Electron work functions of ferrite and austenite phases in a duplex stainless steel and their adhesive forces with AFM silicon probe

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.

Electron work function: an indicative parameter towards a novel material design methodology

Electron work function (EWF) has demonstrated its great promise in materials analysis and design, particularly for single-phase materials, e.g., solute selection for optimal solid-solution strengthening. Such promise is attributed to the correlation of EWF with the atomic bonding and stability, which largely determines material properties. However, engineering materials generally consist of multiple phases. Whether or not the overall EWF of a complex multi-phase material can reflect its properties is unclear. Through investigation on the relationships among EWF, microstructure, mechanical and electrochemical properties of low-carbon steel samples with two-level microstructural inhomogeneity, we demonstrate that the overall EWF does carry the information on integrated electron behavior and overall properties of multiphase alloys. This study makes it achievable to develop "electronic metallurgy"-an electronic based novel alternative methodology for materials design.

Tuning the Conductivity and Electron Work Function of a Spin-Coated PEDOT:PSS/PEO Nanofilm for Enhanced Interfacial Adhesion

We report a novel phenomenon of increasing the adherence of a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS/PEO) nanofilm for Si₃N₄ through cosolvent treatment by DMSO. By varying the w/w% ratio of DMSO, nanofilms with different conductivities were produced. Atomic force microscopy (AFM) analysis showed that the adhesive force between the AFM's Si₃N₄ probe and the nanofilm increased by 35.8% as the conductivity of the nanofilm increased. The conductivity became saturated after the PEDOT:PSS-to-DMSO ratio reached a certain level. This study demonstrates that the variations in the adhesive force are determined by two factors: (1) the difference in EWF between the nanofilm and the counter-body Si₃N₄ and (2) the electrical conductivity of the materials involved. The former is used for establishing a dipole layer at the interface, while the latter determines the degree of ease to achieve the dipole layer. This study demonstrates an approach to tailor interfacial bonding for different types of materials without atomic diffusion, which is promising for applications in various fields such as control of biomedical films on implants and functional films for electronic devices.

Visualization of three different phases in a multiphase steel by scanning electron microscopy at 1 eV landing energy

In this study, we investigated an observation technique by super low energy scanning electron microscopy (SLESEM) at below 5 eV and its contrast mechanism for analyzing complex microstructures of a multiphase steel consisting of ferrite, martensite and austenite. With SLESEM at 1 eV, the three phases were observed as different brightness levels, ferrite as the darkest contrast, martensite as the second brightest and austenite as the brightest. These contrasts disappeared at 2 eV or higher. Similar contrasts and phenomena were also observed in the results of low energy electron microscopy (LEEM). According to the energy dependences of the LEEM intensities of the three phases, the threshold energies of the transition from electron reflection to surface impact were determined to be 0.00 eV, 0.15 eV and 0.39 eV for ferrite, martensite and austenite, respectively. These differences in thresholds indicate that the potentials on the surfaces of each phase are different, which is considered to result in the different brightness of each phase. This potential differences are probably due to the contact potentials generated when phases with different work functions contact each other. Although the sample is covered by a thin native oxide film (several nm thickness), the potentials can affect the incident electrons through the oxide film.

Prevention of Hydrogen Damage Using MoS₂ Coating on Iron Surface

The prevention of hydrogen penetration into steels can effectively protect steels from hydrogen damage. In this study, we investigated the effect of a monolayer MoS₂ coating on hydrogen prevention using first-principles calculations. We found that monolayer MoS₂ can effectively inhibit the dissociative adsorption of hydrogen molecules on an Fe(111) surface by forming a S–H bond. MoS₂ coating acts as an energy barrier, interrupting hydrogen penetration. Furthermore, compared with the H-adsorbed Fe(111) film, the work function of the MoS₂-coated film significantly increases under both equilibrium and strained conditions, indicating that the strained Fe(111) film with the MoS₂ coating also becomes more corrosion resistant. The results reveal that MoS₂ film is an effective coating to prevent hydrogen damage in steels.

Crystallographic anisotropy in surface properties of brass and its dependence on the electron work function

The crystallographic anisotropy of the electric current or conductance, adhesive force, elastic modulus, and deformation magnitude of alpha brass were investigated through property mapping using an atomic force microscope. Surface electron work functions of differently oriented grains in the brass were also analyzed using atomic force microscopy. The mapped surface properties are closely related to the electron work function; the work function reflects the surface activity, which is itself dependent on the surface energy. The anisotropy of the properties is closely correlated to the in situ measured surface electron work function. It is demonstrated that crystallographic planes with higher electron work functions exhibit lower current, smaller adhesive forces, larger elastic moduli and smaller deformation magnitudes. Efforts are made to understand the relationships by connecting the properties with surface energy and electron work function. The dependence of the properties on crystallographic orientation can be elucidated by considering the surface electron behavior using electron work function as a novel probing parameter.

Electron work function-a promising guiding parameter for material design

Using nickel added X70 steel as a sample material, we demonstrate that electron work function (EWF), which largely reflects the electron behavior of materials, could be used as a guide parameter for material modification or design. Adding Ni having a higher electron work function to X70 steel brings more "free" electrons to the steel, leading to increased overall work function, accompanied with enhanced e(-)-nuclei interactions or higher atomic bond strength. Young's modulus and hardness increase correspondingly. However, the free electron density and work function decrease as the Ni content is continuously increased, accompanied with the formation of a second phase, FeNi3, which is softer with a lower work function. The decrease in the overall work function corresponds to deterioration of the mechanical strength of the steel. It is expected that EWF, a simple but fundamental parameter, may lead to new methodologies or supplementary approaches for metallic materials design or tailoring on a feasible electronic base.