Core-shell Composite Powders Research Articles

Mesoscopic Simulation of Core-Shell Composite Powder Materials by Selective Laser Melting.

Mechanical ball milling is used to produce multi-materials for selective laser melting (SLM). However, since different powders have different particle size distributions and densities there is particle segregation in the powder bed, which affects the mechanical properties of the printed part. Core-shell composite powder materials are created and used in the SLM process to solve this issue. Core-shell composite powder materials selective laser melting (CS-SLM) has advanced recently, expanding the range of additive manufacturing applications. Heat storage effects and heat transfer hysteresis in the SLM process are made by the different thermophysical characteristics of the core and the shell material. Meanwhile, the presence of melt flow and migration of unmelted particles in the interaction between unmelted particles and melt complicates the CS-SLM molding process. It is still challenging to investigate the physical mechanisms of CS-SLM through direct experimental observation of the process. In this study, a mesoscopic melt-pool dynamics model for simulating the single-track CS-SLM process is developed. The melting characteristics of nickel-coated tungsten carbide composite powder (WC@Ni) were investigated. It is shown that the powder with a smaller particle size is more likely to form a melt pool, which increases the temperature in the area around it. The impact of process parameters on the size of the melt pool and the distribution of the reinforced particles in the melt pool was investigated. The size of the melt pool is significantly affected more by changes in laser power than by changes in scanning speed. The appropriate control of the laser power or scanning speed can prevent enhanced particle aggregation. This model is capable of simulating CS-SLM with any number of layers and enables a better understanding of the CS-SLM process.

Effects of TiC on the microstructure refinement and mechanical property enhancement of additive manufactured Inconel 625/TiC metal matrix composites fabricated with novel core-shell composite powder

The flexible product shape of additive manufacturing (AM) is attractive, but the process suffers from a lack of material property diversity due to a limited number of printable alloys and post-processing options. To overcome this problem, the AM of metal matrix composites (MMCs) is a highly suitable solution because the properties of MMC can be tailored using various reinforcements. Therefore, extensive research has been conducted on the AM of MMCs; however, the major huddle for this process has been the difficulties in preparing feedstock powder and operating the AM process. This study introduces an easily synthesizable core-shell composite powder, which was fabricated by a recently developed process called the SMART process. The core-shell powder has a novel morphology, consisting of a metal core and composite shell, distinguishing it from the powders used in conventional AM approaches. Inconel 625/TiCp composites were fabricated using the core-shell composite powder, with various fractions of TiCp up to 10 vol.%. Compared to additive-manufactured Inconel 625, the additive-manufactured MMCs showed enhanced strength with significantly fewer defects. The results of this study may accelerate the application of MMC fabricated by AM, which offers superior properties and reliability compared to casting and powder metallurgy due to the higher degree of dislocation density and reinforcement dispersion.

Preparation of MoSi2@ZrO2 core-shell powders by hydrothermal-calcination and evaluation of low-temperature oxidation resistance

MoSi2 is a promising candidate for high-temperature, structural materials. However, their oxidation resistance is poor below 600 °C. To prevent MoSi2 from being oxidized at low temperatures, core-shell structured MoSi2@ZrO2 powder were prepared by hydrothermal-calcination approach. First, MoSi2@Zr(OH)4 core-shell composite powder with nano-sized Zr(OH)4 shell particles were synthesised using a hydrothermal method. Subsequently, the MoSi2@ZrO2 core-shell structured powder were obtained by calcination of the MoSi2@Zr(OH)4 powders at 900 °C for 2h. Finally, microstructure and oxidation behaviour of the MoSi2@ZrO2 powder at 400 °C–600 °C in air were investigated systematically. Microstructural analysis revealed that all samples had a distinct core-shell structure, and the ZrO2 shells coated the surface of the MoSi2 core. Oxidation behaviour studies showed that the dense ZrO2 shell layer could isolate the MoSi2 surface from oxygen, improving the low-temperature oxidation resistance and providing better low-temperature antioxidant properties than those of the MoSi2 and MoSi2@Zr(OH)4 core-shell structures.

Preparation of ZnO/Brucite Functional Composite Powder by the Mechanochemical Method

In this experiment, ZnO/brucite composite powder was prepared through the mechanochemical method; further, the properties, particle morphology, and structure of the composite powder were characterized. The results show that mechanical grinding action can promote the production of a large number of surface ions with unsaturated coordination number on the surface of brucite and ZnO particles, thereby promoting hydroxylation of the particle surfaces. The addition of NaOH to the composite system can also aid the further activation of the surface of the brucite and ZnO particles and the formation of more associated hydroxyl groups. Finally, a core–shell composite powder is formed with weak forces such as hydrogen bonds and van der Waals forces as the connecting bonds.

A Taguchi approach to synthesise Al/Fe core–shell composite powders through electroless deposition

ABSTRACT In the present study, Al/Fe core–shell particles were successfully synthesised by electroless deposition of iron on aluminium particles. The process variables were investigated and optimised to achieve maximum deposition efficiency. These variables, including chemical composition parameters (molar ratios of salt/reducing agent, salt/complexing agent and pH) and process parameters (particle size, temperature and time) were optimised by using the Taguchi method. The chemical composition of iron deposited aluminium particles, their morphology and coating quality were characterised by helium gas pycnometry, scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. The results revealed that, by using the optimised bath chemical composition and optimised process parameters, a maximum of 28.91% iron deposition on aluminium particles can be obtained within 20 min.

Fabrication of Core-Shell Chopped Cf-Phenolic Resin Composite Powder for Laser Additive Manufacturing of Cf/SiC Composites.

Laser additive manufacturing is a promising technique for the preparation of complex-shaped SiC composites. High-quality powders are critical for high-precision laser printing. In this work, core-shell Cf @phenolic resin (PR) composites for selective laser sintering of carbon fiber reinforced silicon carbide (Cf/SiC) composites were fabricated by surface modification using 3-aminopropyltriethoxy silane coupling agent (KH550) in combination with planetary ball milling. PR coated uniformly on the fiber surface to form a core-shell structure. The effects of PR on the morphology, elemental composition, interfacial interactions, and laser absorption of the core-shell composite powder were investigated in detail. Results indicated that the composite powder exhibited good laser absorption within the infrared band.

Preparation and oxidation of aluminum powders with surface alumina replaced by iron coating

ABSTRACT Aluminum powders are well known for excellent release of energy during oxidation upon heating. However, this release of energy becomes limited due to formation of dense oxide (Al2O3) layer on powder surface. In this study, reactivity of aluminum particles was improved by replacing oxide layer with metallic shell of iron using electroless plating technique. Three different bath compositions were selected for deposition of iron on aluminum particles. The surface morphology of prepared Al/Fe core–shell composite powder was characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Further, iron content in the composite powder was measured quantitatively using helium gas pycnometer while elemental analysis was performed by X-ray diffraction (XRD). Finally, thermo-physical behavior of the composite powder was investigated using simultaneous thermogravimetry–differential scanning calorimetry (TG–DSC). The results showed that the synthesized Al/Fe core–shell composite powder exhibits higher energy release in comparison to the uncoated aluminum powder upon heating in air (up to 1180 °C). The powder with the densest iron coating released the highest energy.

Consolidation behaviour of Cu/AlN composites by pulse electric current sintering of copper-coated aluminium nitride precursors

The effect of densification rate on hot consolidation behaviour by pulse electric current sintering (PECS) of AlN-Cu core-shell composite powders (≈13 vol% Cu) is investigated. The ceramic phase was limited to 10, 20 and 30 vol% AlN by adding electrolytic copper particles. The Cu-coating on the ceramic surfaces served as a bridge for the passage of direct current to and between the copper particles of the matrix, making its conversion into heat more efficient. A constant pressure of 50 MPa was applied at the maximum of 850 °C. Three stages of displacement under load were observed, corresponding to particle rearrangement, localized deformation and deformation of the bulk. Thermal softening of the coating promoted a rapid surface-diffusion sintering and neck growth with the copper matrix, reaching a relative density above 97%. The Cu-coated particles led to a more homogeneous interconnected matrix microstructure and isolated the direct-contact between the ceramic particles.

Formation mechanism and enhanced magnetic properties of Fe–Si/Fe2SiO4 soft magnetic composites transformed from Fe-6.5 wt%Si/α-Fe2O3 core-shell composites

In this work, Fe–Si/Fe2SiO4 soft magnetic composites (SMCs) are prepared successfully via sintering Fe-6.5 wt%Si/α-Fe2O3 core-shell composite powders obtained by ball milling. The transformation mechanism from Fe-6.5 wt%Si/α-Fe2O3 to Fe–Si/Fe2SiO4 is revealed in detail. The effect of α-Fe2O3 coating content on microstructure and magnetic properties for the Fe–Si/Fe2SiO4 SMCs is also investigated. During the high temperature sintering process, α-Fe2O3 coating will in situ react with diffused Si from Fe–Si alloy cores, and result in the generation of Fe2SiO4 insulating layer and Fe atoms. When the α-Fe2O3 coating content increases from 0.0 wt% to 7.5 wt%, the insulating layer becomes thicker. Nevertheless, with further increasing the α-Fe2O3 coating content from 7.5 wt% to 12.5 wt%, the generation of excessive Fe breaks the continuity of the insulation layer. As a result, the saturation magnetization decreases gradually, while the coercivity exhibits an opposite trend. The core loss decreases firstly and then increases. Therefore, the Fe–Si/Fe2SiO4 SMCs coated with 7.5 wt% α-Fe2O3 exhibit superior comprehensive performance such as very low core loss (39.6 mW/cm3 at 50 kHz), very high saturation magnetization (191.4 emu/g) and stable permeability (44) from 50 Hz to 200 kHz.

Wave Absorption and Thermal Stability of Nickel Coated Al2O3 Core-shell Composite Powder

As a composite cermet material, the nickel coating on the surface of alumina particles has important application value because of its good electromagnetic wave absorption performance. In this study, nickel was deposited on the surface of alumina nanopowders by hydrothermal method and sintering method. The results show that the XRD results of nickel alumina show that Ni-Al2O3 nanoparticles has FCC face-centered cubic structure. The grain size is about 50 nm. The absorption characteristics of electromagnetic waves were measured at 2-18 GHz by HP 8722 es microwave network analyzer. The results show that Ni-Al2O3 nanoparticles has certain wave-absorbing properties.

Preparation of alumina/polystyrene core-shell composite powder via phase inversion process for indirect selective laser sintering applications

Indirect selective laser sintering (SLS) is one of the promising additive manufacturing (AM) methods that can process conventionally difficult or even impossible materials such as ceramics. In this work, an innovative phase inversion technique is used to fabricate spherical alumina particles coated with a thin layer of polystyrene (PS). Then, indirect SLS is used to fabricate green parts from the 6wt% PS coated alumina particles via a Nd:YAG laser. The assessed SLS process parameters were the scan speed, laser power, scan spacing, pulse frequency, and pulse width. The characterization of the AL2O3/PS core-shell composite particles was described using techniques including SEM (for morphology), FT-IR (for chemical bonding at the interfaces), TGA (for mass loss), and DSC (for glass transition temperature, Tg). 3D green parts were then fabricated using proper process parameters as a proof of the feasibility of using SLS technique for AL2O3/PS core-shell composite powder. The results showed that using a Nd:YAG laser with less absorption by alumina and PS provides greater penetration through a powder bed. In addition, the possibility of sound connections among particles in every direction was observed due to the uniformity of the coating process in spite of a minimal amount of binder. In addition, green part density measurements show high values compared to previously reported results.

Microstructure and electrical properties of SiC dispersed molybdenum silicide composite

In this study, a core–shell composite powder with Mo as the shell material and β-SiC as the core material was produced via chemical vapour transport caused by the hydrogen reduction of Mo oxide. Using this core–shell composite powder, a Mo silicide composite with uniformly dispersed β-SiC was fabricated by pressureless sintering at 1400°C. The relative density of specimens sintered for 10 hours was 96.1% and the electrical resistivity was 4.68 × 10−5 Ω m. It was concluded that a β-SiC dispersed molybdenum silicide composite with enhanced electrical resistivity was successfully fabricated at a relatively low sintering temperature using the core–shell composite powder.

Synthesis Mechanism of ZrB<sub>2</sub>@A1<sub>2</sub>O<sub>3</sub>-Y<sub>2</sub>O<sub>3</sub> Composite Powders with Core-Shell Microstructure

ZrB2 has some excellent high-temperature performance. However, due to it is easily oxidized in the high-temperature air to impact high-temperature strength, which restricts its applied range. To decrease the oxidization and sintering temperature, and improve the strength of ZrB2 at high temperature, in this paper, the prepared composite powders is analyzed with XRD, SEM, EDS and TEM, which proves ZrB2@A12O3-Y2O3 core-shell composite powders are successfully synthesized by co-precipitation method, the synthesis mechanism of ZrB2@A12O3-Y2O3 core-shell composite powders is received through the results discussion.

Core loss reduction in Fe–6.5 wt.%Si/SiO2 core–shell composites by ball milling coating and spark plasma sintering

The Fe–6.5wt.%Si/SiO2 core–shell composite powders and compacts have been prepared by simple ball milling coating and spark plasma sintering process. The microstructure, magnetic and electrical properties of the compact samples obtained at different sintering temperatures were investigated by differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, vibrating sample magnetometer, and impedance analyzer. The relative densities of Fe–6.5wt.%Si/SiO2 composite compacts first increase with sintering temperature from 800°C to 1150°C, and then keep nearly unchanged with higher temperature, while the coercivity and electrical resistivity change in the opposite direction. The saturation magnetization fluctuates around an identical value with increasing the sintering temperature from 800°C to 1250°C. The Fe–6.5wt.%Si/SiO2 composite compact sintered at 1150°C displays excellent soft magnetic properties: saturation magnetization of 153.9emu/g, coercivity of 14Oe, relative permeability of 8.6×103 at 50Hz, respectively. The Fe–6.5wt.%Si/SiO2 core–shell composite compacts clearly exhibit much higher electrical resistivity and lower core loss (1.4×10−5Ωm and 18.1W/kg (P5/400)) than those of Fe–6.5wt.%Si compacts without core–shell structures (9.6×10−7Ωm and 37.9W/kg (P5/400)). The core–shell structures introduced here is a good potential method to increase electrical resistivity and reduce core loss of soft magnetic materials, which can obviously improve energy conversion efficiency of electric–magnetic switching device.

Novel Mo/TiN composites for an alkali metal thermal-to-electric converter (AMTEC) electrode

Core–shell composite powders composed of molybdenum (Mo) coated on titanium nitride (TiN) were synthesized using a Pechini-type polymerizable complex method to improve the microstructural stability and electrochemical performance of alkali metal thermal-to-electric converter (AMTEC) electrodes. The coverage of porous Mo shells was controlled by adjusting the concentration of resin-embedded molybdenum sources around TiN particles. Each composite powder was characterized by microstructural analysis (SEM, TEM) and X-ray spectroscopy (XRD, EDX and XPS). During the synthesis process, thin oxide layers (i.e., TiOx, with x=1.0–2.0) were formed on the TiN surface. Each Mo and TiN phase shares a common intermediate oxide layer, and there are strong interactions between oxygen anions and Mo atoms at the interface. As a result, there are cohesive networks between Mo and TiN at the core–shell interface, and these networks control the grain growth of Mo at high temperatures (>700°C). A single cell composed of Mo/TiN powder (Mo 1.0mol%) was observed to perform well (180mW/cm2) and exhibit a low area specific resistance (ASR at 0.7V) of 0.4Ωcm2 at 700°C.

Modeling of the beam transportation behavior in selective laser transmission sintering the translucent core–shell composite powder

In the paper, selective laser transmission sintering the 2%PF/silica sand core–shell composite powders was studied based on a developed optical model by using a Zemax optical design program. The construction of the powder bed was made by the particle stacking model with the composite particle assumed as a three-component optical system. It showed that simulation results agreed well with the experimental data. A focus point after the first layer particle was produced by the translucent spherical lens of particles and then laser beam was quickly diverged by subsequent particles along the beam transportation path. The occurrence of the vaporization at the focus point had little effect on the whole properties of the sintered sample because of its relatively small size. Since no melting pool was produced in the sintering process owing to the thin polymer binder, the rearrangement of the powder bed was not remarkable which led to a constant porosity of the powder bed before and after laser sintering. Laser transmission sintering of core–shell composite powders was basically “contaction” sintering, where particles near to each other as well as within the beam transportation path had an opportunity to sinter together.

Processing and characterizations of 2%PF/silica sand core–shell composite powders by selective laser sintering with a higher transmittance fiber laser

Previous investigations on selective laser sintering were focused on opaque powders from the primary outer surface absorption of the powder bed. In the paper, the sintering behavior of “transparent” materials with a higher transmittance over 90% is demonstrated. Phenol–formaldehyde resin- (PF-) coated silica sand composite powders were sintered by a fiber laser considering their high transmittance at 1064nm wavelength. The affluences of process parameters on the dimension, shrinkage depth and porosity of the sintered sample were studied, and microstructures by optical microscopy (OM) and scanning electron microscopy (SEM) were observed. Furthermore, the absorption and transmission of laser beam within the powder bed were investigated based on a developed analytical model and the Zemax optical design program. Experimental results showed that composite powders could be successfully sintered by the fiber laser. That was because poor absorption of laser energy could still raise the temperature of the PF shell over its glass transition point. However, the heat absorbed within the powder bed was not uniform by the action of the convergence of the micro spherical lens from the “transparent” composite powder. Finally, some components were manufactured and the feasibility of using this process to build casting molds was verified.

Characterization of microwave processed aluminium powder

Commercial Al powder was exposed to microwave radiation for 45min. The as received and microwave heated Al powders were characterized by X-ray diffraction analysis (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis and transmission electron microscopy (TEM). XRD of the microwave treated Al powder confirmed the formation of Al–Al2O3 composite. FTIR studies and EDX analysis indicated the transformation of Al powder into Al–Al2O3 core–shell composite powder after microwave processing. SEM showed that the morphology of the microwave processed Al powder was quite different from that of the as-received Al powder. TEM image of the microwave treated Al powder supported the FTIR and EDX data.

Al–Al 2O 3 core–shell composite by microwave induced oxidation of aluminium powder

Microwave processing is gaining importance in the preparation of composite materials mainly because of its predominance in the improvement of uniformity for different phases formed. In the present study, commercial Al powder was heat treated using microwave radiation to synthesize Al–Al 2O 3 core–shell composite powder. The as received Al powder and the microwave heated Al powders were characterized by X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis and zeta potential measurement. XRD data confirmed the formation of Al–Al 2O 3 composite. FTIR studies indicated the core–shell type Al–Al 2O 3 composite formation. SEM observations suggested the formation of Al–Al 2O 3 core–shell composite. TEM images, corresponding selected area electron diffraction (SAED) patterns and EDX analysis confirmed the Al–Al 2O 3 core–shell composite development. Zeta potential measurements also indicated that Al core particles were surrounded with Al 2O 3 shell.

SYNTHESIS AND CHARACTERIZATION OF Si3N4@Al(OH)3–Y(OH)3 CORE-SHELL COMPOSITE PARTIC3LES

Silicon nitride ( Si 3 N 4) has attracted substantial interest because of its extreme chemical and physical properties, but the sintering densification of Si 3 N 4 is difficult, and it is easily oxidized in the high-temperature air to impact high-temperature strength, which restricts its applied range. In order to decrease the oxidization and improve the strength of Si 3 N 4 at high temperature, the surface of Si 3 N 4 is coated with Al ( OH )3 and Y ( OH )3 to synthesis Si 3 N 4@ Al ( OH )3– Y ( OH )3 core-shell composite particles. Through TEM, XRD, and BET analysis, when pH is about 9, Si 3 N 4@ Al ( OH )3– Y ( OH )3 core-shell composite particles are successfully synthesized by co-precipitation methods. Coating layer is about 200 nm, which is compaction and conformability. Dispersion of coated Si 3 N 4 with Al ( OH )3 and Y ( OH )3 particles are very good. Synthesis of Si 3 N 4@ Al ( OH )3– Y ( OH )3 core-shell composite powder will lay the foundation for preparing high-performance YAG/Si 3 N 4 multiphase ceramic materials.