of which increased from ~4 nm in case of the virgin powder up to about 38 nm in case of the reused for about 30 month powder, established by XPS. Analysis of the oxide characteristics were consistent with the observed oxygen content in the sampled powder. Columnar oxide scale formation on spatter particles was revealed as well, reaching up to 125 nm in thickness measured using STEM. Results of the XPS and STEM-EDX analysis of oxide composition are shown to be in agreement with the thermodynamic calculations confirming that oxide scale on sputter particles is formed by MgAl2O4 spinel and Al2O3 (corundum) oxides.

Bonding Mechanisms in Cold Spray: Influence of Surface Oxidation During Powder Storage

Cold spray is a solid-state process in which solid particles are subjected to severe plastic deformation to form a coating. The effect of naturally occurring oxides on bonding in the cold spray was investigated in this work. Deposition characteristics of copper powder with different surface oxide thicknesses on steel substrate were examined using a local pull-off test. This enables the investigation of individual particle/substrate interfaces. X-ray photoelectron spectroscopy was used to study thoroughly the powder surface chemistry and the oxide thickness as a function of exposure time. This study revealed that the oxide film thickness grew from 6 nm for the as-received powder to 11.6 nm after 2 months of exposure to atmospheric conditions. Scanning electron microscopy and x-ray energy dispersive spectroscopy were used to characterize the particle/substrate interfacial bonding. Adhesion test results show that the bond strength is highly influenced by the surface oxide thickness. Different bonding behaviors and a drop in coating deposition efficiency were observed as the oxide thickness increased. Finite element simulations supplemented our understanding of the particle adhesion dynamics as the oxide film thickness increases. They were also used to study the surface oxide cleanup and bonding mechanism for particles with different oxide film thicknesses.

Surface chemical analysis of copper powder used in additive manufacturing

Additive manufacturing (AM) has during years gained significant interest owing to its endless component design possibilities. One of the most popular AM techniques is laser powder bed fusion (LPBF), which selectively melts metal powder layer‐by‐layer in a chamber with protective argon atmosphere. This technique is attractive for realizing Cu‐based products in which the high electrical conductivity of Cu is combined with component design possibilities. The successful use of Cu powder not only poses challenges owing to the high reflectivity and thermal conductivity of Cu but also involves the important concern of controlling the powder surface chemistry since the powder surface constitutes the main source of oxygen. It is of crucial importance to control the oxygen level in order to maintain good electrical conductivity and brazing ability of the AM‐fabricated Cu‐part. In LPBF, fine spherical powder with size of 10–60 μm is used, providing significant specific surface area, and this powder is also usually recycled several times, and hence, the role of powder surface chemistry is evident. Two kinds of copper powder with purities 99.70 and 99.95 wt% were analysed in both virgin and in used conditions after numerous printing cycles using LPBF. The powder was analysed by X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A clear difference between the two powder grades in terms of surface chemistry was observed. The oxide thickness and bulk oxygen content increased for both powder grades after recycling. The surface oxides under different conditions are identified and the effect of powder purity on the oxide formed is discussed.

Evolution of surface chemistry during sintering of water‐atomized iron and low‐alloyed steel powder

Water‐atomized iron and steel powder is commonly used as the base material for powder metallurgy (PM) of ferrous components. The powder surface chemistry is characterized by a thin surface oxide layer and more thermodynamically stable oxide particulates whose extent, distribution, and composition change during the sintering cycle due to a complex set of oxidation–reduction reactions. In this study, the surface chemistry of iron and steel powder was investigated by combined surface and thermal analysis. The progressive reduction of oxides was studied using model sintering cycles in hydrogen atmospheres in a thermogravimetric (TG) setup, with experiments ended at intermediate steps (500–1300°C) of the heating stage. The surface chemistry of the samples was then investigated by means of X‐ray photoelectron spectroscopy (XPS) to reveal changes that occurred during heating. The results show that reduction of the surface oxide layer occurs at relatively lower temperature for the steel powder, attributed to an influence of chromium, which is supported by a strong increase in Cr content immediately after oxide layer reduction. The reduction of the stable oxide particulates was shifted to higher temperatures, reflecting their higher thermodynamic stability. A complementary vacuum annealing treatment at 800°C was performed in a furnace directly connected to the XPS instrument allowing for sample transfer in vacuum. The results showed that Fe oxides were completely reduced, with segregation and growth of Cr and Mn oxides on the particle surfaces. This underlines the sequential reduction of oxides during sintering that reflects the thermodynamic stability and availability of oxide‐forming elements.

Immunological response triggered by metallic 3D printing powders

Powder bed fusion-based Additive Manufacturing (3D printing) methods, particularly Selective Laser Melting, are increasingly being employed for customized medical implants. However, partially-melted residual metal powder is difficult to fully remove from the as-printed materials, and the existence of loose powder is another potential defect in the as-printed implant lattice. Understanding what effect loose powder and partially remelted powders have on biocompatibility requires a detailed study of the immunological response associated with the presence of metal powder particles, and the present study is designed to address this critically important concern. Typical powders for implant materials, including 316 L stainless steel, Ti, and Ti-6Al-4 V, with a range of particle sizes (15–100 μm) have been investigated in this study. Macrophage cells were selected to determine the immune response to metallic powder exposure and the cell responses were evaluated using confocal laser scanning microscopy and enzyme-linked immunosorbent assay analysis. Real-time monitoring of the cell reaction to the metal powders, cell viability, and secretion behavior of the inflammatory cytokines was employed for the different metal powders with various particle sizes. The main findings from the study are: (a) low concentrations (e.g. 1 × 104 particles/mL) of metal powders were not found to affect the cell viability, (b) in high particle concentrations (e.g. 1 × 106 particles/mL), the particle size was observed to be a significant factor in determining the cell reaction, and (c) the 316 L powder triggerd macrophages to secret the most pro-inflammatory cytokines and this tends to induce higher inflammatory reaction than the Ti and Ti-6Al-4 V powders. The underlying reasons are discussed from the perspectives of powder surface chemistry and powder morphology, which were investigated by detailed X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy studies. A comparison between the metal powder for 3D printing and conventional wear debris has also been briefly addressed. The results answer some critical questions relating to additively manufactured medical materials and lay a good foundation for better and more systematic assessment of the biological response of metal implants containing residual powders for potential orthopedic applications.

The effect of milling additives on powder properties and sintered body microstructure of NiO

The evolution of powder particle size, crystal structure, and surface chemistry was evaluated for micron scale NiO powders subjected to impact milling with commonly employed milling additives: methanol, Vertrel XF, and amorphous carbon. The effect of the different comminution protocols on sintered body microstructure was evaluated for high temperature sintering in inert atmosphere (N2). X-ray photoelectron spectroscopy showed that NiO powder surface chemistry is surprisingly sensitive to milling additive choice. In particular, the proportion of powder surface defect sites varied with additive, and methanol left an alcohol or alkoxy residue even after drying. Upon sintering to intermediate temperatures (1100 °C), scanning electron microscopy (SEM) showed that slurry milled NiO powders exhibit hindered sintering behaviors. This effect was amplified for NiO milled with methanol, in which sub-500 nm grain sizes dominated even after sintering to 1100 °C. Upon heating to high temperatures (1500 °C), simultaneous differential scanning calorimetry/thermogravimetric analysis (DSC/TGA) showed that the powders containing carbon residues undergo carbothermal reduction, resulting in a melting transition between 1425 and 1454 °C. Taken together, the results demonstrated that when processing metal oxide powders for advanced ceramics, the choice of milling additive is crucial as it exerts significant control over sintered body microstructure.

Role of Surface Chemistry and Energetics in High Shear Wet Granulation

This article presents a detailed study of the role of solid state surface chemistry on the high shear granulation performance of organic pharmaceutical solids. The sole effect of powder surface chemistry on granulation performance was investigated by coagglomerating untreated and silanized d-mannitol powder from a narrow sieve fraction in a high shear process. The wetting performance by polyvinylpyrrolidone (PVP) binder solutions was quantified using experimentally determined thermodynamic spreading coefficients, calculated from surface energy heterogeneity distributions obtained via inverse gas chromatography (IGC) measurements. Increases in binder wettability of the formulations correlated with increases in both granule size and compressive elastic moduli, which are attributed to an increase in interparticle contacts. In conclusion, starting materials surface chemistry plays a crucial role in final granule properties, even in highly agitated processes such as high shear granulation. The comprehensive ch...

The surface chemistry of silicon nitride powder in the presence of dissolved ions

Colloidal processing of silicon nitride (Si3N4) powders depends largely on the control of reactions at the solid-solution interface. The role of dissolved ions in the surface chemistry of Si3N4 powders has been investigated, and the implications of these results for the effects of impurities, contaminants and additives in processing are discussed. The interaction of ions at the solid-solution interface was characterized by particle electrokinetic behaviour determined from electroacoustic measurements in moderately concentrated suspensions. Ions were classified according to chemical similarity and surface specificity. Specific adsorption was inferred from the movement of the isoelectric point relative to the endemic “native” value. Most simple univalent electrolytes behaved indifferently towards the Si3N4 surface, with the exception of fluoride which specifically adsorbed and may have formed a strong complex with surface silicon sites. The alkaline-earth cations exhibited a similar weak specificity. In the presence of hydrolysable transition metal cations, powder surface chemistry was controlled by the adsorption of hydroxy metal complexes and by the solubility of a surface-precipitated metal hydroxide phase. Oxo anions, such as sulphate and carbonate, adsorbed specifically on the Si3N4 surface, but the interactions were weaker than previously observed on metal oxide surfaces.

Surface chemistry of fine silica powders

A range of the most important problems of fine silica powder surface chemistry are considered in terms of a unified approach based on the analysis of the electronic structure of the surface moieties. The amorphous silica hydroxylic cover topography is analyzed, and the peculiarities of the cover thermal destruction are studied. Different forms of adsorbed water have been identified, and characteristic properties of the hydrated film structure have been ascertained. The dehydration and dehydroxylation process mechanisms have been suggested. General regularities of different compound adsorption have been established, which are caused by the sterical compatibility of adsorbates and silica surface structural elements. Various aspects of the reagent structure deformation during elementary chemical transformations in the surface layer have been considered. Simple methods of estimation for chemical reaction activation barriers have been developed, which are of great importance for the silica surface modification processes.

ChemInform Abstract: Effect of Powder Surface Chemistry on the Stability of Concentrated Aqueous Dispersions of Alumina.

AbstractIt is shown that the initial dispersion pH and the magnitude and sign of the initial mobility (zeta potential) of the dispersion are strongly influenced by the surface chemistry of ceramic powders.

Effect of Powder Surface Chemistry on the Stability of Concentrated Aqueous Dispersions of Alumina

The surface chemistry differences between two alumina powders is shown to have a significant effect on the properties of concentrated aqueous dispersions of the powders. The initial dispersion pH and magnitude and sign of the initial mobility of the particles (determined by acoustophoresis) are different for the two powders because of differences in surface impurity contents. As a result, the two powders require different amounts of dispersant to achieve stability in dispersions near their isoelectric points. The surface chemistry differences between these two aluminas illustrate the importance of examining ceramic dispersion properties at the concentrations used in processing the material.

Characterization of INCONEL alloy MA 6000 powder

Mechanically alloyed powders of INCONEL* alloy MA 6000 have been characterized in order to understand the mechanical alloying process. Various analytical techniques including transmission electron microscopy, X-ray diffraction, electron microprobe analysis, and Auger electron spectroscopy were used to reveal the microstructural changes, chemical homogeneity, degree of mechanical working, and surface chemistry of powders during the powder processing. The current investigation has brought about a better understanding of the basic mechanism of the mechanical alloying process as well as a guideline for quality control of mechanically alloyed powder.