Sintering Of Powders Research Articles

Compression behavior of nanoparticle powder considering fractal aggregate for additive manufacturing

Compression behavior of nanoparticle powder is still unclear, while it could largely affect its applications in additive manufacturing. Considering the fractal structure of aggregates in nanoparticle powder, from the van der Waal's state equation, the relation between porosity and compression pressure of nanoparticle powder was modeled theoretically, i.e. (P + c)(φ-a)=(1-a)c, here P is pressure, φ is porosity, a and c are parameters defined in main text. The model could capture the compression behavior of nanoparticle powders well. Under the same compression pressure, smaller nanoparticles will lead to larger porosity and thus lower thermal conductivity of nanoparticle packed beds, due to the easier aggregate of smaller nanoparticles. The compression behavior and the thermal conductivity of nanoparticle powders or packed beds are expected to supply information for powder spreading and sintering treatment respectively in additive manufacturing.

Effects of Cu/SnAgCu Powder Fraction and Sintering Time on Microstructure and Mechanical Properties of Transient Liquid Phase Sintered Joints.

The effects of the sintering duration and powder fraction (Ag-coated Cu/SnAgCu) on the microstructure and reliability of transient liquid phase sintered (TLPS) joints are investigated. The results show that two main intermetallic compounds (IMCs, Cu6Sn5 and Cu3Sn) formed in the joints. The Cu6Sn5 ratio generally decreased with increasing sintering time, Cu powder fraction, and thermal treatment. The void ratio of the high-Cu-fraction joints decreased and increased with increasing sintering and thermal stressing durations, respectively, whereas the low-Cu-fraction counterparts were stable. We also found that the shear strength increased with increasing thermal treatment time, which resulted from the transformation of Cu6Sn5 and Cu3Sn. Such findings could provide valuable information for optimizing the TLPS process and assuring the high reliability of electronic devices.

Thermal Conductivity and Sintering Mechanism of Aluminum/Diamond Composites Prepared by DC-Assisted Fast Hot-Pressing Sintering.

A novel DC-assisted fast hot-pressing (FHP) powder sintering technique was utilized to prepare Al/Diamond composites. Three series of orthogonal experiments were designed and conducted to explore the effects of sintering temperature, sintering pressure, and holding time on the thermal conductivity (TC) and sintering mechanism of an Al-50Diamond composite. Improper sintering temperatures dramatically degraded the TC, as relatively low temperatures (≤520 °C) led to the retention of a large number of pores, while higher temperatures (≥600 °C) caused unavoidable debonding cracks. Excessive pressure (≥100 MPa) induced lattice distortion and the accumulation of dislocations, whereas a prolonged holding time (≥20 min) would most likely cause the Al phase to aggregate into clusters due to surface tension. The optimal process parameters for the preparation of Al-50diamond composites by the FHP method were 560 °C-80 MPa-10 min, corresponding to a density and TC of 3.09 g cm-3 and 527.8 W m-1 K-1, respectively. Structural defects such as pores, dislocations, debonding cracks, and agglomerations within the composite strongly enhance the interfacial thermal resistance (ITR), thereby deteriorating TC performance. Considering the ITR of the binary solid-phase composite, the Hasselman-Johnson model can more accurately predict the TC of Al-50diamond composites for FHP technology under an optimal process with a 3.4% error rate (509.6 W m-1 K-1 to 527.8 W m-1 K-1). The theoretical thermal conductivity of the binary composites estimated by data modeling (Hasselman-Johnson Model, etc.) matches well with the actual thermal conductivity of the sintered samples using the FHP method.

Effect of phase evolution and pyroplastic formation behavior on glass-ceramic foam derived from silicomanganese slag and feldspar tailings

Utilizing solid waste to produce foam material has become a significant direction in resource recycling. This study focuses solely on using silicomanganese slag and feldspar tailings as raw materials, utilizing a powder sintering process to produce autocatalytic foaming glass–ceramic foam. We formulated four mix ratios based on the compositional characteristics of the silicomanganese slag and feldspar tailings. The study then explored the evolution of phase formation and macroscopic pore morphology under varying preparation temperatures. Moreover, through multi-phase co-melting experiments, this work unveiled the high-temperature molten mass generation mechanism between silicomanganese slag and feldspar tailings. Our findings reveal that the glass melt generated from silicomanganese slag particles and the 'solid–liquid erosion behavior' of feldspar particles play pivotal roles in the formation of high-temperature co-melt substances. As the sintering temperature increases, the mixed powder exhibits three different states of high-temperature molten mass, which are the reasons for the evolution of the macroscopic pore structure. It is suggested that the addition of silicomanganese slag should be less than 50 %, and the heat treatment temperature should be below 1145 °C in preparing glass–ceramic foam with silicomanganese slag and feldspar tailings. The sample with 40 % silicomanganese slag and 60 % feldspar tailings, treated at 1140 °C, yielded glass–ceramic foam with a compressive strength of 1.68 MPa, apparent density of 0.279 g/cm3, porosity of 81.7 %, and thermal conductivity of 0.074 W·m−1·K−1. In summary, this study provides a novel approach for the high-value utilization of silicomanganese slag and offers an economical route for developing lightweight, eco-friendly structural materials.

Rapid densification of low-positive thermal expansion Al2W3O12 ceramics

The Al2W3O12 ceramic with near-zero thermal expansion is a promising candidate for thermal shock resistance applications. However, the often-reported microcracks in the microstructure hinder the practical use of Al2W3O12. This work focuses on optimising the material processing—powder synthesis, calcination, milling, and sintering—to prepare dense polycrystalline Al2W3O12 with fine microcrack-free microstructure. The amorphous powder produced by the co-precipitation method had the highest specific surface area reported for this material, resulting in improved sinterability. Two rapid sintering methods were employed—Rapid Pressure-less Sintering and Spark Plasma Sintering. Both methods led to shorter processing times and lower sintering temperatures. However, Al2W3O12 samples produced by Spark Plasma Sintering were nearly fully dense with a density of ∼98% of theoretical density, the best value reported to date for this phase. The obtained fine crack-free microstructure of the samples led to a superior mechanical response compared to that achieved by Rapid Pressure-less Sintering.

A facile one step route that introduces functionality to polymer powders for laser sintering

Laser Sintering (LS) is a type of Additive Manufacturing (AM) exploiting laser processing of polymeric particles to produce 3D objects. Because of its ease of processability and thermo-physical properties, polyamide-12 (PA-12) represents ~95% of the polymeric materials used in LS. This constrains the functionality of the items produced, including limited available colours. Moreover, PA-12 objects tend to biofoul in wet environments. Therefore, a key challenge is to develop an inexpensive route to introduce desirable functionality to PA-12. We report a facile, clean, and scalable approach to modification of PA-12, exploiting supercritical carbon dioxide (scCO2) and free radical polymerizations to yield functionalised PA-12 materials. These can be easily printed using commercial apparatus. We demonstrate the potential by creating coloured PA-12 materials and show that the same approach can be utilized to create anti-biofouling objects. Our approach to functionalise materials could open significant new applications for AM.

Study of Galena Ore Powder Sintering and Its Microstructure

Galena is a natural mineral enriched with lead sulfide (PbS). It typically forms in hydrothermal veins associated with igneous rocks and can also occur as a gangue mineral in other ore deposits. PbS is of special importance for scientific research applications due to the possibility of tuning its semiconductor energy gap using nanotechnology in conjunction with powder metallurgy as an easy, controllable production route. In this paper, almost pure PbS was successfully produced starting from a high ratio of PbS phase galena ore. As-received galena lumps were roughly pulverized and milled to produce four particle size ranges of 38, 63, 125, and 250 µm prior to compaction and sintering in a vacuum (pre-flushed with argon gas). SEM coupled with the EDAX analysis unit was employed to investigate the microstructure and chemical composition of the as-received galena and the subsequent products after sintering. The chemical analysis confirmed the high ratio of PbS compound in the as-received galena and sintered products with approximately 85% Pb and 13% S mass ratio. The sintering process of the galena powder was carried out at different values of temperature, time, and compaction pressure. Additionally, the effect of length to diameter ratio of compacted and sintered samples was investigated. XRD analysis confirmed the existence of the PbS phase in the as-received and sintered samples at 700 °C with approximately 98 wt.%, as well as a new phase that is formed at 800 °C with a lower percentage. The micro-hardness of the as-received and sintered samples was measured and compared with the as-received galena ore. The results showed a significant reduction in the hardness of sintered galena powder compared with the bulk as-received galena by 52%. Furthermore, a relative sintered density of 99.3% for the as-received galena density signifies a novel result using powder metallurgy techniques.

Compact SiOC ceramic composite nanofiltration membranes by slow dip coating for water purification

In this work, novel silicon oxycarbide (SiOC) ceramic composite nanofiltration membranes with high performance were successfully synthesized at low sintering temperatures by gradual dip-coating approach for the first time. The slow dip-coating process facilitated improved wetting and even spreading of the precursor solution across the substrate. This controlled method ensured the formation of uniform and continuous SiOC ceramic membranes without the introduction of substantial defects or irregularities. Consequently, the systematic approach minimized the occurrence of pinholes, cracks and other structural imperfections in the SiOC ceramic membranes. The synthesis of the SiOC separation layer involved the sintering of polymeric polysilicon as precursor, which caused a lower decomposition temperature in contrast to the conventional method employing silicon carbide powder for sintering. The specific feature enabled a sharp decrease in the sintering temperature which was required for the formation of the separation layer. The prepared SiOC ceramic composite nanofiltration membranes exhibited good permeability with high rejection of dye molecules (>95%) and maintained stable water flux for at least 24 h, showing good long-term stability during cross-flow filtration. This progress provides new approach for the fabrication of compact ceramic nanofiltration membranes with low sintering temperatures, high separation efficiency and enhanced stability.

Hygroscopicity in Epoxy Powder Composites

Epoxy powders offer a low-cost way of manufacturing thick-section composite parts, such as those found in wind and tidal turbines. Currently, their processing cycle includes a lengthy drying stage (≥15 h) to remove ambient moisture. This drying stage prevents void defect formation and, thereby, a reduction in mechanical properties; however, it constitutes up to 60% of the processing time. Little research has been published which studies the drying stage or its optimisation. In the present work, experimental and simulated analyses are used to investigate the effects of hygroscopicity in epoxy powder composites. Tests are performed to quantify the void content of dried and undried laminates and to measure its impact on transverse flexural strength. Dynamic vapour sorption analysis is used to study the sorption behaviour of the epoxy powder. It is shown that the epoxy powder is slightly hygroscopic (1.36 wt%) and exhibits sorption behaviour that is characteristic of glassy polymers. This results in up to 4.8% voids (by volume) if processed in an undried state, leading to a 43% reduction in transverse flexural strength. A modified linear driving force model is fitted to the desorption data and then implemented in existing process-simulation tools. The drying of a thick epoxy powder composite section is simulated to investigate the influence of powder sintering on the duration of the drying stage. Process simulations reveal that a standard drying cycle prematurely sinters the powder, which inhibits moisture release. By maintaining the powder state, simulations show that the drying cycle can be reduced to 5 h.

Ultrahigh-strengthened intragranular κ-Al2O3 nanoparticles dispersed Mo composite prepared via SPS sintering of core-shell Mo nanocomposite powder

Oxide dispersion strengthening (ODS) is an effective method to improve the mechanical properties of Mo-based materials. However, the mechanical properties of traditional ODS-Mo composites are always limited by the coarsening and intergranular distribution of second-phase particles. In this work, an effective nano-reinforcement dispersion strategy was developed to fabricate an ODS-Mo composite with ultrafine grain and intragranular distribution of second-phase particles. Core-shell structural Mo nanocomposite powders, with internally distributed sub-10 nm Al2O3 dispersoids, were prepared by nano atomization doping (AD) followed by a chemical vapor transport growth strategy. Then, ODS-Mo composites with ultra-fine Mo grain (below 700 nm) and high-density intragranular κ-Al2O3 (below 20 nm) nanoparticles were prepared via spark plasma sintering (SPS), in which a coherent interface between κ-Al2O3 and Mo matrix was formed. The composites present remarkably improved hardness (above 500 HV), bend strength, and compressive yield strength (above 1664 MPa) at room temperature, with a suitable strain to fracture of 27.1 %. The calculation of strengthening mechanisms indicates that the enhancement was mainly attributed to the intragranular κ-Al2O3 nanoparticles. This nano-sized reinforcement distributed within the grain can more effectively pin dislocations and achieve dispersion strengthening in ODS-Mo composites. Therefore, this strategy can efficiently construct intragranular second-phase nanoparticles and open up new avenues to fabricate high-performance ODS-Mo composites.

Sintering densification behavior of nano-ITO powder with high oxygen vacancy induced via plasma stimulation

With the growing electronics display industry, indium tin oxide (ITO) powder plays a crucial role in target preparation. To enhance the sintering performance of ITO powder, a high-activity nano ITO powder was prepared using plasma spraying, and compared to a commercial powder. This study aimed to investigate the effects of different preparation methods on powder morphology, dispersibility, oxygen vacancy concentration, and the physical properties of powders on target density, surface morphology, grain size, element distribution, and solid solubility. The results indicate that the plasma spraying high-activity nano ITO powder exhibits better dispersibility, higher oxygen vacancy concentration, and a specific surface area of up to 88.39 m2/g compared to the commercial powder. Under normal pressure sintering conditions, plasma spraying nanoscale ITO powder exhibits higher sintering activity, leading to easier densification of ceramic targets with reduced porosity, more uniform surface grains, and significant stepped growth between surface grains. High-resolution transmission electron microscopy (HRTEM) analysis suggests that targets prepared with high-activity powder have greater tin solubility in indium oxide compared to traditional commercial ceramic targets. This article further explores the relationship between powder specific surface area, oxygen vacancy, and sintering activity, along with the formation mechanism of high oxygen vacancy.

A numerical model for investigating the effect of material and process parameters on the running of flash sintering

Flash sintering of ceramic powders is a complex process that involves electric, thermal and mechanical phenomena. Modelling should help to better understand and control it, with a view to producing safe and homogeneous parts. A finite element model that couples all relevant phenomena, including the densification and shrinkage of the sintering material, has been worked out for this purpose. This model is used here to investigate the flash sintering of disk-shaped parts. Particular attention is paid to the appearance and growth of heterogeneities within the part, known as hot spots, which are highly dependent on material and process parameters. Leads are provided for limiting the development of such heterogeneities.

Study on the Effect of Nanoporous Copper Particle Size on Copper-Based Azide.

Preparing copper-based azide by in situ reaction is well-suited for MEMS processing technology and holds promising prospects in the field of MEMS micro-initiators. This study involved the preparation of porous copper with particle sizes of approximately 30 nm, 60 nm and 100 nm through powder sintering. These were used as precursors for a gas-solid in situ azide reaction to produce copper-based azide with varying morphologies and compositions. Copper-based azide micro-initiators were designed, and their output performance was evaluated using CL-20 and HNS-IV explosives. Analytical results revealed that the product from the reaction of the 100 nm precursor exhibited a lumpy and uneven structure with a conversion rate of 90.36%. The product from the 60 nm precursor reaction had a dense surface with a conversion rate of 94.56%, while the 30 nm precursor resulted in a needle-like form with a conversion rate of 92.82%. Detonation experiments demonstrated that the copper-based azide micro-initiators prepared with 100 nm of a porous copper precursor exhibited unstable output performance, requiring a 1.6 mg charge to successfully detonate CL-20 explosives. On the other hand, copper-based azide micro-initiators prepared from 60 nm and 30 nm of porous copper precursors exhibited stable output performance. A charge of 0.8 mg was adequate for reliably and consistently detonating CL-20 and HNS-IV explosives. The reduced particle size of the precursor enhanced the output performance of the copper-based azide micro-initiators, providing increased energy redundancy during detonation and improving overall usage reliability.

Combined Effect of Particle Reinforcement and T6 Heat Treatment on the Compressive Deformation Behavior of an A357 Aluminum Alloy at Room Temperature and at 350 °C

Solid state sintering of cast aluminum powders by resistance heating sintering (RHS), also known as spark plasma sintering or field-assisted sintering technique, creates a very fine microstructure in the bulk material. This leads to high performance material properties with an improved strength and ductility compared to conventional production routes of the same alloys. In this study, the mechanical behavior of an RHS-sintered age-hardenable A357 (AlSi7Mg0.6) cast alloy and a SiCp/A357 aluminum matrix composite (AMC) was investigated. Aiming for high strength and good wear behavior in tribological applications, the AMC was reinforced with a high particle content (35 vol.%) of a coarse particle fraction (d50 = 21 µm). Afterwards, separated and combined effects of particle reinforcement and heat treatment were studied under compressive load both at room temperature and at 350 °C. At room temperature compression, the strengthening effect of precipitation hardening was about twice as high as that for the particle reinforcement, despite the high particle content. At elevated temperatures, the compressive deformation behavior was characterized by simultaneously occurring temperature-activated recovery, recrystallisation and precipitation processes. The occurrence and interaction of these processes was significantly affected by the initial material condition. Moreover, a rearrangement of the SiC reinforcement particles was detected after hot deformation. This rearrangement lead to a homogenized dispersion of the reinforcement phase without considerable particle fragmentation, which offers the potential for secondary thermo-mechanical processing of highly reinforced AMCs.

Porous titanium alloys for medical application: Progress in preparation process and surface modification research

Excellent mechanical properties and biocompatibility are the most sought-after attributes in biomedical materials for the regeneration of damaged tissues. However, conventional dense titanium alloys possess a modulus significantly higher than that of human tissues, leading to potential stress-shielding effects. Medical porous titanium alloys can reduce the elastic modulus of the material, promote tissue fixation and vascular regeneration, and improve the suitability for human tissue properties. With the continuous development of technology, the preparation process of porous titanium alloys has undergone a series of multifaceted transformations and improvements in the aspects of powder sintering, fiber preparation, and additive manufacturing processes, and its structural characteristics and mechanical properties are constantly evolving in a controllable direction. Alongside the enhancement of the material’s mechanical properties through porous design, optimization of the properties at the implant-tissue interface also leads to improved antimicrobial and osteogenic properties of porous titanium. Due to the complex internal structure of porous titanium alloys, surface modification is mainly carried out in fluid media, which is realized by morphological modification and the introduction of functional substances. Over time, the surface modification of porous titanium alloys for medical applications has progressed from morphological modification and introduction of chemical composition to the loading of bioactive substances. This evolution aims to enhance safety and efficiency in the use of these materials. This paper reviews the preparation and surface modification processes of porous titanium alloys for medical use and summarizes the advantages, disadvantages, and influencing factors among different processes, with a view to providing new ideas for the development of porous implants for medical use.

Effect of Copper Content on the Microstructure and Properties of the Sintered Porous Aluminum Wick

Porous aluminum has been widely used as a wick, an electrode, and in other products, due to its advantages of a light weight, pore uniformity, and corrosion resistance. However, the dense alumina layer on the surface of Al powder hinders its densification during sintering. In this paper, porous aluminum was prepared via loose powder sintering with the addition of Cu as a sintering aid. The effects of Cu content on the microstructure and wick properties of the porous aluminum were investigated. The results showed that, with increasing Cu content, the porosity and capillary properties of porous Al decreased, while the compressive strength improved. The optimal Cu content was determined to be 3 wt.% and the prepared porous Al has a porosity of 47.5%, plateau stress of 11.82 MPa, and capillary characteristic parameter of 6.72 × 10−8 N, meeting the requirements for wicks in heat pipes. These findings contribute to the demand for the lightweight design of heat pipes for aerospace applications.

Fabrication of a stacked Archimedean spiral reactor with porous carbon walls using 3D-printed PLA as internal sacrificial template and carbonized whey powder as porous carbon matrix

This study introduces a method to create porous carbon structures with intricate internal voids. 3D-printed PLA acts as an internal sacrificial template, combined with carbonized whey powder as the porous carbon matrix. Sintering whey powder at 150°C yields solid pieces that, upon carbonization, result in highly porous carbon objects while maintaining the original mold shape. Temperature control ensures successful whey powder sintering before PLA melting. The use of PLA sacrificial templates, along with whey carbonization, allows for developing devices with finely tailored internal voids, as demonstrated through a double Archimedean spiral reactor with porous carbon walls.Graphical abstract

Microstructure and tribological properties of copper/graphite composites with Ti3AlC2 addition prepared by rapid hot press sintering

Copper/graphite composites are widely used due to their excellent electrical, thermal, and self-lubricating properties. However, their mechanical and anti-wear properties are usually limited by brittle graphite and soft copper matrix. Enhancing the properties of composites by further introducing harder ceramic particles is an effective way. However, these particles are usually poorly wetted to metal and can easily separate from the matrix during friction, causing more severe abrasive wear. Herein, TiCx-reinforced Cu/graphite composites were prepared by rapid hot press sintering of a hybrid powder consisting of Ti3AlC2, Cu-plated graphite, and Cu powder. Al atoms diffused out of Ti3AlC2 and solid-solved into the Cu matrix during the sintering process, forming TiCx and Cu(Al) alloys. Such in situ formed TiCx particles have good interfacial bonding with the matrix. With the increase of Ti3AlC2 addition from 0 to 15 wt%, the average grain size of the composites decreased, the relative density increased, the hardness became larger, the electrical conductivity decreased, and the average coefficient of friction were all maintained at about 0.14, but the wear rate was reduced from 3.6569 × 10−4 mm3·N−1·m−1 to 9.35013 × 10−6 mm3·N−1·m−1. The in situ Ti3AlC2-derived TiCx particles improved the deformation resistance and shear resistance of the Cu/graphite composites, thus contributing to the wear resistance. This research may offer fresh perspectives on how to enhance the properties of copper/graphite composites.

Controllable Technology for Thermal Expansion Coefficient of Commercial Materials for Solid Oxide Electrolytic Cells.

Solid oxide electrolysis cell (SOEC) industrialization has been developing for many years. Commercial materials such as 8 mol% Y2O3-stabilized zirconia (YSZ), Gd0.1Ce0.9O1.95 (GDC), La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), La0.6Sr0.4CoO3-δ (LSC), etc., have been used for many years, but the problem of mismatched thermal expansion coefficients of various materials between cells has not been fundamentally solved, which affects the lifetime of SOECs and restricts their industry development. Currently, various solutions have been reported, such as element doping, manufacturing defects, and introducing negative thermal expansion coefficient materials. To promote the development of the SOEC industry, a direct treatment method for commercial materials-quenching and doping-is reported to achieve the controllable preparation of the thermal expansion coefficient of commercial materials. The quenching process only involves the micro-treatment of raw materials and does not have any negative impact on preparation processes such as powder slurry and sintering. It is a simple, low-cost, and universal research strategy to achieve the controllable preparation of the thermal expansion coefficient of the commercial material La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) through a quenching process by doping elements and increasing oxygen vacancies in the material. Commercial LSCF materials are heated to 800 °C in a muffle furnace, quickly removed, and cooled and quenched in 3.4 mol/L of prepared Y(NO3)3. The thermal expansion coefficient of the treated material can be reduced to 13.6 × 10-6 K-1, and the blank sample is 14.1 × 10-6 K-1. In the future, it may be possible to use the quenching process to select appropriate doping elements in order to achieve similar thermal expansion coefficients in SOECs.

Customized linear refractive index GRIN prepared by rapid sintering of multilayer chalcogenide glass powders

AbstractGradient refractive index (GRIN) lenses have recently become a hot topic in compact imaging and achromaticity. Current research focuses on exploiting GRIN materials and upgrading the preparation technology. In this study, infrared (IR) GRIN materials were successfully prepared by spark plasma sintering of multilayer chalcogenide glass (ChGs) powders. ChGs powders were derived from the Ge20Se80‐xTex series glasses (x = 0–28 mol%) with a maximum refractive index difference (Δn) of 0.25. The similar glass transition temperatures (ΔTg < 10°C) promote efficient sintering of multilayer powders. Six‐, eight‐, and ten‐layer powders of different compositions were rapidly sintered with a thickness of 600, 300, and 200 µm per layer, respectively. Such tiny layer thickness enables the GRIN materials to exhibit nearly linear refractive index variation. Energy dispersive spectroscopy line scans well characterized the gradient variation of GRIN. The maximum deviation of the refractive index gradient from the fitted target line was only ± 0.016 for the 10‐layer GRIN. The sintered GRIN shows excellent IR transparency, which is of great interest for mid‐wave and long‐wave infrared imaging.