Pulsed Laser Melting In Liquid Research Articles

Size distribution evolution and viscosity effect on spherical submicrometer particle generation process by pulsed laser melting in liquid

Spherical submicrometer particles (SMPs) can be fabricated by pulsed laser melting in liquid (PLML). In this process, raw nanoparticles dispersed in liquid are irradiated by a pulsed laser, heated over the melting point to generate molten droplets, and quenched to form SMPs. The particle size increment is caused by aggregating and merging raw particles in liquid by transient heating. In this study, the viscosity effect on the formation process of SMPs by PLML process is studied by changing aqueous glycerin solution concentration. In highly viscous liquid, smaller SMPs are generated due to the ineffective aggregation process. The temporal evolution of cumulative volume frequency of particles can be simulated, considering the constraints of the non-melting at larger size due to high heat capacity and evaporation for smaller particles by overheating. The SMP size can be controlled by viscosity of the liquid, prior disaggregation time, and liquid temperature.

Fracture and Embedment Behavior of Brittle Submicrometer Spherical Particles Fabricated by Pulsed Laser Melting in Liquid Using a Scanning Electron Microscope Nanoindenter.

Generally, hard ceramic carbide particles, such as B4C and TiC, are angulated, and particle size control below the micrometer scale is difficult owing to their hardness. However, submicrometer particles (SMPs) with spherical shape can be experimentally fabricated, even for hard carbides, via instantaneous pulsed laser heating of raw particles dispersed in a liquid (pulsed laser melting in liquid). The spherical shape of the particles is important for mechanical applications as it can directly transfer the mechanical force without any loss from one side to the other. To evaluate the potential of such particles for mechanical applications, SMPs were compressed on various substrates using a diamond tip in a scanning electron microscope. The mechanical behaviors of SMPs were then examined from the obtained load–displacement curves. Particles were fractured on hard substrates, such as SiC, and fracture strength was estimated to be in the GPa range, which is larger than their corresponding bulk bending strength and is 10–40% of their ideal strength, as calculated using the density-functional theory. Contrarily, particles can be embedded into soft substrates, such as Si and Al, and the local hardness of the substrate can be estimated from the load–displacement curves as a nanoscale Brinell hardness measurement.

Behavior of Thermally Induced Nanobubbles during Instantaneous Particle Heating by Pulsed Laser Melting in Liquid.

Pulsed laser melting in liquid (PLML) is a technique to produce submicrometer spherical particles (SMPs). In this process, raw particles dispersed in liquid are selectively heated, and thermally induced nanobubbles (TINBs) at the particle surface are generated and act as a thermal barrier to enhance the temperature increase during heating. However, monitoring TINBs is difficult since PLML is a low-temperature, nonplasma process. Simple transmittance measurements of monodisperse Au SMP (250 nm) colloidal solutions on a transient time scale were used to monitor the temporal dependence of the TINB thickness and the pressure within the bubble. By applying this technique for ZnO and Sn SMP formation, TINBs in the PLML process are important in promoting the formation of large particles via particle merging during laser heating.

Reduction Mechanism of Transition Metal Oxide Particles in Thermally Induced Nanobubbles during Pulsed Laser Melting in Ethanol.

Pulsed laser melting in liquid (PLML) is a technique to fabricate spherical submicrometer particles (SMPs) wherein nanosecond pulsed laser (several tens to several hundreds of mJ pulse-1 cm-2 ) irradiates raw particles dispersed in liquid. Raw particles are transiently heated above the melting point to form spherical particles, which enables pulsed heating of surrounding liquid to form thermally induced bubbles by liquid vaporization. These transient bubbles play an important role as a thermal barrier to rapidly heat the particle. Reduced SMPs are generated from raw metal-oxide nanoparticles by PLML process in ethanol. This reduction cannot be explained by high-temperature thermal decomposition, but by mediation of molecules decomposed from ethanol. Computational simulations of ethanol decomposition by pulsed heating for 100 ns at the temperature 1000-4000 K revealed that ethylene is generated as the main product. Gibbs free energies of oxide reduction reactions mediated by ethylene greatly decreased compared to those without ethylene mediation. This explanation can be applied to reductive SMP formation from various transition metal oxides by PLML.

P.504 Targeting nitric oxide synthase 1 adaptor protein interactions as a possible treatment for schizophrenia: an initial proof-of-concept study in mice

Nonporous, submicrometer spherical particles can be fabricated by pulsed laser melting in liquid. In this process, particles dispersed in liquid absorb laser light, instantaneously heated above melting point, and quenched to form submicrometer spherical particles. The dispersed raw particles can only be heated from the laser beam direction, and thus the surface layer facing laser beam is reaction zone. Two automated iterative batch processing apparatuses with different cell sizes were developed to fabricate large quantities of submicrometer spherical particles by efficient use of liquid surface layer. Model calculations indicated that the formation rate will be higher when smaller cells and frequent cell exchange are adopted. This is experimentally confirmed with the fabrication of submicrometer spherical silver particles. The overall production rate of the iterative process with smaller cells was 5.15 mg h−1. However, the smaller cells imposed several limitations, including difficult sample agitation and stray light effects.

Laser-induced nano-heater performance of B4C submicrometer spherical particles fabricated by pulsed laser melting in liquid

Gold nanoparticles as promising nano-heater particles utilizing surface plasmon resonance have been extensively studied for various chemical and medical applications, even though the suitable wavelength for nano-heater is only in the visible range and possible maximum temperature is limited due to its relatively low melting point. About a decade ago, a technique called pulsed laser melting in liquid (PLML) was developed to fabricate submicrometer spherical particles as main products. In this process, particles in the submicrometer size range are effectively heated over the melting point by laser irradiation, indicating that submicrometer spherical particles can be developed as novel heater particles that may overcome the drawback of gold nanoparticles. This study examined the potentials of PLML-fabricated B4C submicrometer spherical particles as nano-heater particles since B4C has a wider optical absorption wavelength range and higher melting point than gold. From the thermal modification induced by melting the materials in contact with the B4C particles, the highest attained temperature was estimated by laser irradiation. This experiment showed that B4C particles have sufficient response in 300–1100 nm wavelength range and can act as nano-heater even at temperatures over 2000 K. Thus, B4C submicrometer spherical particles can be used as novel space-selective heater particles.

Automated iterative batch processing of submicrometer spherical particles by pulsed laser melting in liquid

Nonporous, submicrometer spherical particles can be fabricated by pulsed laser melting in liquid. In this process, particles dispersed in liquid absorb laser light, instantaneously heated above melting point, and quenched to form submicrometer spherical particles. The dispersed raw particles can only be heated from the laser beam direction, and thus the surface layer facing laser beam is reaction zone. Two automated iterative batch processing apparatuses with different cell sizes were developed to fabricate large quantities of submicrometer spherical particles by efficient use of liquid surface layer. Model calculations indicated that the formation rate will be higher when smaller cells and frequent cell exchange are adopted. This is experimentally confirmed with the fabrication of submicrometer spherical silver particles. The overall production rate of the iterative process with smaller cells was 5.15 mg h−1. However, the smaller cells imposed several limitations, including difficult sample agitation and stray light effects.

Hydrofluoric acid pretreatment effect on the formation of silicon submicrometer particles by pulsed laser melting in liquid and their optical scattering property

Recently, the optical properties of silicon (Si) submicrometer spherical particles have been investigated to understand the dielectric nano-photonic function. Herein, we fabricated Si submicrometer spherical particles with high scattering efficiency using pulsed laser melting in deionized water or ethanol by irradiating laser at 66 mJ pulse−1 cm−2 via third harmonic of Nd:YAG laser. Hydrofluoric acid pretreatment was effective to remove surface oxide of raw Si particles; the laser fluence to obtain well crystallized spherical particles was lowered to 20 mJ pulse−1 cm−2 and the crystallinity of particles obtained were greatly improved without forming unwanted byproducts. The amount of particles was much more than those obtained by conventional fabrication technique. The particle size can be controlled by changing the laser fluence, and the scattering wavelength of colloidal solution can be controlled from visible to the near infrared range by increasing the laser fluence.

Spherical Particle Formation Mechanism in Pulsed Laser Melting in Liquid under Controlled-Pulse-Number Irradiation using a Slit Nozzle Flow System

A detailed mechanism of pulsed laser melting in liquid (PLML) has remained a controversial issue because of the difficulty associated with tracking and observing particles irradiated with a specific number of pulses in conventional batch-style irradiation. In a previous study, we developed a flow irradiation technique with a new slit nozzle for PLML mass-production. The slit nozzle can precisely control pulse numbers irradiated onto particles flowing through the slit nozzle. In the present study, this unique feature of the slit nozzle was used to clarify the PLML mechanism by a specific pulse number irradiation. According to a numerical analysis of the flow with the slit nozzle, 79% of the particles flowing through the slit nozzle were irradiated with two or three pulses and other particles that were irradiated with three or more pulses upon one passage through the slit nozzle at a 1.1 mL s–1 volume flow rate for a particle suspension in 60 wt % glycerol aqueous solution. Sub-micrometer-sized hollow spher...

Determining the Composite Structure of Au-Fe-Based Submicrometre Spherical Particles Fabricated by Pulsed-Laser Melting in Liquid.

Submicrometre spherical particles made of Au and Fe can be fabricated by pulsed-laser melting in liquid (PLML) using a mixture of Au and iron oxide nanoparticles as the raw particles dispersed in ethanol, although the detailed formation mechanism has not yet been clarified. Using a 355 nm pulsed laser to avoid extreme temperature difference between two different raw particles during laser irradiation and an Fe2O3 raw nanoparticle colloidal solution as an iron source to promote the aggregation of Au and Fe2O3 nanoparticles, we performed intensive characterization of the products and clarified the formation mechanism of Au-Fe composite submicrometre spherical particles. Because of the above two measures (Fe2O3 raw nanoparticle and 355 nm pulsed laser), the products—whether the particles are phase-separated or homogeneous alloys—basically follow the phase diagram. In Fe-rich range, the phase-separated Au-core/Fe-shell particles were formed, because quenching induces an earlier solidification of the Fe-rich component as a result of cooling from the surrounding ethanol. If the particle size is small, the quenching rate becomes very rapid and particles were less phase-separated. For high Au contents exceeding 70% in weight, crystalline Au-rich alloys were formed without phase separation. Thus, this aggregation control is required to selectively form homogeneous or phase-separated larger submicrometre-sized particles by PLML.

Heating process control of pulsed-laser melting in liquid via a burst-mode laser

The inhomogeneous heating effect caused by picosecond laser irradiation to colloidal particles will form nanoparticles as a byproduct. A method is therefore needed for controlling the particle heating process to suppress byproduct formation through partial evaporation. In this work, we used a burst-mode laser to control the temperature increase of irradiated particles by modifying the burst pulse. Consequently, fewer nanoparticle byproducts formed as the number of burst pulses was increased because the heat distribution by particles was homogenized during the pulse interval. Burst pulse heating suppressed the inhomogeneous particle heating and particle-evaporation-induced byproduct formation.

Guided Slow Continuous Suspension Film Flow for Mass Production of Submicrometer Spherical Particles by Pulsed Laser Melting in Liquid

Pulsed laser melting in liquid (PLML) is a technique to fabricate submicrometer crystalline spherical particles of various materials by laser irradiation of suspended raw particles with random shapes. To fully exploit the unique features of PLML-fabricated particles (crystalline and spherical) in practice, a mass-production PLML technique is required. To this end, the present study develops a new slit nozzle that guides the suspension film flow into a non-droplet continuous stream with a low flow rate. These two incompatible flow properties (continuity and slowness) are difficult to be realized for a liquid jet to free space. The suspension film flow was irradiated with a typical laboratory scale-flash lamp pumping laser at 30 Hz pulse frequency. Only a single flow passage of the slit nozzle with a few laser pulse irradiation transformed 95% of the raw particles into spherical particles. This spheroidizing ratio exceeded those of low-rate drip flow and high-rate cylindrical laminar flow directly jetted into free space through a Pasteur pipette nozzle. Extrapolating the data obtained from a 20-ml suspension, the average production rate was determined as 195 mg h−1. The high spheroidizing ratio and yield through the slit nozzle is attributable to the uniquely slow but continuous liquid film flow. The structure of the slit nozzle also prevents particles from adhering to the slit wall during continuous laser irradiation. Thus, the suspension film flow through the newly developed slit nozzle can potentially scale up the PLML technique to mass production.

High‐Strength Sub‐Micrometer Spherical Particles Fabricated by Pulsed Laser Melting in Liquid

AbstractSub‐micrometer spherical particles that are obtained by pulsed laser melting in liquid (PLML) are usually observed to be single crystalline, and it is suggested that they are mechanically very strong. In this study, fracture tests of various sub‐micrometer spherical particles are performed by compressive force application. The results indicate that B4C and TiO2 sub‐micrometer spherical particles exhibit brittle fracture behavior under tensile fracture mode at the center of the particles. The fracture strength of the sub‐micrometer spherical particles is larger than that of the bulk material reported in the literature by about one order of magnitude. TiO2 sub‐micrometer spherical particles obtained by PLML are stronger than the commercially available TiOx sub‐micrometer spherical particles with a porous structure. In addition, due to the single crystallinity of particles, smaller particles have larger fracture strength, becoming up to 10–40% of ideal tensile fracture strength calculated based on density functional theory. Thus, these results demonstrate that sub‐micrometer spherical particles obtained using PLML exhibit fairly strong and unique mechanical properties, and therefore they are very promising for various mechanical applications at the sub‐micrometer size scale.

Comparison of picosecond and nanosecond lasers for the synthesis of TiN sub-micrometer spherical particles by pulsed laser melting in liquid

In this study, TiN sub-micrometer spherical particles were fabricated by pulsed laser melting in liquid using picosecond and nanosecond lasers applied to colloidal nanoparticles. The sizes of the obtained sub-micrometer spherical particles decreased as the pulse width decreased from nanoseconds to picoseconds. Furthermore, the laser fluence required for fabricating sub-micrometer spherical particles by irradiation was lower with a picosecond laser than with a nanosecond laser. This result suggests that the heat loss from the particles during pulsed laser heating is lower with shorter laser pulse durations. Therefore, picosecond laser irradiation is an energy-efficient method for fabricating sub-micrometer spherical particles.

Influence of pulse frequency on synthesis of nano and submicrometer spherical particles by pulsed laser melting in liquid

Submicrometer spherical particles (SMSPs) are reported to be fabricated by pulsed laser irradiation with a frequency of 10 or 30Hz onto raw nanoparticles dispersed in liquid. Here, the effect of the pulse frequency on particles obtained by laser irradiation onto the suspension in a vessel, especially at higher pulse frequencies up to 800Hz, is investigated. At 200Hz or lower, SMSPs of similar size can be fabricated, as at 10 or 30Hz, by the same number of pulses. This indicates that the time required for particle fabrication can be greatly reduced and production efficiency can be improved using a high-frequency laser. In contrast, at 400Hz or above, nanospherical particles (NSPs) are formed in addition to SMSPs, and the mass fraction of SMSPs is drastically decreased. This result suggests that consecutive laser pulse irradiation induces heat accumulation in particles and suspensions, resulting in a temperature increase and partial evaporation of the particles at 400Hz or above. From the temperature increase of the suspension, the local temperature of the liquid surrounding the particles is believed to be increased by heat dissipation from the heated particles. Calculations suggest that an increase in the local liquid temperature would cause further heating of the particles.

Germanium Sub-Microspheres Synthesized by Picosecond Pulsed Laser Melting in Liquids: Educt Size Effects

Pulsed laser melting in liquid (PLML) has emerged as a facile approach to synthesize submicron spheres (SMSs) for various applications. Typically lasers with long pulse durations in the nanosecond regime are used. However, recent findings show that during melting the energy absorbed by the particle will be dissipated promptly after laser-matter interaction following the temperature decrease within tens of nanoseconds and hence limiting the efficiency of longer pulse widths. Here, the feasibility to utilize a picosecond laser to synthesize Ge SMSs (200~1000 nm in diameter) is demonstrated by irradiating polydisperse Ge powders in water and isopropanol. Through analyzing the educt size dependent SMSs formation mechanism, we find that Ge powders (200~1000 nm) are directly transformed into SMSs during PLML via reshaping, while comparatively larger powders (1000~2000 nm) are split into daughter SMSs via liquid droplet bisection. Furthermore, the contribution of powders larger than 2000 nm and smaller than 200 nm to form SMSs is discussed. This work shows that compared to nanosecond lasers, picosecond lasers are also suitable to produce SMSs if the pulse duration is longer than the material electron-phonon coupling period to allow thermal relaxation.

Nano- and Submicrometer-Sized Spherical Particle Fabrication Using a Submicroscopic Droplet Formed Using Selective Laser Heating

Detailed characterization of products by pulsed laser melting in liquid was performed for TiO2 particles obtained at different laser fluences. The size, crystal structure, and inner structure of obtained spherical particles depended on the irradiating laser fluence. Single crystalline submicrometer spherical TiO2 particles of 200 nm were obtained using nanosecond pulsed laser irradiation at 100 mJ cm–2 pulse–1 onto raw particles dispersed in ethanol using cross-sectional high-resolution transmission electron microscopy observation. At higher laser fluence (e.g., 225 mJ cm–2 pulse–1), large submicrometer spheres with strain and twin structures 500 nm in diameter and nanometer spheres of 70 nm were simultaneously observed. The formation of such bimodal size distribution of obtained particles was explained based on the phase transition fluence curves deduced by Mie theory and the optical absorption increase induced by the formation of nonstoichiometric TiO2 particles. Thus, with the appropriate laser fluence...

Raw Particle Aggregation Control for Fabricating Submicrometer-sized Spherical Particles by Pulsed-laser Melting in Liquid

Abstract We have been studying a new method of pulsed-laser melting in liquid, which is a unique and promising technique for the fabrication of submicrometer-sized spherical particles of various materials. However, spherical particles sometimes did not form depending on the raw particles, even when we used the same material. In this study, we clarified experimentally that particle aggregation in a suspension is important for spherical particle formation, and we found that the addition of an electrolyte to induce particle aggregation effectively promotes spherical particle formation.