Sintering Of Nanopowders Research Articles

Effect of Adding W Nanopowder to Inhibit Sintering of Ni Nanopowder for MLCC

Multilayer ceramic capacitors (MLCCs) are essential fundamental components in electronic devices, enabling miniaturization and high capacitance for energy storage and filtering. A key focus in MLCC research is enhancing capacitance density, for which improving the connectivity of Ni internal electrodes is imperative. The primary cause of reduced electrode connectivity in MLCCs is the sintering shrinkage mismatch between Ni internal electrodes and dielectric materials. In this study, we propose a method to mitigate the sintering shrinkage mismatch between the Ni internal electrodes and dielectric materials by incorporating W nanopowder into Ni to retard the sintering shrinkage of the Ni nanopowder. The degree of sintering retardation due to the increase in W content was quantified through measurements of shrinkage rate, density, and porosity, confirming the retarded sintering of Ni. Particularly, microstructural analysis and phase analysis were conducted to elucidate the role of W during the dissolution and sintering processes. Furthermore, the activation energy of each sintering stage was determined, with an analysis of the sintering retardation mechanism induced by W. Additionally, similar sheet resistance values before and after the addition of W were obtained through electrical resistance measurements, and results suggested that the incorporation of W effectively retards the sintering of Ni while enabling its function as an internal electrode in MLCCs.

Structural, spectroscopi c and laser properties of transparent Tm:YScO3 ceramic based on gas-phase synthesized nanoparticles

Owing to the combination of good thermo-optical properties and inhomogeneously broadened fluorescence spectra suitable for ultrashort laser pulse generation, rare-earth doped solid solutions of sesquioxides (Y,Lu,Sc)2O3 have attracted considerable interest in recent years. In this study, compositionally disordered Tm3+-doped YScO3 transparent ceramic was fabricated using solid-state vacuum sintering of nanopowder produced using laser ablation of solid target in air flow. The as-synthesized powder consisted of weakly aggregated spherical particles having 20 nm diameter and a mostly monoclinic (space group C2/m) crystal structure, whose symmetry was transformed into cubic (space group Ia-3) by calcination at 1050 °C. From high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX), we observed a homogeneous distribution of components both throughout individual nanoparticles and 100-nm-thick ceramic foil resulting in high structural and optical quality of the obtained samples. The sintered ceramic featured an optical transmission of 77% at 600 nm and above 81% near 2 μm, an average grain size of 24.2 μm and a content of scattering centers of 26.1 ppm. The absorption spectra at several temperatures, from 93 K to 293 K, were measured and the corresponding absorption coefficients were obtained by Lambert-Beer formula in the range from 600 nm to 900 nm and from 1400 nm to 2100 nm. In particular, the room temperature absorption cross section at 794 nm and 1634 nm were determined to be σabs = 4.25 × 10−21 cm2 and σabs = 5.01 × 10−21 cm2 respectively. Concerning the emission cross section it was calculated by Fuchtbauer-Ladenburg equation starting from the fluorescence spectrum acquired at the laser work temperature: at 1951 nm and 2106 nm corresponding to the main two emission peaks belonging the curve, it was found σem = 9.2 × 10−21 cm2 and σem = 4.9 × 10−21 cm2, respectively. Finally, it was tested the laser behavior of the ceramic: pumped at 793 nm in quasi-CW mode it delivered 1.27 W of output power with a slope efficiency of 10.33% at 2081 nm. The tunability curve spans from 1942 nm to 2110 nm (i.e. 168 nm).

Fine-grained high-performance Ba0.85Ca0.15 Zr0.1Ti0.9O3 piezoceramics obtained by current-controlled flash sintering of nanopowders

Due to environmental concerns, extensive research has been carried out to develop high-performance lead-free piezoceramics capable of replacing commercial lead-based materials. The lead-free (Ba0.7Ca0.3)TiO3 −Ba(Zr0.2Ti0.8)O3 system has emerged as a candidate for room temperature transducer applications because a high piezoelectric charge coefficient is achieved in this system for compositions at the morphotropic phase boundary. However, conventional ceramic processing of these eco-friendly piezoceramics demands high energy consumption because long-lasting, high-temperature heat treatments are needed, which often lead to microstructural degradation that compromises the material reliability. Field-assisted flash sintering has started to be explored since the application of an adequate electric field was shown to significantly reduce the sintering time and temperature, thereby controlling grain growth. In this work, Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics are obtained by current-controlled flash sintering of mechanosynthesized nanopowders. Exhaustive control of the sintering parameters allows tailoring of the microstructure, which allows dense fine-grained flash-sintered ceramics exhibiting a high electric field-induced strain response to be obtained.

Highly transparent cubic γ-Al2O3 ceramic prepared by high-pressure sintering of home-made nanopowders

Highly transparent gamma-aluminum oxide (γ-Al2O3) ceramics were fabricated for the first time, by combining homogeneous precipitation and high-pressure sintering in the absence of exogenous dopants. The resulting cubic γ-Al2O3 transparent ceramic material exhibits a promising replacement for single-crystal sapphire. The optimum optical properties are achieved in response to sintering at 5 GPa and a temperature of 300 °C and include maximum transmittance of 86% in the range of 0.6–1.2 µm which are properties that are comparable to those of single-crystal sapphire (∼86%). Vickers hardness (16 GPa) and compressive strength (350 MPa) in response to high-pressure sintering are also similar to those of a conventional sapphire single crystal. Meanwhile, the dielectric constant (9.46) is comparable to that of sapphire in the C-axis direction. These findings will facilitate further development of transparent Al2O3 ceramics for use in a wider range of optical applications.

Water-Repellent Galvanized Steel Surfaces Obtained by Sintering of Zinc Nanopowder.

Galvanized steel surfaces are widely used in industry as a solution to prevent corrosion of steel tools that operate in outdoor or corrosive and oxidative environments. These objects are coated with a zinc protective layer deposited by hot dip galvanization. Turning the surface of galvanized steel tools into superhydrophobic may lead to very useful functionalities, although it may be a difficult task, because the preservation of the thin zinc layer is a claim. We propose herein the use of a bottom-up approach based on sandblasting, followed by sintering of zinc nanoparticles on the galvanized steel substrate, which allowed us to produce a zinc-made hierarchical structure required for superhydrophobicity. These samples acquired a double-scale structure that led to superhydrophobicity when they were later hydrophobized with a thin fluoropolymer layer. We found that sandblasting might be useful but not mandatory, unlike the sintering process, which was essential to reach superhydrophobicity. We found that, under certain experimental conditions, the surfaces showed outstanding water-repellent properties. We observed that the sandblasting on galvanized steel caused more damage than the sintering process. Sintering of low-melting-point metal nanoparticles was revealed as a promising strategy to fabricate functional metallic surfaces.

Critical role of water in fused silica glass by silica nanopowder sintering: Nanoparticle dispersion

Thermoplastic nanocomposite preparation using a silica nanopowder/polyethylene glycol (PEG) / polyvinyl butyral (PVB) system followed by heat-treatment sintering to obtain fused silica glass has high precision, scalable manufacturing processes, and short processing time. However, conventional solvents are toxic and harmful to the environment, and for this reason, the ethanol/water solvent system was proposed in this study. In this study, the effect of water content on the dispersion effect of nano-silica suspensions was investigated through particle size and zeta potential tests and FTIR spectra analysis; in addition, the study proposes a dispersion theory based on the solvation layer of the ethanol/water competitive adsorption and viscosity effect. Finally, optical photographs and SEM characterization showed that the obtained fused silica glass was transparent and had no seam defects, and the XRD and hydroxyl group tests showed that the glass had a low hydroxyl group content of only 2.55 ppm.

In-situ oxide dispersion strengthened ultrafine-grained molybdenum alloy with enhanced hardness and bending strength

The refinement of grain size and introduction of the second phase particles are the main pathways to improve the mechanical properties of Mo materials. In this work, ultrafine-grained Mo alloy with in-situ formed intragranular MoO2 particles (average particle size, 34 nm) was prepared via spark plasma sintering (SPS) of Mo nanopowder. This ultrafine-grained Mo sample possessed excellent hardness and bending strength of 464 HV1.0 and 909 MPa, respectively, which were more than twice of the traditional coarse grained Mo alloy. It was found that these intragranular MoO2 nanoparticles were converted from the oxygen on the surface of Mo nanoparticles and had a coherent relationship with Mo matrix (MoO2 (110) // Mo (102)), which can improve strength via inhibiting grain growth, pinning and storing dislocations.

Oxygen Aspects in the High-Pressure and High-Temperature Sintering of Semiconductor Kesterite Cu2ZnSnS4 Nanopowders Prepared by a Mechanochemically-Assisted Synthesis Method

We explore the important aspects of adventitious oxygen presence in nanopowders, as well as in the high-pressure and high-temperature-sintered nanoceramics of semiconductor kesterite Cu2ZnSnS4. The initial nanopowders were prepared via the mechanochemical synthesis route from two precursor systems, i.e., (i) a mixture of the constituent elements (Cu, Zn, Sn, and S), (ii) a mixture of the respective metal sulfides (Cu2S, ZnS, and SnS), and sulfur (S). They were made in each system in the form of both the raw powder of non-semiconducting cubic zincblende-type prekesterite and, after thermal treatment at 500 °C, of semiconductor tetragonal kesterite. Upon characterization, the nanopowders were subjected to high-pressure (7.7 GPa) and high-temperature (500 °C) sintering that afforded mechanically stable black pellets. Both the nanopowders and pellets were extensively characterized, employing such determinations as powder XRD, UV-Vis/FT-IR/Raman spectroscopies, solid-state 65Cu/119Sn NMR, TGA/DTA/MS, directly analyzed oxygen (O) and hydrogen (H) contents, BET specific surface area, helium density, and Vicker's hardness (when applicable). The major findings are the unexpectedly high oxygen contents in the starting nanopowders, which are further revealed in the sintered pellets as crystalline SnO2. Additionally, the pressure-temperature-time conditions of the HP-HT sintering of the nanopowders are shown (in the relevant cases) to result in the conversion of the tetragonal kesterite into cubic zincblende polytype upon decompression.

Two-step sintering of alumina nano-powders: A discrete element study

Avoiding grain growth during sintering of ceramic nano-powders is of great technological interest. Although two-step sintering is an effective technique to achieve this goal, the mechanisms at play are not well understood. This study adapts our previous discrete model to investigate the conventional and two-step sintering of nano-powders. The densification and grain growth results agree qualitatively well with experimental data on α-alumina. Simulations confirm that faster heating rates retard grain growth in conventional sintering of nano-alumina. Our results support the hypothesis that the success of nano-alumina two-step sintering relies on the sharp increase in the activation energy of the grain boundary mobility at low temperatures. Simulations indicate a transition temperature of 1100 °C and that at least a 2.5-fold increase in activation energy is required to explain the suppression of grain growth. The relative weights of surface diffusion and of grain boundary motion for grain growth are clarified.

(Invited) A Novel, Ultra-Fast Fabrication of SOFCs By Flash Light Sintering Process

Solid oxide fuel cells (SOFCs) have strong potential for next-generation energy conversion systems because of their high energy conversion efficiency, fuel flexibility and low emission of pollutant. In the last decades, the SOFC technology has rapid progress in terms of performance and durability aspect, expanding its application ranges from stationary system to portable devices. In the fabrication of SOFCs, a simple and mass productive fabrication process such as screen-printing, tape casting and sol-gel method are widely used in the industry because it is easy, scalable and economical process. However, a high temperature post heat-treatment process is required to ensure material properties of the ceramic electrolyte and electrode in these processes. In particular, to achieve a highly dense electrolyte layer, a high sintering temperature above 1300oC is usually employed in the fabrication process, which causes considerable amount of time and significant manufacturing costs. Therefore, reducing the sintering temperature and long process time is one of the major challenges to reduce the fabrication cost and facilitate the commercialization of SOFCs. To overcome this problem, many efforts have been made to demonstrate alternative sintering process to reduce the sintering temperature and process time. For example, several researchers proposed spark plasma sintering process as an alternative sintering process because of the high heating rate within a short time based on Joule heating mechanism [1, 2]. However this process has a limitation of complex vacuum system configuration and high equipment cost for commercialization. In addition, excimer laser ablation annealing and microwave sintering process have been also suggested but there are still limitations such as extremely narrow spot size and complexity of microwave interactions with materials respectively [3, 4]. On the other hand, the flash light sintering process presented here uses a white flash light generated by a xenon arc lamp discharge as a heat treatment sources and has many advantages of an extremely short process time, scalability and simple process condition as standard ambient and temperature. Most of all, because this process conducted within a few seconds, it can significantly improve the productivity and cost-effectiveness of the SOFCs. Our previous works successfully demonstrated that the solution-based YSZ thin films was annealed by flash light sintering and it showed comparable material properties to the conventional thermal sintering process [5]. Nevertheless, due to the low light absorption property and difficulty to sinter the ceramic particles, studies on the flash light sintering process for the powder-based films are still limited. In this work, we demonstrate for the first time rapid fabrication of SOFCs using a novel flash light sintering process. The ceramic electrolyte and electrode was fabricated by using mass productive screen-printing method and flash light sintering process was employed as an alternative sintering process of conventional thermal sintering method. In the sintering process, the effect of various pulse conditions on the sinterbility of the ceramic components were investigated, and the material properties such as microstructure, crystallinity and electrochemical property were analyzed. The optimized ceramic components were employed to fabricate SOFC unit cell and their cell performance and impedance behavior was evaluated to demonstrate the flash light sintering process. Reference [1] Deepash Shekhar Saini et al. “Improved conductivity of spark plasma sintered Ho-Substituted Ba-ZrO3 electrolyte ceramics for IT-SOFCs.” ACS Applied Energy Materials. 1, 7, 3469-3478 (2018)[2] Tatiana L. Simonenko et al. “Spark plasma sintering of nanopowders in the CeO2-Y2O3 system as a promising approach to the creation of nanocrystalline intermediate-temperature solid electrolytes.” Ceramics International, 44, 16, 19879-19884 (2018)[3] Hao, H., Xu, L., Huang, Y., Zhang, X. & Xie, Z. “Kinetics mechanism of microwave sintering in ceramic materials.” Science in China Series E: Technological Sciences 52, 2727–2731 (2009).[4] Bayati, R. et al. “Modifcation of properties of yttria stabilized zirconia epitaxial thin flms by excimer laser annealing.” ACS applied materials & interfaces 6, 22316–22325 (2014).[5] Park, J.S. et al. Rapid, cool sintering of wet processed yttria-stabilized zirconia ceramic electrolyte thin films. Scientific reports 7, 12458 (2017)

Mechanochemical synthesis and cold sintering of mussel shell-derived hydroxyapatite nano-powders for bone tissue regeneration

According to the circular economy principles, processing routes aiming at reducing the natural resources consumption and the energy demand can be addressed as ‘green’. In this framework, mussel shells, a natural feedstock of calcium carbonate, were successfully transformed into nano-crystalline hydroxyapatite by mechanochemical synthesis at room temperature after mixing with a phosphoric acid solution. The as-synthesized powder was then consolidated up to 82 % relative density by cold sintering (600 MPa, 200 °C). The materials were fully investigated by physical, chemical and thermal characterization techniques. Cold-sintered samples were also subjected to biaxial flexural strength test, showing a flexural resistance of 23 MPa. Cell viability assessment revealed that cold sintered hydroxyapatite derived from mussel shells promotes faster adhesion and spreading of human bone marrow-derived mesenchymal stem cells, in comparison to a commercial hydroxyapatite sintered at 1050 °C. Therefore, cold-sintered mussel shells-derived hydroxyapatite can be a promising future candidate scaffold for bone tissue regeneration.

Spark plasma sintering of UO2 nanopowders: Pressure, heating rate and current effects

We analysed with different methods the densification of UO2 nanopowders in SPS under constant heating rate (CHR) and isothermal sintering conditions. The apparent activation energy of densification in SPS (75 kJ/mol with CHR method) is significantly smaller than in conventional sintering. It is shown that this is likely not an effect of the applied current. We also observed a threshold stress at 64 MPa for the transition from pressure-insensitive sintering (stress exponent n≈0) to pressure-assisted sintering, suggesting that the contribution of the capillary stresses in such nanopowders is comparable with the typical stress applied in SPS.

Nanocrystalline high-entropy hexaboride ceramics enable remarkable performance as thermionic emission cathodes

The development of high-entropy borides with combined structural and functional performance holds untold scientific and technological potential, yet relevant studies have been rarely reported. In this work, we report nanocrystalline (La0.25Ce0.25Nd0.25Eu0.25)B6 high-entropy rare-earth hexaboride (HEReB6-1) ceramics fabricated through the high-pressure sintering of self-synthesized nanopowders for the first time. The as-fabricated samples exhibited a highly dense (96.3%) nanocrystalline (94 nm) microstructure with major (001) fiber textures and good grain boundaries without any impurities, resulting in a remarkable mechanical, electrical, and thermionic emission performance. The results showed that the samples possessed outstanding comprehensive mechanical properties and a high electrical resistivity from room temperature to high temperatures; these were greater than the average values of corresponding binary rare-earth hexaborides, such as a Vickers hardness of 23.4 ± 0.6 GPa and a fracture toughness of 3.0 ± 0.4 MPa•m1/2 at room temperature. More importantly, they showed high emission current densities at elevated temperatures, which were higher than the average values of the corresponding binary rare-earth hexaborides. For instance, the maximum emission current density reached 48.3 A•cm−2 at 1873 K. Such superior performance makes the nanocrystalline HEReB6-1 ceramics highly suitable for potential applications in thermionic emission cathodes.

Characterization and modeling of the temperature-dependent thermal conductivity in sintered porous silicon-aluminum nanomaterials

Nanostructured silicon and silicon-aluminum compounds are synthesized by a novel synthesis strategy based on spark plasma sintering (SPS) of silicon nanopowder, mesoporous silicon (pSi), and aluminum nanopowder. The interplay of metal-assisted crystallization and inherent porosity is exploited to largely suppress thermal conductivity. Morphology and temperature-dependent thermal conductivity studies allow us to elucidate the impact of porosity and nanostructure on the macroscopic heat transport. Analytic electron microscopy along with quantitative image analysis is applied to characterize the sample morphology in terms of domain size and interpore distance distributions. We demonstrate that nanostructured domains and high porosity can be maintained in densified mesoporous silicon samples. In contrast, strong grain growth is observed for sintered nanopowders under similar sintering conditions. We observe that aluminum agglomerations induce local grain growth, while aluminum diffusion is observed in porous silicon and dispersed nanoparticles. A detailed analysis of the measured thermal conductivity between 300 and 773 K allows us to distinguish the effect of reduced thermal conductivity caused by porosity from the reduction induced by phonon scattering at nanosized domains. With a modified Landauer/Lundstrom approach the relative thermal conductivity and the scattering length are extracted. The relative thermal conductivity confirms the applicability of Kirkpatrick’s effective medium theory. The extracted scattering lengths are in excellent agreement with the harmonic mean of log-normal distributed domain sizes and the interpore distances combined by Matthiessen’s rule.

Synthesis and dielectric properties of K1.6Fe1.6Ti6.4O16 ceramics produced by the Pechini method

Potassium titanate modified with Fe3+ ions matching the stoichiometry of K1.6Fe1.6Ti6.4O16 (KFTO) was obtained by the polymer complex precursor method (Pechini method). The phase composition and morphology of the obtained sample were characterized using XRD, laser diffraction, and SEM technique. X-ray phase analysis shows single-phase formation of potassium titanate with a tetragonal hollandite-like structure. The Rietveld method was used to clarify the crystal lattice parameters. The parameters of the crystal lattice are: a = b = 10.1510 Å и c = 2.9659 Å. The results of SEM and laser diffraction showed that the particles of the studied nanopowder have a cubic shape and an average size of 400 nm. The dielectric properties in the frequency range of 0.1 Hz to 1.0 MHz were studied for ceramic disks obtained by sintering of compressed nanopowder at 1080 oC. It was found that ceramics based on KFTO solid solutions have high dielectric permittivity and low dielectric losses and characterized with increased polarizability. The high polarizability of the material is explained by the relatively high mobility of K+ ions in the tunnel of the considered hollandite structure that is accompanied by the redistribution of electrons in the crystal lattice. A discussion of the contribution of various processes to the permittivity is presented.

Joining and cycling performance of ultra-thick tungsten coatings on patterned steel substrates for fusion armour applications

Tungsten is the leading candidate to provide a critical protective coating for copper and steel-based plasma facing components in nuclear reactors. However, fabrication of thick tungsten coatings on copper or steel is challenged by the significant difference in the melting points and the thermal expansion coefficients of the materials, which leads to severe thermal expansion mismatch strains during manufacture and in service. This challenge is investigated using a new processing approach — field assisted sintering of tungsten nanopowders directly onto pre-sculptured steel component surfaces to induce controlled, vertical segmentation cracks that provide strain relief. Tungsten coatings up to 2 mm thick were fabricated with uniform density and with a microstructure consisting of ultrafine grains of ∼200 nm. The coatings showed outstanding thermal cycling durability and survived for at least 50 cycles under pulsed temperature cycles between 300 to 800 °C. Microscopy and in situ thermal imaging revealed that a previously unachievable thermal expansion mismatch strain tolerance was provided by a combination of enhanced tungsten coating adhesion and deliberate coating segmentation by vertical cracking. The approach should be readily adaptable to other dissimilar materials systems to facilitate a range of durable, ultra-thick low ductility coatings on metallic substrates.

Optimization of magnesia sintering parameters fabricated by spark plasma sintering method for infrared transparency

In this study, sintering of magnesia nanopowder was investigated using spark plasma sintering (SPS), and the effects of time and pressure on microstructure, physical properties, density, and infrared transparency were studied. Densification of magnesia nanopowder was examined at 1100 °C and the pressure of 40, 60, and 80 MPa, and at different times. Findings indicated that MgO ceramics’ density rapidly increased with increasing the sintering time up to 5 min, while it increased with a slow speed after 5 min. Analysis of grain growth rate at different times indicated that, in the initial stages of sintering, the process was done through a non-diffusional process and, in the final 15 min of the process, it was done through lattice diffusion. In addition, examining the yield stress of the sintered sample at 1100 °C and considering its conformity with the results of theoretical density showed that it underwent plastic deformation up to 15 min. In general, the results revealed that the main mechanism of magnesia sintering was plastic deformation. The highest infrared transmittance of 65% was obtained for the sample sintered at 1100 °C for 20 min at 80 MPa.

Site dependence of surface dislocation nucleation in ceramic nanoparticles

The extremely elevated strength of nanoceramics under compression arises from the necessity to nucleate highly energetic dislocations from the surface, in samples that are too small to contain pre-existing defects. Here, we investigate the site dependence of surface dislocation nucleation in MgO nanocubes using a combination of molecular dynamics simulations, nudged-elastic-band method calculations and rate theory predictions. Using an original simulation setup, we obtain a complete mapping of the potential dislocation nucleation sites on the surface of the nanoparticle and find that, already at intermediate temperature, not only nanoparticle corners are favorable nucleation sites, but also the edges and even regions on the side surfaces, while other locations are intrinsically unfavorable. Results are discussed in the context of recent in situ TEM experiments, sheding new lights on the deformation mechanisms happening during ceramic nanopowder compaction and sintering processes.

Investigation of Aspects of High-Speed Sintering of Plasma-Chemical Nanopowders of Tungsten Carbide with Higher Content of Oxygen

Investigation of Aspects of High-Speed Sintering of Plasma-Chemical Nanopowders of Tungsten Carbide with Higher Content of Oxygen

Multiscale phase homogeneity in bulk nanocrystalline high entropy oxides

X-ray diffraction, energy dispersive X-ray spectroscopy, and atom probe tomography (APT) were used to assess the phase homogeneity of bulk sintered (Cu, Co, Mg, Ni, Zn)O transition metal high entropy oxides (TM-HEOs) across multiple length scales. These three methods were applied to both coarse-grain and nanocrystalline TM-HEO samples sintered using two different processing routes. Coarse-grain samples were prepared by reaction sintering of unreacted nanopowders using conventional sintering, while nanocrystalline samples were prepared from solid-state synthesized nanopowders consolidated using spark plasma sintering. Despite the different processing methods and grain sizes, the TM-HEO samples exhibit little-to-no chemical or phase segregation across all length scales. Critically, APT confirms that there is no chemical segregation at the nm scale even after solid-state synthesis and high temperature consolidation.