Sintering Of Nanopowders Research Articles

Microstructure and mechanical properties of Al2O3–20 wt%Al2TiO5 composite prepared from alumina and titania nanopowders

In the present work, Al2O3–20wt%Al2TiO5 composite was prepared from reaction sintering of alumina and titania nanopowders. The nano-sized raw powders were reconstituted into nanostructured particles by ball milling. Then, the nanostructured reconstituted powders were pressed and pressureless-sintered into bulk ceramics at 1300, 1400, 1500°C for 2h. The phase composition and microstructures of reconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope and energy-dispersive spectrometer (EDS). The microstructural analysis of the ceramic showed that the average grain size of the alumina–aluminium titanate composite increases with increasing the temperature. Also, SEM proved the existence of a proper interface between Al2TiO5 and Al2O3 grains and preferential distribution of aluminium titanate particles in the grain boundaries. XRD analysis indicated the absence of rutile titania in the sintered composite ensuring complete formation of aluminium titanate. The hardness of the samples sintered at 1300, 1400, 1500°C were 4.8, 6.2 and 8.5GPa, respectively.

Rapid dye-sensitized solar cell working electrode preparation using far infrared rapid thermal annealing

TiO2 nanopowder sintering during dye-sensitized solar cell working electrode preparation usually takes hours and is the rate-limiting step to rapid production of dye-sensitized solar cells. Here, we show that by far infrared rapid thermal annealing (RTA) method, TiO2 working electrode sintering time reduces from hours to minutes. 5min binders/solvents removal time at 250°C plus 10min sintering time at 500°C in a RTA system gives a cell with the efficiency of 4.37%. Although the rapid removal of binders/solvents causes the TiO2 film structure collapsing and gives a more compact working electrode structure, the far infrared RTA method directly radiates TiO2 layer giving binders/solvents removal and sintering within minutes and is ideal for rapid DSSC fabrication.

플라즈마 이온질화에 의한 Fe 나노분말소결체의 표면경화 가능성 연구

This study has been performed on the full density sintering of Fe nanopowder and the surface hardening by plasma ion nitriding. The Fe sintered part was fabricated by pressureless sintering of the Fe nanopowder at <TEX>$700^{\circ}C$</TEX> in which the nanopowder agglomerates were controlled to have 0.5-5 <TEX>${\mu}m$</TEX> sized agglomerates with 150 nm Fe nanopowders. The green compact with 46% theoretical density(T.D.) showed a homogeneous microstructure with fine pores below 1 <TEX>${\mu}m$</TEX>. After sintering, the powder compact underwent full densification process with above 98%T.D. and uniform nanoscale microstructure. This enhanced sintering is thought to be basically due to the homogeneous microstructure in the green compact in which the large pores are removed by wet-milling. Plasma ion nitriding of the sintered part resulted in the formation of <TEX>${\gamma}$</TEX>'-<TEX>$Fe_4N$</TEX> equilibrium phase with about 12 <TEX>${\mu}m$</TEX> thickness, leading to the surface hardening of the sintered Fe part. The surface hardness was remarkably increased from 176 <TEX>$H_v$</TEX> for the matrix to 365 <TEX>$H_v$</TEX>.

Synthesis, processing, and thermoelectric properties of bulk nanostructured bismuth telluride (Bi2Te3)

Bismuth telluride (Bi2Te3) is the best-known commercially used thermoelectric material in the bulk form for cooling and power generation applications at ambient temperature. However, its dimensionless figure-of-merit-ZT around 1 limits the large-scale industrial applications. Recent studies indicate that nanostructuring can enhance ZT while keeping the material form of bulk by employing an advanced synthetic process accompanied with novel consolidation techniques. Here, we report on bulk nanostructured (NS) undoped Bi2Te3 prepared via a promising chemical synthetic route. Spark plasma sintering has been employed for compaction and sintering of Bi2Te3 nanopowders, resulting in very high densification (>97%) while preserving the nanostructure. The average grain size of the final compacts was obtained as 90 ± 5 nm as calculated from electron micrographs. Evaluation of transport properties showed enhanced Seebeck coefficient (−120 μV K−1) and electrical conductivity compared to the literature state-of-the-art (30% enhanced power factor), especially in the low temperature range. An improved ZT for NS bulk undoped Bi2Te3 is achieved with a peak value of ∼1.1 at 340 K.

Preparation of transparent Y2O3 ceramic by slip casting and vacuum sintering

In the present work transparent Y2O3 ceramics were made by slip casting and vacuum sintering of nanopowders with sodium polyacrylic acid (PAA-Na) as dispersant. The rheological properties of Y2O3 nanopowder slurry were investigated using different amounts of dispersant and solid contents. The microstructures and transmittance of the sintered ceramics were also studied by means of scanning electron microscopy (SEM) and ultra-violet visible spectrometry. The results showed that rheological behaviors of the Y2O3 nanopowder slurry were effectively promoted by sodium polyacrylic acid. Highly dispersive and stable slurries were obtained as the dispersant was added over 1.0 dwb% under the fixed conditions of pH 11 and 45 wt.% solid content. All the slip cast green bodies were sintered into highly dense ceramics after sintering at 1700 °C for 5 h in vacuum, wherein the sample added with 1.1% sodium polyacrylic acid exhibited the highest relative density of 99.36% and transmission of 30% at 800 nm wavelength.

Ferroelectric ceramics with enhanced remnant polarization by ordered coalescence of nano-crystals

An exceptional high ferroelectric remnant polarization (Pr) was observed in BaTiO3 ceramics owing to the formation of micron-sized grains possessing nano-scale mosaicity. Such a structural hierarchy was developed via a novel crystal-growth mechanism, namely ordered coalescence of nano-crystals achieved by synergetic atomic epitaxial growth and self-assembly of nano-crystals. The accommodating lattice defects in sub-grain boundaries due to the imperfect assembly of nano-crystals significantly contribute to the Pr enhancement by stimulating the dynamics of ferroelectric domain formation and switching. This finding defines a new approach to nanopowder sintering leading to enhanced properties sensitive to lattice defects.

Grain‐Boundary Enthalpies of Cubic Yttria‐Stabilized Zirconia

The change in enthalpy with grain size of dense nanograined yttria‐stabilized zirconia (YSZ) was measured by oxide melt solution calorimetry of cubic 10 YSZ and 15 YSZ dense nanograined ceramic samples prepared by spark plasma sintering of nanopowders. The derived grain‐boundary enthalpies are 0.68 J/m2 for 10 YSZ and 0.64 J/m2 for 15YSZ, with an estimated uncertainty of about+0.13 J/m2, arising mostly from the roughly +20% uncertainty in interface area calculated from the particle size distribution. These values are similar to a previous value of 0.73 ± 0.19 J/m2 for 8 YSZ, suggesting a possible weak concentration dependence (decrease of grain‐boundary energy with increasing dopant concentration) that may not be outside experimental error. Previously published interface enthalpies of 0.6–1.0 J/m2 for partially agglomerated nanopowders of 8, 10, and 12 YSZ, are in general agreement with the present data on densified nanoceramics. These data, taken together, suggest the ratio of surface to interface energy increases slightly with doping level, providing a slightly higher driving force for densification and pore elimination as the yttria content is increased.

Sintering kinetics analysis of molybdenum nanopowder in a non-isothermal process

Compared with commercial micro-sized molybdenum powder, the sintering of molybdenum nanopowder fabricated by high-energy ball milling and subsequent hydrogen reduction was considerably more activated due to its extended surface area. In this study, the sintering kinetics of a molybdenum nanopowder were evaluated by measuring the linear shrinkage in a non-isothermal process and calculating the activation energy for the initial sintering stage. To reach a linear shrinkage of 2 %, the activation energy of the molybdenum nanopowder sintering was 180 kJ, primarily due to the grain boundary diffusion, while the activation energy of the commercial micro-sized powder sintering was 345 kJ, predominantly due to the volume diffusion.

Sintering of WC and WC-Co nanopowders with different inhibitor additions by the SPS method

Sintering of WC and WC-Co nanopowders with different inhibitor additions by the SPS method

Spark plasma sintering of TiCN nanopowders in non-linear heating and loading regimes

Consolidation of commercially available nanostructured titanium carbonitride (TiCN) powder has been performed by Spark Plasma Sintering (SPS) in the temperature range from 1300 to 1600°C. The effect of non-linear heating and loading regimes on consolidation of high melting point nanocomposites has been investigated. SPS consolidated TiCN material has demonstrated near fully dense and fine homogeneous microstructure with average grains size about 150nm. Nanohardness and fracture toughness of the TiCN nanocomposite have been measured as 33±0.9GPa and 3.2MPam1/2 respectively.

Microwave sintering of ZnO nanopowders and characterization for gas sensing

Thick film gas sensors based on ZnO nanopowders were fabricated by using microwave sintering. The surface and cross section morphologies were characterized by field-emission scanning electron microscopy (FE-SEM). The stability of the microstructure was studied by impedance spectroscopy. The results showed that the shape of the nanoparticles was not changed through microwave sintering, and the thick films had the more dense microstructures than that by muffle oven sintering. The resistance–temperature characteristic and the responses to toluene, methanol and formaldehyde revealed that the microwave sintering technique could effectively control the growth of ZnO nanoparticles, realize the uniform sintering of thick film, gain the stable microstructure and improve the response of sensor. In addition, the formative mechanism of the thick film microstructure was proposed according to microwave sintering mechanism.

Effect of process conditions on the properties of conductive tracks in direct imprinting and sintering of metal nano-powders

A direct patterning process which uses imprinting and sintering of metal nano-powder to make conductive tracks where sintering was performed during imprinting process was designed. A silicon mold for making conductive tracks was designed and fabricated using photolithography and reactive ion etching (RIE). The effect of process conditions on pattern formation was investigated experimentally. To confirm the feasibility of the direct patterning method as an industrial process, the microstructure and electrical resistivity of the fabricated conductive tracks were analyzed. Various defects were analyzed and based on the analysis, the histories of temperature and pressure were optimized.

Explosive compaction and low-temperature sintering of alumina nanopowders

Physical aspects of explosive compaction of alumina nanopowders with different phase compositions are studied experimentally. Physical processes that occur during consolidation of nanoparticles under pulsed loading are considered. Conditions of retaining of the material nanostructure after compaction and subsequent low-temperature sintering are determined. Physicomechanical properties of explosive compacts and ceramics on the basis of these compacts are studied. A ceramic material characterized by a nanostructure (grain size of ≈200 nm) and high values of density (97% of the theoretical value) and microhardness (up to 23.5 GPa) is obtained.

Effect of TiO2 Nanopowder on the Sintering Behavior of Nickel–Alumina Composites for Functionally Graded Materials

The use of TiO2 nanopowder as a sintering aid in a pressureless sintering process has been investigated to improve the compatibility of the shrinkage rate in graded nickel–alumina composites. The results indicate that a 3 wt% TiO2 nanopowder ball mixed into Al2O3 powder substantially reduces sintered porosity in nickel–alumina composite specimens, improving the compatibility of shrinkage rates while increasing Vickers microhardness. Previously developed porosity reduction and sintering models were used to quantify improvement in shrinkage rates. However, the porosity reduction model had to be modified to account for sintering of agglomerated reinforcing particles in alumina‐rich composites. A power law sensitivity of Vickers microhardness to relative density was determined from homogeneous specimens. Vickers microhardness profiles of graded composites were then used to provide an insight into the effects of hydrostatic internal stresses that develop due to differential shrinkage, indicating that substantial hydrostatic stress retards shrinkage, which reduces microhardness at the interface of the nickel‐ and alumina‐rich regions and leads to crack formation without a nanopowder sintering aid.

Grain size control by pressure application regime during spark plasma sintering of Nd-YAG nanopowders

Grain size control by pressure application regime during spark plasma sintering of Nd-YAG nanopowders.

Triple junction limited grain growth in nanomaterials

Grain growth in nanocrystals can be limited by triple junctions, during both sintering of nanopowders and annealing of dense materials. An analysis is developed to determine experimental conditions under which triple junctions can limit grain growth.

Cyclic strength of submicrocrystalline nickel obtained by nanopowder sintering

The laws of fatigue hardening, softening, and fracture of coarse-grained, submicrocrystalline (SMC), and nanocrystalline nickel-based metallic materials are briefly reviewed. The cyclic strength of SMC nickel obtained by means of hydrostatic pressing has been studied.

Mechanosynthesis, Radiation-Thermal Modification and Characterization of Nanostructured Scandia Stabilized Zirconia Ceramics

AbstractSSZ-based ceramics were obtained by sintering of nanopowders derived at room temperature by mechanochemical synthesis from refined technical grade ZrO2 nano-precursors. RT-treatment by 2.5 MeV electrons up to 1563 K was used for the modification of ceramics. Powders and ceramics were characterized by XRD, Raman, SEM and EDS, TEM, SIMS techniques. The phase composition of Zr0.89Sc0.1Ce0.01O1.95 ceramics was very close to cubic structure but better fitting of XRD patterns was obtained for rhombohedral lattice. Conductivity of solid electrolytes for IT SOFC was studied by complex impedance method. To stabilize cubic structure and increase conductivity at operation temperature of To ∼ 1000 K, the composition of SSZ solid electrolyte was optimized by addition of yttria and sintering aids. The interaction of admixtures with minor dopants leading to intergrain phase was revealed. During fast sintering, ceramics keep a memory about inhomogeneous disordered solid solutions in a form of nanostructuring. Conductivity data indicate nanostructuring of ceramics too: activation energies of bulk and grain boundary conductivities are close (Eb ∼ 0.9 eV, Egb ∼ 1.05 eV). Annealing of ceramics at high temperatures increases conductivity at To and promotes grain growth.

Effect of the conditions of sintering W-Ni-Fe-Co heavy alloy nanopowders on the structure and density of compacted samples

A compact material made of a heavy tungsten alloy W-Ni-Fe-Co nanopowder is produced. The nanopowders are synthesized by the treatment of a solid tungstic acid in aqueous solutions of Ni, Fe, and Co salts followed by the reduction of the solid residue by hydrogen at 800°C (the average size of the powder conglomerates is ∼300 nm, and the conglomerates consist of 100-nm particles). Solid-phase sintering is performed in stages. An increase in the temperature at the last stage from 1300 to 1350 and 1450°C increases the density from 16.7 to 17.2–17.4 g/cm3 and the average tungsten grain size to 2.4–4.6 µm. The samples after solid-phase sintering at 1350°C have no porosity. Liquid-phase sintering of nanopowders with high surface and interface energies occurs at 1480°C.

Fabrication, properties and sintering of ZnO nanopowder

Pure ZnO nanopowder was unintentionally prepared by a solvothermal synthesis method in dimethylformamide (DMF) solvent, using zinc chloride, potassium hydroxide, and potassium borohydride as starting materials. The size of the powder synthesized at 180 °C for 48 h is about 35 nm. The nanopowder exhibits much better absorbance in ultraviolet band than commercial ZnO powder and shows a strong violet light emission excited by a 325-nm line. The nanopowder can be pressurelessly sintered to high density (>98% of theoretical density) at 950 °C in atmosphere.