Pressure-assisted Sintering Research Articles

Mechanical property and heavy metal leaching behavior enhancement of municipal solid waste incineration fly ash during the pressure-assisted sintering treatment

The conventional sintering process of municipal solid waste incineration (MSWI) fly ash is always energy intensive. The process forms a cracked structure because of the difficulty in forming the liquid phase to enhance the mass transfer process. Therefore, exploring a new disposal method to simultaneously decrease the sintering temperature and improve the mechanical and heavy metal leaching properties of sintered samples is necessary. In this study, a pressure-assisted sintering treatment was introduced to dispose fly ash by varying the chemical composition and mechanical pressure at relatively low temperatures (300–500 °C). The results revealed that the compressive strength of treated samples increased with the CaO/SiO2 molar ratio increasing from 0.5 to 1.0, and a maximum value of 238.28 ± 8.50 MPa was obtained. The heavy metal leaching concentration results demonstrated a low risk of contamination in the treated samples. Microstructure analyses suggested that the densification process was enhanced with increased mechanical pressure, and the formed calcium silicates and aluminosilicates positively affected the compressive strength. Moreover, smaller crystal lattices were observed during aggregation formation, suggesting the restraint of anomalous crystal growth, which accelerated the densification process and increased the compressive strength. Moreover, the mass transfer process during the pressure-assisted sintering process was enhanced compared with the conventional thermal process, which was reflected by the transformation of elements from homogeneous to heterogeneous distribution. Therefore, the improved mechanical properties and leaching behavior of heavy metals were attributed to the densified microstructure, formation of new minerals, and enhanced driving force during the pressure-assisted sintering process. These findings suggest that pressure-assisted sintering is a promising method for maximizing the reutilization and minimizing the energy consumption simultaneously to dispose fly ash.

Modelling of elimination of strength-limiting defects by pressure-assisted sintering at low stress levels

The structural reliability of sintered products depends on large defects introduced during powder processing, which cannot be removed by pressureless sintering. Here, we present a model how a large single ellipsoidal void is deformed, and finally disappears by pressure-assisted sintering. Taya-Seidel’s model is applied to predict the shrinkage of a large void in a compressible linear viscous material by using bulk viscosity, shear viscosity, and sintering stress that are determined experimentally for sintering of alumina powder at low stress levels. The application of mechanical stress promotes the densification rate. Its effect is maximum for hot isostatic pressing (HIP) and minimum for sinter forging. The effect is intermediate for hot pressing (HP) and spark plasma sintering (SPS), because the hydrostatic component of stress varies with densification. While a crack-like defect can be removed during densification, a spherical void must be eliminated by shear deformation in the final stage during dwell time.

Numerical investigation into pressure-assisted sintering using fully coupled mechano-diffusional phase-field model

To investigate the effect of contact stress between particles on the evolution of microstructure in pressure-assisted sintering, an improved fully coupled mechano-diffusional phase-field model (PFM) was presented, in which the design of the interpolation method for elastic modulus ensured that the normalised contact stress distribution was approximated to that in conventional contact problems. The immediate advantage was that the consequent strain energy in the contact and neck area could be directly characterised without additional approximate treatment.The simulation results showed that the sintering neck evolved under the joint action of the mechanical mechanism of elastic contact and the mass diffusion mechanism for pressure-assisted sintering. Elastic contact played a dominant role before significant neck growth in the early stage of sintering. As the neck grew, mass diffusion became the dominant factor, but the normalised contact stress distribution in the contact area always satisfied that of the elastic contact problem. Owing to the additional contribution of strain energy, the greater the pressure, the faster the sintering neck growth. Further study showed that the diffusion-induced stress hindered the growth of the sintering neck according to the presented fully coupled model presented, and that the strain energy stored within the contact area will accelerate the sintering process.

Optimization of Process Parameters of Boron Carbide-Reinforced Al-Zn-Mg-Cu Matrix Composite Produced by Pressure-Assisted Sintering

Optimization of Process Parameters of Boron Carbide-Reinforced Al-Zn-Mg-Cu Matrix Composite Produced by Pressure-Assisted Sintering

Promoting the sintering densification and mechanical properties of gas-atomized high-entropy alloy powder by adding boron

Powder metallurgy (PM) with pressure-assisted sintering is a versatile process for producing high-entropy alloys (HEAs) with a fine grain size and high strength. Unfortunately, HEA powders are extremely difficult to sinter with a general press-and-sinter PM process due to the sluggish diffusion effect in HEAs. One economical means to effectively densify the PM materials is liquid phase sintering (LPS). According to the phase diagrams, boron (B) is a feasible element for facilitating LPS of an AlCoCrFeMoNi powder because B can react with Co, Cr, Fe, Mo, and Ni to generate a low-temperature liquid phase. The objective of this study was thus to conduct the first investigation of the influences of 0.8 wt% B on the sinterability, microstructure, and mechanical properties of the AlCoCrFeMoNi HEA powder. The results showed that after 1200 °C sintering, the densification behaviors of both the AlCoCrFeMoNi and AlCoCrFeMoNi+0.8B alloys were undesirable. However, adding 0.8 wt% B to AlCoCrFeMoNi and sintering at 1220 °C significantly increased the densification and reduced the porosity from 11 vol% to 1 vol%. The onset temperature for liquid generation was identified as 1205 °C by differential scanning calorimetry curve. The microstructure of the AlCoCrFeMoNi alloy sintered at 1220 °C was face-centered cubic and sigma phases. The 0.8 wt% B additive led to the generation of Mo2FeB2 boride at the expense of sigma phase. Furthermore, adding 0.8 wt% B to the AlCoCrFeMoNi alloy sufficiently improved the yield strength by 42%, ultimate compressive strength by 66%, and fracture strain by 92%, due to the LPS effect. The AlCoCrFeMoNi powder was successfully sintered by adding B to facilitate LPS. In the future, this alloy design can be also applied to the other HEA powders that contain principal elements that react with B to form a low-temperature liquid.

Efficiently sintering of MSWI fly ash at a low temperature enhanced by in-situ pressure assistant: Process performance and product characterization

Municipal solid waste incineration (MSWI) fly ash disposal is an urgent task with some technical bottlenecks. In this study, a novel pressure-assisted sintering method was employed to treat the MSWI fly ash. A series of pressure-assisted sintering experiments were carried out by varying mechanical pressures and sintering temperatures, and their properties of compressive strength, density and heavy metals leaching behavior were determined to screen out the optimal conditions. Instrumental analysis of XRF, SEM, XRD and TEM-EDX and calculation kinetics were conducted to explore the enhancement mechanism of pressure-assisted sintering. With the help of mechanical pressure, a high-strength ceramic product was produced from MSWI fly ash sintered at a low temperature (400 °C), which never occurred in the conventional low-temperature sintering process. Maximum compressive strength of 218.30 ± 4.08 MPa was obtained at 400 °C and 100 MPa, which was much higher than conventional construction materials of brick and cement. In addition, the leaching concentrations of heavy metals obtained from pressure-assisted sintering process were lower than the standard limitation. The SEM and XRD results revealed that the increased mechanical properties and the decreased heavy metals leaching concentration were mainly attributed to the increased density and crystalline degree. The kinetics calculation results indicated that the sintering activation energy was much lower than the sintering process without pressure, suggesting surface diffusion and grain boundary diffusion were main sintering mechanisms in the pressure-assisted sintering process. These findings proved that pressure-assisted sintering could be a promising method to treat fly ash together with producing high-value building materials.

Influences of TiO2 or Y2O3 doping on the homogeneity of polycrystalline Al2O3 produced by pulsed electric current sintering

ABSTRACT Influences of TiO2 or Y2O3 dopants on pressure-assisted sintering of polycrystalline Al2O3 were investigated to control the microstructure of sintered Al2O3 as well as to improve the homogeneity of transparent Al2O3 samples. The transparent polycrystalline Al2O3 samples with and without the dopants were fabricated by two-step pulsed electric current sintering (TS-PECS). By the grain boundary segregation by the dopants, both TiO2 and Y2O3 depressed the densification and grain growth processes during the sintering of polycrystalline Al2O3. The principle originalities of this work include the relationship between the homogeneities of microstructure and of optical transmittance of large-size polycrystalline Al2O3 and the consequent doping strategy for controlling of homogeneity of transparent polycrystalline Al2O3 prepared by two-step PECS. Doping of Y2O3 dramatically reduced the grain size of polycrystalline Al2O3 and improved its optical transparency. The homogeneity of the microstructure and the optical transparency of the large-sized polycrystalline Al2O3 with 0.1 mol% Y2O3 was improved. The addition of TiO2 also slightly retarded densification and reduced the grain size of the sintered Al2O3 samples. However, TiO2 dopant is not good for applications of transparent Al2O3 because it caused a dark color in transparent polycrystalline Al2O3 samples by uncertain reasons.

A pressure-assisted low temperature sintering of particulate bismuth chalcohalides BiSX (X = Br, I) for fabricating efficient photoelectrodes with porous structures

Metal mixed-anion compounds (e.g., metal oxyhalides and sulfahalides) have recently emerged as promising photo-absorber materials that harvest a wide range of solar spectrum, whereas fabrication of their porous-structured photoelectrodes for efficient wet-type photoelectrochemical (PEC) reactions remains a challenge. This prime issue is that although partial sintering between the particles in the porous electrodes is required for efficient charge transport, conventional (air-)calcinations for such sintering often cause damage to mixed-anion materials. We herein applied pressure assisted treatment (PAT) to fabrication of porous-structured photoelectrodes based on BiOI and BiSX (X=Br, I). PAT for BiSX electrodes resulted in forming particle layer of densely-packed arraignment accompanied by an obvious decrease in thickness of the particle layers, along with partial fusion of BiSX particles only around the electrode surfaces. The resulting decreased thickness was greatly larger than the theoretical one estimated as a fully filled film, supporting the presence of substantial pores to which liquid electrolytes and reactants can access. In terms of PEC performance, PAT even at 323 K, close to room temperature, for BiSX electrodes led to an apparent increase in the anodic photocurrent, strongly suggesting that PAT enhanced the electron transport through the sintered and enlarged contacts between the particles. On the other hand, PAT for BiOI electrodes did not show any improvement of the PEC performance. This stark contrast suggests that softer nature of BiSX originated from the composition of S2– compared to BiOI is one of the keys to produce an effect on fabricating intended porous structure by the pressure-assisted sintering at low temperature.

Synthesis of Bulk Zr48Cu36Al8Ag8 Metallic Glass by Hot Pressing of Amorphous Powders

The critical cooling rate necessary for glass formation via melt solidification poses inherent constraints on sample size using conventional casting techniques. This drawback can be overcome by pressure-assisted sintering of metallic glass powders at temperatures above the glass transition, where the material shows viscous-flow behavior. Partial crystallization during sintering usually exacerbates the inherent brittleness of metallic glasses and thus needs to be avoided. In order to achieve high density of the bulk specimens while avoiding (or minimizing) crystallization, the optimal combination between low viscosity and long incubation time for crystallization must be identified. Here, by carefully selecting the time–temperature window for powder consolidation, we synthesized highly dense Zr48Cu36Ag8Al8 bulk metallic glass (BMG) with mechanical properties comparable with its cast counterpart. The larger ZrCu-based BMG specimens fabricated in this work could then be post-processed by flash-annealing, offering the possibility to fabricate monolithic metallic glasses and glass–matrix composites with enhanced room-temperature plastic deformation.

Distribution and chemical species transition behavior of chlorides in municipal solid waste incineration fly ash during the pressure-assisted sintering treatment

High chloride content in municipal solid waste incineration (MSWI) fly ash is a critical restriction for the recycling and re-utilization due to the acceleration of the corrosion process in construction material. Therefore, the investigation of chloride distribution and transition behavior in fly ash is of great significance. In this study, the distribution and chemical species of chloride in fly ash were first analyzed through varying mechanical pressures (10, 30, 50, 70, and 100 MPa) and sintering temperatures (300 °C, 400 °C and 500 °C) during the pressure-assisted sintering process. Instrumental analyses of SEM-EDX, element mapping, XPS, XRD and chloride leaching concentration were conducted to explore the chloride transition behavior. The results suggested that chlorides were homogeneously distributed on the surface of fly ash particles, which was significantly influenced by the microstructure change during the pressure-assisted sintering process. In addition, the chloride migration was enhanced by the pore shrinkage under different mechanical pressures and the increased atomic mobility at various temperatures. The mineral results showed that the main crystals in raw fly ash were NaCl, KCl and CaClOH, and the minerals were transformed into more complex and stable crystal phases, such as Ca10(Si2O7)2SiO4Cl2(OH)2 and Ca10(SiO4)3(SO4)3Cl. The transformation of chloride from soluble salts to stable Cl-containing minerals is beneficial to the immobilization of chloride, which suggested that pressure-assisted sintering could effectively decrease the chloride leaching concentration. This study provides a further understanding of chloride transition and distribution behavior during pressure-assisted sintering treatment.

A Lamination Model for Pressure-Assisted Sintering of Multilayered Porous Structures

This work describes a lamination model for pressure-assisted sintering of thin, multilayered, and porous structures based on the linear viscous constitutive theory of sintering and the classical laminated plate theory of continuum mechanics. A constant out-of-plane normal stress is assumed in the constitutive relation. The lamination relations between the force/moment resultants and the strain/curvature rates are presented. Numerical simulations were performed for a symmetric tri-layer laminate consisting of a 10% gadolinia doped ceria (Ce0.9Gd0.1O1.95-δ) composite structure, where porous layers were adhered to the top and bottom of a denser layer under uniaxially-applied pressures and the sinter forging conditions. The numerical results show that, compared with free sintering, the applied pressure can significantly reduce the sintering time required to achieve given layer thicknesses and porosities. Unlike free sintering, which results in a monotonic decrease of the laminate in-plane dimension, pressure-assisted sintering may produce an in-plane dimension increase or decrease, depending on the applied pressure and sintering time. Finally, the individual layers in the laminate exhibit different stress characteristics under pressure-assisted sintering.

Pressure-assisted sintering and characterization of Nd:YAG ceramic lasers

Spark plasma sintering (SPS) is an advanced one-stage, rapid, near-net shape densification technique combining uniaxial pressure with resistive heating. Various transparent ceramics have been successfully fabricated by SPS, despite the existence of inherent carbon contamination and residual pores. Due to the disk-shape of SPS-processed samples, the technique may be suited for producing thin-disk ceramic laser materials. Nevertheless, an in-depth study of these materials has never been reported. With that goal in mind, the major focus of this study was to characterize the laser performance of Nd:YAG ceramics fabricated by one-stage SPS under conventional (60 MPa) and high (300 MPa) applied pressures. In addition to measuring the lasing slope efficiency and threshold, the passive losses associated with each sample were also evaluated. Surprisingly, it was found that in-line transmittance spectra do not provide accurate predictions of laser performance due to the nature of residual porosity. Moreover, homogeneity and beam quality were assessed, and comparisons were drawn between conventional and high-pressure SPS ceramics. This study lays the groundwork for the future of laser materials fabricated by SPS or similar pressure-assisted techniques.

In-air sintering of copper nanoparticle paste with pressure-assistance for die attachment in high power electronics

There is a high demand for the implementation of metallic nanoparticle (NP) sintering technology for die attach in high-power electronics. The performance of this technology is superior to that of the technology involving the use of lead-free solders. Although Cu NP paste is potentially a low-cost material, it faces the challenge of oxidation during sintering. This may result in a significant deterioration of the mechanical, thermal, and electrical properties. Therefore, there are limited studies on the in-air sintering of Cu NP pastes. The present study demonstrated the in-air pressure-assisted low-temperature sintering of a commercial Cu NP paste. Furthermore, the sintering was performed without using a protective atmosphere, unlike that in most of the previously reported investigations. The sintering behavior was investigated at three levels of temperatures (200–240 °C) and five levels of pressures (5–25 MPa). The joints that were sintered at high temperatures and pressures exhibited condensed microstructures and high bonding strengths. High sintering temperatures accelerated the diffusion between Cu NPs, while high sintering pressure facilitated the removal of evaporated organic compounds and the air between NPs. This not only facilitated sintering but also prevented the oxidation of Cu. The optimal sintering conditions promoted the formation of 3D connections between the Cu NPs, thereby increasing the shear strength of the sample. The samples that were sintered at 240 °C and 10 MPa experiences the highest increase in the shear strength, furthermore, the microstructures were optimized under this condition. The shear strength of 28.1 ± 8.47 MPa was achieved under this condition, which satisfied the requirements for die attach in high power electronics applications, moreover, the sintering process was moderate and cost-effective. Therefore, the optimal sintering temperature and pressure for the in-air sintering of the Cu NP paste was concluded to be 240 °C and 10 MPa, respectively. The results indicated that in-air sintering with pressure assistance can be applied for die attach in the high-power electronics.

Elucidation of Densification Experienced By LLZO Solid Electrolytes

In order to improve the energy density and minimize the safety concerns associated with the liquid electrolytes used in the present day lithium ion batteries, solid electrolytes are being actively considered for use in the next generation energy storage devices.[1] Among various ceramic based solid state electrolytes being investigated, Li7La3Zr2O12 (LLZO) with a garnet crystal structure demonstrates very high ionic conductivity and is extremely stable against lithium.[2] Hence, LLZO is being actively considered as the electrolyte for next generation lithium ion batteries. However, due to poor processing of these ceramics, they contain pores and voids.[3] These open spaces within the bulk not only decrease the effective conductivity of the electrolyte, but also provides open pathways for Li dendrites to propagate. Presence of pores within the ceramic structure can be estimated from the final relative density of the solid electrolyte. High temperature sintering induced densification of the LLZO particles are conducted to improve the relative density of the final solid electrolyte, and this process helps to minimize the void space within the structure.[3, 4] Sometime external pressures are applied to further improve the relative densities.[5] Even though several experimental studies have been conducted for constructing the LLZO solid electrolytes, very little effort has been devoted to actually understand the physical mechanisms that play major role during the densification process.In the present context, a computational methodology has been developed at the mesoscale level to understand the impact of surface and grain-boundary diffusion mechanisms on the overall grain ripening and densification experienced by the LLZO solid electrolytes during the sintering process conducted at elevated temperatures. During densification, the extent of increase in relative density depends on the activation energy associated with the diffusive mass transport processes. Here, the activation energy for LLZO will be estimated from direct comparison of relative densities obtained from the developed computational model and experimental observations.[6] Some strategies, in terms of particle sizes and size distributions, that can lead to improved relative densities within the LLZO solid electrolytes, will also be discussed in this presentation. References Zhang, Z., et al., New horizons for inorganic solid state ion conductors. Energy & Environmental Science, 2018. 11(8): p. 1945-1976. Ramakumar, S., et al., Lithium garnets: synthesis, structure, Li+ conductivity, Li+ dynamics and applications.Progress in Materials Science, 2017. 88: p. 325-411. Rangasamy, E., J. Wolfenstine, and J. Sakamoto, The role of Al and Li concentration on the formation of cubic garnet solid electrolyte of nominal composition Li7La3Zr2O12. Solid State Ionics, 2012. 206: p. 28-32. Sharafi, A., et al., Controlling and correlating the effect of grain size with the mechanical and electrochemical properties of Li7La3Zr2O12 solid-state electrolyte. Journal of Materials Chemistry A, 2017. 5(40): p. 21491-21504. Dzepina, B., D. Balint, and D. Dini, A phase field model of pressure-assisted sintering. Journal of the European Ceramic Society, 2019. 39(2-3): p. 173-182. Shin, R.-H., et al., Effect of Li3BO3 additive on densification and ion conductivity of garnet-type Li7La3Zr2O12 solid electrolytes of all-solid-state lithium-ion batteries. Journal of the Korean Ceramic Society, 2016. 53(6). Figure 1

Influence of pressure and dwell time on pressure‐assisted sintering of calcium cobaltite

AbstractCalcium cobaltite Ca3Co4O9, abbreviated Co349, is a promising thermoelectric material for high‐temperature applications in air. Its anisotropic properties can be assigned to polycrystalline parts by texturing. Tape casting and pressure‐assisted sintering (PAS) are a possible future way for a cost‐effective mass‐production of thermoelectric generators. This study examines the influence of pressure and dwell time during PAS at 900°C of tape‐cast Co349 on texture and thermoelectric properties. Tape casting aligns lentoid Co349. PAS results in a textured Co349 microstructure with the thermoelectrically favorable ab‐direction perpendicular to the pressing direction. By pressure variation during sintering, the microstructure of Co349 can be tailored either toward a maximum figure of merit as required for energy harvesting or toward a maximum power factor as required for energy harvesting. Moderate pressure of 2.5 MPa results in 25% porosity and a textured microstructure with a figure of merit of 0.13 at 700°C, two times higher than the dry‐pressed, pressureless‐sintered reference. A pressure of 7.5 MPa leads to 94% density and a high power factor of 326 µW/mK2 at 800°C, which is 11 times higher than the dry‐pressed reference (30 MPa) from the same powder.

Effect of Thermomechanical Treatment Conditions on the Structure and Properties of MgB2

The effect of pressure-assisted sintering and preliminary densification by hydrostatic pressure of commercial powder on the microstructure and superconducting properties of MgB2 ceramics is studied in this work. Various thermomechanical actions are shown to result in the formation of different structural defects, which can be considered probable centers of magnetic flux pinning.

Fundamental principles of spark plasma sintering of metals: part III – densification by plasticity and creep deformation

ABSTRACT The mechanisms of densification during spark plasma sintering (SPS) of spherical copper particles are investigated both experimentally and analytically. Experimentally measured densification rates are compared to expected contributions to densification coming from Coble creep, Nabarro-Herring creep, power-law creep, and pressure-assisted sintering described by the two-particle model. The results indicate densification by Coble creep or a low-temperature version of power-law creep in the investigated range of temperature and pressure at a relative density of 70%. The findings are supported by (i) activation energies obtained for various conditions using the Dorn-method as well as (ii) the pressure dependence of the densification rate.

Thermal kinetic and mechanical behaviors of pressure-assisted Cu nanoparticles sintering: A molecular dynamics study

A molecular dynamics (MD) simulation was performed on the coalescence kinetics and mechanical behavior of the pressure-assisted Cu nanoparticles (NPs) sintering at low temperature. To investigate the effects of sintering pressure and temperature on the coalescence of the nanoparticles, sintering simulations of two halve Cu NPs were conducted at the pressure of 0–300 MPa and the temperature of 300–500 K. A transition of the dominant coalescence kinetics from slight surface diffusion to intensive grain boundary diffusion and dislocation driven plastic flows were found when pressure was applied. Furthermore, atomic trajectories showed the effect of temperature on sintering was strongly dependent on the microstructures of Cu NPs. The atomic diffusion around defects can be significantly promoted by the elevated temperature. Additionally, based on the sintered structures, uniaxial tension simulation was implemented with a constant strain rate. Stress–strain curves and evolution of dislocation activities were derived. Improved mechanical behaviors, including larger elastic modulus and larger tensile strength, were obtained in the structure sintered under higher pressure and temperature. Among this study, sintering temperature and pressure consistently exhibited the same relative impact on affecting both coalescence and the mechanical properties of the sintered structure.

Discrete Element Framework for Determination of Sintering and Postsintering Residual Stresses of Particle Reinforced Composites.

In this paper, the discrete element method framework is employed to determine and analyze the stresses induced during and after the powder metallurgy process of particle-reinforced composite. Applied mechanical loading and the differences in the thermal expansion coefficients of metal/intermetallic matrix and ceramic reinforcing particles during cooling produce the complex state of stresses in and between the particles, leading to the occurrence of material defects, such as cracks, and in consequence the composite degradation. Therefore, the viscoelastic model of pressure-assisted sintering of a two-phase powder mixture is applied in order to study the stress field of particle assembly of intermetallic-ceramic composite NiAl/AlO. The stress evaluation is performed at two levels: macroscopic and microscopic. Macroscopic averaged stress is determined using the homogenization method using the representative volume element. Microscopic stresses are calculated both in the body of particles and in the contact interface (necks) between particles. Obtained results are in line with the cooling mechanism of the two-phase materials.

Brief review of silver sinter-bonding processing for packaging high-temperature power devices

Silver sintering is receiving increasing attention due to its novel die-attach technique for high-temperature power electronics. Excellent thermal conductivity, high melting point/remelting temperature and low-temperature sintering behaviors of the silver sintered attachment meet the requirements of high-temperature applications for power devices, specifically SiC devices. The merits and demerits of the existing pressure-assisted sintering and pressure-less sintering techniques using nano-scale, micro-scale and micro-nano-scale hybrid silver sintered materials are separately presented. The emerging rapid sintering approaches, such as the electric-assisted approach, are briefly introduced and the technical outlook is provided. In addition, the study highlights the importance of creating a brief resource guide on using the correct sintering methods.