Metal Microparticles Research Articles

Printable Liquid Metal Microparticle Ink for Ultrastretchable Electronics.

Liquid metal confined in the elastomer represents an ideal platform for stretchable electronics with ultimate deformability. To enable facile and scalable patterning of conductive features, bulk liquid metal is typically dispersed into fine particles to formulate printable inks. The presence of native oxide or organic ligands stabilizing these liquid metal particles unfortunately inhibits their direct coalescence to recover the metallic conductivity and liquid-state deformability. Here, we report a chemical sintering process that converts printed liquid metal microparticles into a highly deformable conductor. The process involves the removal of surface passivating oxide by a short exposure to acid fume and subsequent selective wetting of liquid metal microparticles onto copper nanoplates present in the ink formulation. The chemical reaction provides the basis for a facile and scalable procedure to print conductive features over a large area with exceptional conductivity (>104 S cm-1) and ultrahigh stretchability (∼1000% strain). Their practical suitability is demonstrated by the fabrication of an ultrastretchable ribbon cable and an epidermal heater.

Site-specific study of jetting, bonding, and local deformation during high-velocity metallic microparticle impact

In the cold-spray process, jetting is often considered a precursor to particle-substrate adhesion, but the conditions under which these phenomena occur remain unclear. This paper presents a systematic site-specific study of single-particle impact across a wide range of impact velocities for Cu particles impacting a Cu substrate. A high degree of velocity control enables identification of new behaviors that emerge in distinct velocity ranges. At the lowest velocities, we observe a fully plastic impact regime with rebound of the incident particle. As velocity increases, substrate jetting emerges even while the particle still rebounds. At the critical adhesion velocity, vcr, jetting only occurs on the substrate, leading to poor metallurgical bonding. Bonding improves as velocity increases and the particles also begin to jet, but there is an apparent maximum in bonding extent at ~1.3 vcr. This is followed by a drop in metallurgical bonding due to the ‘backward’ jetting of the particle that is associated with peeling forces on the interface. At ~1.6 vcr and above, we observe hydrodynamic penetration of the particle and possible erosion, as indicated by petalling of the substrate and burial of the particle below the substrate impact plane. Taken together, this catalog of phenomena, correlated to specific velocities, points to an impact-structure-based approach to establishing processing windows for cold spray.

Hierarchically-porous metallic scaffolds via 3D extrusion and reduction of oxide particle inks with salt space-holders

3D ink-extrusion of powders followed by sintering is an emerging additive manufacturing method capable of creating metallic microlattices. Here, we study the creation of hierarchically porous Fe or Ni scaffolds by 3D extrusion of 0/90° lattices from inks consisting of fine oxide powders (Fe 2 O 3 or NiO, < 3 µm), coarse space-holder particles (CuSO 4, < 45 µm) and a polymer binder within a solvent. After space-holder leaching and debinding of the lattices, a sintering step densifies the metallic Fe or Ni powders created by oxide reduction with H 2 , while maintaining the larger pores templated by the space-holder particles within the printed struts. The resulting Fe or Ni lattices exhibit a hierarchical porosity: (i) open submillimeter channels within the lattice between the printed struts; (ii) open micropores (5–50 µm) within the struts, from leaching of space-holder particles; (iii) closed submicron pores within the micropore walls, from incomplete sintering of the metal microparticles created by oxide reduction. While the channel porosity is defined by the printed geometry, strut porosity is controlled by the sintering temperature, with the finest porosity removed first. Sintering at 873–1373 K for 1 h leads to strut porosities of 38–19% in Fe and 59–29% in Ni. In situ synchrotron X-ray tomography reveals the sintering kinetics and porosity evolution upon debinding, oxide reduction by H 2 and sintering of the porous metal, which, combined, lead to ~50% shrinkage of the scaffolds as compared to the printed state. The space-holder templated pores are not eliminated by sintering and remain in the struts, but they shrink upon reduction and sintering of the surrounding metal matrix (e.g., by 40% in Fe and by 30% in Ni upon sintering at 1173 K for 1 h).

In situ observations of jetting in the divergent rebound regime for high-velocity metallic microparticle impact

Metallic bonding of certain materials can be achieved via high-velocity impact, and this type of bonding is often linked to an intense extrusion of material from the impact site, known as jetting. We present in situ observations of hydrodynamic jetting and ejection of matter from high-velocity metallic microparticle impacts that still result in rebound of the particle. While the occurrence of jetting has been linked to permanent particle deposition in prior studies, the present findings reveal that any bonds that may form upon jetting can be rebroken by particle rebound in some circumstances. We further present asymmetric occurrences of jetting associated with localized delays in particle detachment followed by rotation during particle rebound, which may speak to transient bond formation at the jetting sites.

Metal particles filled gratings with high diffraction efficiency for terahertz wave by traditional mask production

Terahertz (THz) grating is a widely used as a modulation device. In this paper, we propose a low cost and fast fabrication method of THz diffraction grating, which is utilized by a conventional mask as mold composed with metallic micro- and nano- particle fillers. Three kinds of metal microparticle gratings based on Cu, Sn and Ni were prepared by this method. Experimental and computational results demonstrated that the first-order diffraction efficiency for the metal microparticle gratings could be significantly improved than that for the conventional rectangular metal gratings. The physical mechanism is attributed to the enhanced transmittance and the concentration feature within the metallic microparticle strips of the gratings.

Efficient Co@Co3O4 core-shell catalysts for photocatalytic water oxidation and its behaviors in two different photocatalytic systems

Co@Co 3 O 4 photocatalyst was prepared in situ to investigate its water splitting performance and mechanism in two photocatalytic systems. • Photocatalytic activity of catalyst 1 in two systems were compared for the first time. • Catalyst 1 exhibited excellent photocatalytic performances. • Core-shell structure of Catalyst 1 can promote light capture and electron transmission. The photocatalytic performances of water oxidation were usually carried out in two different systems, photosensitizer and non-photosensitizer systems. There is few report about the same catalyst used in two systems and therefore it is of great significant to compare its role of the same catalyst in two systems and explore its different reaction mechanisms. In this work, first 4 kinds of metallic Co microparticles were obtained by different reduction methods through hydrothermal processes, and Co@Co 3 O 4 core-shell microparticles (1–4) were obtained from these metallic Co microparticles oxidized in air or in the reacting solution in situ. The core-shell structure of 1 was characterized by a series of analytical techniques. 1–4 exhibited excellent activities and stabilities in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system when they were used as catalysts for the photocatalytic water oxidation. The maximum O 2 evolution of 1 after 20 min’s illumination was 98.2 μmol, the O 2 yield was 65.5%, the initial turnover frequency was 6.6 × 10 −3 , the initial quantum efficiency ( Φ QY initial ) was 15.0% higher than Co (8.3%), CoO (11.7%), Co 2 O 3 (1.2%), Co 3 O 4 (2.8%) and 5 (2.2%). Even after the sixth run, the catalytic activity of recovered 1 still remained 85.1% of initial activity. In addition, the photocatalytic performances of 1 in the [Ru(bpy) 3 ] 2+ /S 2 O 8 2− /light system and S 2 O 8 2− /light system were compared for the first time. In the non-photosensitizer system, 1 shows bifunctional roles and acts as optical absorption center and active catalytic site, and oxygen evolution rate is lower and it takes longer time. In the photosensitizer system, 1 only acts as a catalyst, the photosensitizer enhances the light absorption and promotes water oxidation reaction with higher O 2 yield and QE, meanwhile the photosensitizer brings the defect of high cost and instability into the system. Based on the results the two different reaction mechanisms were deeply discussed.

Morphology of random packing of micro-particles and its effect on the absorption of laser radiation during selective melting of powders

The results of analytical and numerical studies of the internal structure of random packed beds of metal micro-particles are presented. To construct packed beds from mono- and polydisperse solid spheres, an algorithm for their random generation based on the discrete element method is used. Analysis of the structure of the packed bed includes the determination of porosity, specific surface area and tortuosity of the pores when changing powder particle sizes. A theoretical conclusion of these structural parameters is proposed in the form of formulas. The data obtained characterize the percolation properties of the granular medium, which affects the propagation and absorption of laser radiation in the volumetric layer of particles. Analysis is made of the influence of the physical properties of particles on the limiting characteristics of the penetration of laser radiation into a packed powder layer; the effects of radiation absorption and scattering in the pore space of a granular medium are estimated. A comparative analysis of approximate analytical and numerical solutions with the data of known experiments is carried out.

Optimization of silver recovery from waste X-ray radiographic films by oxalic acid extraction with response surface methodology

Waste X-ray radiographic films contain an appreciable amount of silver (Ag) that can be recovered and reprocessed. Most of the existing methods of Ag recovery from waste radiographic films are expensive and harmful to the environment. In this study, the extraction of Ag from waste radiographic films was done using oxalic acid solution heated at 90–100 °C. XRD and SEM results reveal that the recovered residue contains pure metallic Ag spherical microparticles. According to the response surface methodology (RSM) based on three-level, three-factorial Box-Behnken design, the optimized stripping conditions are oxalic acid concentration of 9.97% w/v, extraction temperature of 99.51 °C, and extraction time of 16.62 min. The predicted and actual % Ag recovery are of 94.94 and 90.07%, respectively. Reusability study confirms that the oxalic acid solution can be reused for several runs of Ag extraction. Initial Ag recovery of ~68.9% decreased to ~28.2% after 4 cycles of reusing the extraction solution.

Liquid Metal Microparticles Phase Change Medicated Mechanical Destruction for Enhanced Tumor Cryoablation and Dual‐Mode Imaging

AbstractMechanical forces are crucial for normal living organisms as well as formation of tumor microenvironments. However, to date, there are rather limited trials to regulate the mechanical factors toward tumor treatment or imaging. Here, a synergistic antitumor therapy of cryoablation and gallium microparticles (GMs) mediated bomb‐explosion‐like mechanical destruction is proposed for the first time. Moreover, the GMs are demonstrated to enhance the T2 magnetic resonance imaging (MRI) effect and mediate the dual‐mode imaging of computerized tomography (CT) and MRI. The GMs are found to exert a mechanical force to surrounding chitosan ice crystals during freezing, which is attributed to the volume expansion during the phase transition process. Meanwhile, the phenomenon of piercing gallium materials via a sword‐like shape into the solid ice crystals is observed with a penetration length of 150 µm within 1 ms, which further shows the remarkable mechanical destruction to frozen ice crystals. Furthermore, a series of in vitro and in vivo results prove the negligible biotoxicity and good biocompatible of GMs. The in vivo synergistic therapy exhibits effective destructive results with reduced recurrence rate and prolonged survival. The present methods are expected to not only open an efficient tumor destructive approach, but also hold potential for advanced theranostic systems.

Development of the Multi-Material Inspection for Closed-Loop Rapid Optimization (MICRO) Sensor for Extrusion-Based Additive Manufacturing of Metal-Polymer Composite Inks

Additive Manufacturing (AM), or 3D printing, is a powerful technology that is revolutionizing the way designers create products across many industries. AM technology is severely limited by the lack of effective methods for in situ characterization of multi-material properties and composition during printing. The ability to detect the composition of multi-material printed inks in real time is an emerging need for a wide range of manufacturing applications. In this study, dielectric properties of embedded metal microparticles in a dielectric matrix are measured and characterized as a function of particle size, shape, volume percentage and frequency. Unexpectedly, there was no observation of any percolation threshold. The impedance was found to decrease with the percentage of metal filler as expected. While particle shape seemed to have significant effect on the impedance, there was little to no correlation between particle size and impedance. The resulting data can be used to generate a calibration curve correlating metal loading with impedance or capacitance. We discuss how this data can be used for in situ sensing of local ink composition, which does not currently exist, to facilitate greater control over the resulting properties and functionality of printed materials.

Liquid Metal Composites with Anisotropic and Unconventional Piezoconductivity

Summary Anisotropic elastic composites—that is, elastomers containing aligned fillers—often have enhanced properties in the direction of alignment. Depending on the fillers, these composites can have desirable electrical, thermal, or mechanical properties. Here, a silicone composite filled with both solid magnetic metal microparticles and liquid metal microdroplets has been developed. Aligning the solid particles within a magnetic field during curing imparts anisotropy in several properties. Thus, the composite is called an anisotropic liquid metal-filled magnetorheological elastomer (ALMMRE). Compared with isotropic liquid metal-filled composites, the conductivity of the ALMMRE is significantly enhanced in all directions. The ALMMRE also exhibits anisotropic piezoconductivity and a significantly enhanced electrical anisotropy under mechanical deformation; these properties are not observed in conventional anisotropic composites. The ALMMRE also shows anisotropic mechanical, thermal, and magnetic properties and demonstrates its several proof-of-concept applications. The sensitivity of the ALMMRE's properties to strain may help advance future flexible sensors and soft electronics.

Recent Developments on Noble Metal Based Microparticles for Their Applications in Organic Catalysis

Noble metal microparticles have been employed as desired catalysts for a number of classical organic chemical reactions due to their unique physicochemical properties. Currently, in order to obtain more benefits for practical applications such as low cost, easy separation and high selectivity, many efforts of scientists are devoted to constructing composite microparticles in which noble metals are coupled with other materials. In this paper, we summarize some recent research developments on noble metal based microparticles for their catalytic applications in organic synthesis. Among them, application of the gold and silver based microparticles is the focus of this paper for their relatively low cost and the diversity of preparation methods. Furthermore, the challenges and prospects of noble metal based microparticles for their applications in organic catalysis are also discussed.

The Transition From Rebound to Bonding in High-Velocity Metallic Microparticle Impacts: Jetting-Associated Power-Law Divergence

Abstract A metallic microparticle impacting a metallic substrate with sufficiently high velocity will adhere, assisted by the emergence of jetting—the splash-like extrusion of solid matter at the periphery of the impact. In this work, we compare real-time observations of high-velocity single-microparticle impacts to an elastic–plastic model to develop a more thorough understanding of the transition between the regimes of rebound and bonding. We first extract an effective dynamic yield strength for copper from prior experiments impacting alumina spheres onto copper substrates. We then use this dynamic yield strength to analyze impacts of copper particles on copper substrates. We find that up to moderate impact velocities, impacts and rebound velocities follow a power-law behavior well-predicted on the basis of elastic-perfectly plastic analysis and can be captured well with a single value for the dynamic strength that subsumes many details not explicitly modeled (rate and hardening effects and adiabatic heating). However, the rebound behavior diverges from the power-law at higher impact velocities approaching bonding, where jetting sets on. This divergence is associated with additional lost kinetic energy, which goes into the ejection of the material associated with jetting and into breaking incipient bonds between the particle and substrate. These results further support and develop the idea that jetting facilitates bonding where a critical amount of bond formation is required to effect permanent particle deposition and prevent the particle from rebounding.

Recycling of polymer laminated aluminum packaging (PLAP) materials into carbonaceous metallic microparticles

This research describes the recycling of laminated metal packaging into multiple products by utilizing three different techniques. PLAP materials have been thermally disengaged at 550 °C for 20 min in an argon gas created inert atmosphere to recycle into solid Al and graphitic carbon. In a different route, in the presence of gaseous N2 at 600 °C for 1h, the PLAP materials are degraded into bio-oil. A new technique to produce powdered materials from the ductile metal (Al) has been introduced in this study. In this technique, recycled Al and C from the first route at 550 °C, undergoes mechanical milling at cryogenic temperature (−196 °C) created by liquid N2. The translatory high energy ball milling process varied for 2, 4, 6, and 8 cycles (time: 17, 29, 41, and 53 min respectively). As the milling process is extensively high impact loaded at a very low temperature, the foil-like Al recycled from the packaging waste transforms into smaller particles within a very short time (20 min). Upon further milling (41 min), the microparticles (particle size below 100 μm) of Al can be observed. The microparticles received from the cryo-mill are characterized to investigate the particle size, morphology, crystallographic structure, thermal behavior, chemical state, and dispersibility in different liquid media. Flake shaped, and micro-sized carbonaceous Al microparticles are contamination-free and thermally stable and can be useful in the fields of additive manufacturing.

Dipolophoresis and Travelling-Wave Dipolophoresis of Metal Microparticles.

We study theoretically and numerically the electrokinetic behavior of metal microparticles immersed in aqueous electrolytes. We consider small particles subjected to non-homogeneous ac electric fields and we describe their motion as arising from the combination of electrical forces (dielectrophoresis) and the electroosmotic flows on the particle surface (induced-charge electrophoresis). The net particle motion is known as dipolophoresis. We also study the particle motion induced by travelling electric fields. We find analytical expressions for the dielectrophoresis and induced-charge electrophoresis of metal spheres and we compare them with numerical solutions. This validates our numerical method, which we also use to study the dipolophoresis of metal cylinders.

High temperature characterization of a CMOS based infra-red source using thermal-incandescence microscopy

This paper presents the high temperature thermal characterization of a Micro-Electro-Mechanical Systems (MEMS) infra-red (IR) thermal source, using non-contact optical approaches, based on IR and thermo-incandescence microscopy. The IR thermal source was fabricated using a CMOS based processing technology and consists of a miniature micro-heater, fabricated using tungsten metallization. The performance and reliability of the IR source is highly dependent on its operating temperature. For short-wave (1.4 µm−2.5 µm) infra-red emission, the operating temperature is in excess of 800 °C. Work will be presented in this paper in which spot temperature measurements (>700 °C) were made on the IR source using thermal-incandescence microscopy. Thermal-optical calibration was achieved by utilizing the known melting point (MP) of different metal micro-particles. Optical measurements were compared to those obtained using an electrical approach. The thermal measurements suggest good temperature uniformity across the micro-heater of the IR source.

Facile fabrication of semiconducting oxide nanostructures by direct ink writing of readily available metal microparticles and their application as low power acetone gas sensors

In this work, a facile two-step fabrication and characterization of printed acetone sensors based on mixed semiconducting metal oxides is introduced. The devices are fabricated by Direct Ink Writing metal microparticle (MP) stripes of commercially available pure iron and copper particles onto the surface of a glass substrate, forming a bridging multi-phase semiconducting oxide net by subsequent thermal annealing. The open, highly porous bridging structures consist of heterojunctions which are interconnected via non-planar CuO/Cu2O/Cu nanowires and Fe2O3/Fe nanospikes. Morphological, vibrational, chemical and structural studies were performed to investigate the contact-forming Fe2O3–CuO nanostructures on the surface of the MPs. The power consumption and the gas sensing properties showed selectivity to acetone vapor at an operating temperature of around 300 °C with a high gas response of about 50% and the lowest operating power of around 0.26 μW to a concentration of 100 ppm of acetone vapor. The combination of the possibility of acetone vapor detection, the controllable size and geometry and their low power make these printed structures important candidates for next developments of accessible detection devices, as well as acetone vapor monitoring (even below 1 ppm). The printing of MPs in general paves the way for a new generation of printed different devices, even in “home-made” conditions, for a manifold of applications tailored by the composition and geometry of the printed MP stripes, enabled through the simplicity and versatility of the fabrication method.

Liquid Metal Composites with Anisotropic and Unconventional Piezoconductivity

Anisotropic elastic composites−that is, elastomers containing aligned fillers−often have enhanced properties in the direction of alignment. Depending on the fillers, these composites can have desirable electrical, thermal, or mechanical properties. Here, a silicone composite filled with both solid magnetic metal microparticles and liquid metal microdroplets has been developed. Aligning the solid particles within a magnetic field during curing imparts anisotropy in several properties. Thus, the composite is called an anisotropic liquid metal-filled magnetorheological elastomer (ALMMRE). Compared to isotropic liquid metal-filled composites, the conductivity of the ALMMRE is significantly enhanced in all directions. The ALMMRE also exhibits anisotropic piezoconductivity and a significantly enhanced electrical anisotropy under mechanical deformation; these properties are not observed in conventional anisotropic composites. The ALMMRE also shows anisotropic mechanical, thermal and magnetic properties and demonstrates its several proof-of-concept applications. The sensitivity of the ALMMRE’s properties to strain may help advance future flexible sensors and soft electronics.

Polarizability-Dependent Sorting of Microparticles Using Continuous-Flow Dielectrophoretic Chromatography with a Frequency Modulation Method

The separation of microparticles with respect to different properties such as size and material is a research field of great interest. Dielectrophoresis, a phenomenon that is capable of addressing multiple particle properties at once, can be used to perform a chromatographic separation. However, the selectivity of current dielectrophoretic particle chromatography (DPC) techniques is limited. Here, we show a new approach for DPC based on differences in the dielectrophoretic mobilities and the crossover frequencies of polystyrene particles. Both differences are addressed by modulating the frequency of the electric field to generate positive and negative dielectrophoretic movement to achieve multiple trap-and-release cycles of the particles. A chromatographic separation of different particle sizes revealed the voltage dependency of this method. Additionally, we showed the frequency bandwidth influence on separation using one example. The DPC method developed was tested with model particles, but offers possibilities to separate a broad range of plastic and metal microparticles or cells and to overcome currently existing limitations in selectivity.

Aspects concerning the mathematical distribution of metal microparticles on the textile surfaces with electroconductive properties obtained by printing method

This paper presents the aspects concerning the mathematical distribution of metal microparticles on the textile surfaces to model the electroconductive properties of the textiles obtained by a printing method using electroconductive paste based on polyethylene glycol (PEG)/polyurethane (PU) and micro/nanoparticle (nickel, silver, and copper). A direct relationship between surface conductivity and surface pH and an inverse relation surface resistance and pH was observed. Besides this, in this paper, we analyzed the dependences between conductivity, pH and surface resistance by using covariance between two vectors (cov(pH, Rs), cov(C, pH), cov(C, Rs)). The purpose of this research is to define a particles distribution that could be useful in establishing the correct distribution of the microparticles for obtaining the surface with antistatic/dissipative, and conductive properties for sensors or electromagnetic shields.