Sintered Bulk Materials Research Articles

Preparation of Sub-Micron Bi Alloy Powers with the Ultrasonic Mixed Crushing

The powders of the BiInSn alloy were produced by the ultrasonic atomization and the ultrasonic mixed crushing using the different dispersants. In this study, the composition, microstructure, melting point, and size of these powders were observed. The viscosity of different solutions of the dispersants and the mechanical properties of the sintered bulk materials were also tested. From the data analysis and results, we found that the composition of the powders using the different methods was consistent with the as-cast state. In addition, the size of powder produced by ultrasonic mixed crushing was significantly smaller than that ultrasonic atomization. And during the ultrasonic crushing process, with the increase of the viscosity of the dispersant, the size of the final powder also decreased, and even submicron powder were produced. The product of submicron powder could effectively improve the density and mechanical properties of sintered materials. And the principles of ultrasonic atomization and ultrasonic mixed crushing were discussed. We found that the mechanism of ultrasonic mixed crushing to produce powder was the micro-shock-wave theory of ultrasonic cavitation. At the same time, these dispersants were effective in keeping the droplets separate from each other and preventing them from merging back into the larger droplets. The droplet was solidified into a powder by rapid cooling in the end.

Realization of high thermoelectric power factor in Ta-doped ZnO by grain boundary engineering

Electrical conductivity in nanostructured ZnO bulks is limited by the inherently low carrier mobility caused by the high density of grain boundaries and interfaces. In this study, Zn1−xTaxO (x = 0, 0.01, 0.02, 0.03) with micro/nanoplatelet structures composed of nearly coherent dense grain boundaries with a low misorientation angle of ∼4° between the grains was successfully fabricated. Despite the presence of a significant amount of grain boundaries and interfaces in the sintered bulk material, a high carrier mobility (52.2 cm2 V−1 s−1) was obtained in the composition Zn0.99Ta0.01O, which is comparable to the value shown by ZnO single crystals and far higher than their ordinary nanostructured counterparts (<15 cm2 V−1 s−1). In addition, the distortion of the density of states increased effective mass induced by Ta 5d hybridization in ZnO caused a Seebeck coefficient of −290 μV K−1 at 1002 K resulting in a high power factor of 15.2 × 10−4 W m−1 K−2 at 1002 K.

Ball-milling properties and sintering behavior of Al-based Gd2O3–W shielding materials used in spent-fuel storage

To overcome the disadvantages of commercial neutron absorbers used for spent-fuel storage, novel Al-based Gd2O3–W composites were designed by calculating the neutron absorption and γ-ray transmission and synthesized via ball milling, cold isostatic pressing, and sintering. During the ball milling, the cubic Gd2O3 was partially transformed into monoclinic Gd2O3, and new Al3Gd and Gd3Al5O12 phases were formed. The sintering induced chemical reactions that formed Gd3Al5O12, Al3Gd, and Al12W phases. Nanoparticles with sizes of 100–300 nm were uniformly distributed in the sintered bulk materials. Moreover, with increasing sintering time, sintering temperature, and W content, the microhardness of the Al-based Gd2O3–W metal matrix composites (MMCs) increased. The compressive strength of the Al-based Gd2O3–W MMCs increased with W content, in contrast to the thermal expansion coefficient. Overall, the properties of the synthesized composites were better than those of the reference Al–B4C.

Chemical Stability between NiCr₂O₄ Material and Molten Calcium-Magnesium-Alumino-Silicate (CMAS) at High Temperature.

NiCr2O4 as a potential protection for thermal barrier coatings (TBCs) against the attack of molten calcium-magnesium-alumino-silicate (CMAS) was studied by a CMAS-contacting experiment. Atmospheric plasma sprayed coatings and sintered bulk materials were fabricated, covered with CMAS deposits, and exposed to 1200 °C for 24 h. Nano-sized CMAS-NiCr2O4 mixed powder was manufactured by ball milling and then conducted heat treatment under the same condition. The results show that no reacting product was found at the border between molten CMAS and NiCr2O4 and no element transportation occurred. It can be inferred that NiCr2O4 has outstanding chemical stability with the molten CMAS.

Novel Approaches to Prepare the High Performance Dense Electolyte and Interconnect Applicable to SOFC by Atmospheric Plasma Spraying

Plasma spraying has been widely applied to deposit ceramic coatings and is also considered as promising cost-effect manufacturing process to deposit electrolyte and ceramic interconnect for solid oxide fuel cells. However, it is generally difficult to deposit highly dense electrolyte and interconnect which can be directly applied to SOFC operation in the as-prepared state following the conventional process routine. In this paper, a novel approach based on the critical bonding temperature concept is presented to prepare dense electrolytes by plasma spraying. YSZ and LSGM are used as typical electrolytes, and LaxSr1-xCrO3 and LaxSr1-xTiO3 are used as typical interconnect. The individual ceramic functional layers are deposited at a deposition temperature higher than the critical bonding temperature. The compositions, microstructure, electric properties and gas permeability of different ceramic deposits are characterized. It is revealed that both plasma-sprayed electrolytes and interconnects present a gas permeability less than 1x10-7 cm4gf-1s-1 and can be directly applied to SOFC high temperature operation. Moreover, the deposited electrolytes present an ionic conductivity of over 80% sintered bulk materials. Along with the composition control approach during plasma spraying, high performance electrolyte and ceramic interconnect are deposited by atmospheric plasma spraying. Moreover, the excellent performance of the electrolyte and interconnect is also demonstrated by the output performance of assembled SOFC cells with the APSed functional layers.

Novel Approaches to Prepare the High Performance Dense Electolyte and Interconnect Applicable to SOFC by Atmospheric Plasma Spraying

Plasma spraying is considered as promising cost-effect manufacturing process to deposit electrolyte and ceramic interconnect for solid oxide fuel cells. However, it is generally difficult to deposit highly dense electrolyte and interconnect with desirable compositions for multiple components ceramics following the conventional process routine. In this paper, a novel approach based on the critical bonding temperature concept is presented to prepare dense electrolytes by plasma spraying. The spray particle size was utilized to control preferential element evaporation and subsequently composition of spray deposits. YSZ and LSGM are used as typical electrolytes. It is revealed that both plasma-sprayed electrolytes and interconnects present a gas permeability less than 1x10-7 cm4gf-1s-1 and can be directly applied to SOFC high temperature operation. Moreover, the deposited electrolytes present an ionic conductivity of over 80% sintered bulk materials which can be used to assembly SOFC with excellent performance.

Microstructural evolution and mechanical properties of hot pressed WC–α-Al2O3 with Y2O3 and CeO2

WC–α-Al2O3 composites, with the rare earth oxides (Y2O3 or CeO2) as sintering additives, were fabricated by mechanical alloying and hot pressing sintering. The microstructure and mechanical properties of WC–Al2O3 with and without oxides were investigated. The appearance of rare oxides increased the density of sintered bulk materials and suppressed the decarburisation due to the decrease of surface activation energy in sintering process. The hardness and fracture toughness of the samples first increased and then decreased, indicating that an appropriate addition of Y2O3 or CeO2 could strengthen the properties of samples. Microstructure observation results revealed that the grain growth of WC was significantly retarded, and the homogeneity of Al2O3 particulate dispersion was improved with the addition of Y2O3 and CeO2.

Segregation and precipitation formation for in situ oxidised 9Cr steel powder

Powder and sintered bulk materials of a reduced activation steel with oversaturated Ti and Y are prepared by gas atomisation and sintering. Segregation on both the surface and interior of the powder particles are observed after an thermal treatment. The oxidation of the segregates promotes formation of clusters of large Y oxide crystals and thick continuously distributed Ti oxide layers on the surface of the powder particles. The oxidation also occurs within the powder particles and mainly forms larger Ti oxide dispersoids in the grain boundaries of the powder. The coarsening and aggregation of the oxide dispersoids occur with increasing duration of the thermal treatment. Yttrium is also segregated and precipitated in the grains of the powder particles.

Melt spinning synthesis of p-type skutterudites: Drastically speed up the process of high performance thermoelectrics

In this study, via a home-made induction melting process combined with spark plasma sintering technique, we successfully synthesize the nanostructured skutterudites NdxYbyFe3CoSb12 (x+y=1.0, x=0.3, 0.4, 0.5, 0.6, 0.7) in less than 3h, which is significantly faster than that of the solid state reaction (168h) and the traditional melt spinning method (40h). Systematical microstructures analysis of the sintered bulk materials reveals the largely refined matrix grains (100–300nm) and ubiquitous YbSb precipitates (~30nm), resulting in a very low lattice thermal conductivity. A peak ZT up to 1.02 at 760K is obtained for Nd0.6Yb0.4Fe3CoSb12 compound.

Oxide coating fabrication by metal organic decomposition method for liquid blanket systems

Performance tests and process improvement for ceramic coatings fabricated by the metal organic decomposition (MOD) method have been conducted to obtain tritium permeation barrier and MHD insulator in liquid cooled blanket systems. In the present study, fabrication of MOD Er2O3 coating on a low activation ferritic steel substrate has been tested under reduced pressure condition. Measured hydrogen permeation reduction factors of >100 indicate that MOD coating fabrication under reduced pressure would be effective for suppression oxidation of a substrate and achieving superior coating performances. While crystallinities of MOD coating layers are significantly lower compared with sintered bulk materials, results of electrical conductivity and breakdown voltage measurements indicate that MOD Er2O3 coatings would have sufficient performances as an MHD insulation coating in a Li/V-alloy blanket system. Cathodoluminescence measurement using scanning electron microscope (SEM) is successfully applied to microscopic characterization of MOD coating layers.

Rapid preparation of CeFe4Sb12 skutterudite by melt spinning: rich nanostructures and high thermoelectric performance

In this work, we adopt a non-equilibrium melt spinning technique combined with a subsequent spark plasma sintering technique to successfully synthesize a p-type nanostructured CeFe4Sb12 skutterudite compound with high homogeneity in less than 24 hours. Microstructures of the melt-spun ribbons and the sintered bulk material are systematically investigated. The evolution of multiple-phase melt-spun ribbons into a single-phase skutterudite compound during the heating process is also carefully examined. Greatly refined matrix grains (300–500 nm) and numerous FeSb2 nanodots with sizes below 50 nm are evenly distributed inside the grains, and together contribute to the experimentally observed low lattice thermal conductivity of the sintered bulk material. Both absolute and average ZT values of this melt-spun skutterudite are about 10% higher than in the material of the same composition prepared by traditional melting and long-term annealing. The markedly shortened preparation time coupled with the enhanced thermoelectric performance should make this synthesis process of interest for commercial applications.

In situ synthesis and thermoelectric properties of PbTe–graphene nanocomposites by utilizing a facile and novel wet chemical method

In this work, a facile and novel wet chemical method is adopted to synthesize PbTe–graphene nanocomposites, and the thermoelectric properties of the sintered bulk materials are discussed in detail. An intercalative nanostructure is formed by using commercial graphene oxide nanosheets as both the dispersant and the two-dimensional growth template for PbTe nanoparticles in the synthesis process, where PbTe nanoparticles are in situ synthesized and graphene oxide nanosheets are reduced to graphene at the same time. FESEM and TEM measurements indicate that PbTe nanoparticles with sizes of 20–60 nm are uniformly anchored on the surface of graphene, and the nanostructure is retained in the bulk. These novel nanocomposites show enhanced thermoelectric properties as compared to bare PbTe prepared by the same route, as well as to samples prepared by traditional methods. The ultra-high electron mobility of graphene improves the electrical conductivity of the PbTe–graphene nanocomposites, and their conductivity exceeds not only that of bare PbTe but also the samples prepared by the traditional melt-quenching and melt-cooling processing techniques. Moreover, the much-decreased size of the PbTe particles in the bulk material, caused by the intercalative structure, increases the concentration of interfaces which results in the thermal conductivity of the nanocomposite being lower compared with the bare PbTe sample. Therefore, a much higher ZT value of the PbTe–graphene nanocomposites is obtained, reaching a value of 0.7 at 670 K. This is 6 times the value of the bare PbTe sample and a significantly higher ZT than for any n-type PbTe samples prepared by any of the traditional synthesis routes.

Characterization of TiAl-based alloy with high-content Nb by powder metallurgy

Powders with nominal composition of Ti-45Al-10Nb (mole fraction, %) were prepared by mechanical ball milling and were then consolidated by hot-pressed sintering in vacuum atmosphere. The microstructures of 12 h-milling composite powders and sintered bulk materials were characterized by OM, XRD, SEM and EDX. The results showed that the composite powders were completely homogeneous, and fine-grained Ti/Al/Nb composite powders were made. The microstructure of the consolidated alloy was composed of fine equiaxed grains with large pure Nb particles. And the results also indicated that the compressive strength and ductility of the sintered TiAl-based bulk material at RT could be improved effectively by adding 10% Nb element. The yield strength (σ0.2) and fracture strength (σb) of the alloy were 842 MPa and 1314 MPa, respectively, and the compressive ductility (δp) was 12.4%.

Formation mechanism and magnetic properties of barium hexaferrite quasi-single crystals fabricated via magnetic forming and liquid participation sintering

Highly textured barium hexaferrite material is a good candidate as the high performance gyromagnetic ferrite for self-biased and low loss millimeter wave devices. To fabricate it, a magnetic forming and liquid phase participation sintering route has been developed and its formation mechanism has been discussed based on the heating behaviors of starting powders and micro-structural characteristics of sintered bulk materials. The results indicated that the diffusion and re-crystallization of liquid phase from melting of small crystallites promoted fast densification and high orientation and led to the formation of highly textured structures. The highest grain orientation degree achieved to 0.986 at the optimum sintering temperature of 1280°C. As the grain orientation degree increased, the coercive forces of the obtained barium hexaferrite materials dramatically decreased from 5185Oe to 120Oe, while the apparent magneto-crystalline anisotropic fields linearly decreased to the value very similar to the theoretical value of single crystals.

Thermoelectric Properties of Polycrystalline Ca0.9Yb0.1MnO3 Prepared from Nanopowder Obtained by Gas-Phase Reaction and Its Application to Thermoelectric Power Devices

Ca0.9Yb0.1MnO3 nanopowder prepared by a gas-phase reaction (GPR) consisted of well-dispersed particles with an average diameter of 47 nm. Sintering of this GPR powder proceeded rapidly and at a lower temperature than that required for a comparable powder prepared by conventional solid-state reaction (SSR). The sintered bulk material from the GPR powder (GPR-bulk) consisted of small grains with an average diameter of 620 nm; this morphology is completely different to that of the SSR-bulk in which larger grains bind together to form a network-like structure. A maximum power factor of 0.19 mW·m-1·K-2 was obtained for GPR-bulk at 973 K; this value is higher than that of SSR-bulk, mainly as a result of the lower electrical resistivity of GPR-bulk. The thermal conductivity of GPR-bulk is also lower than that of SSR-bulk, possibly because of increased phonon scattering at the grain boundary. The maximum value of the dimensionless figure of merit of 0.13 was obtained for GPR-bulk at 1073 K; this value is about 1.5-fold higher than that for SSR-bulk at 773 K. A unicouple device consisting of a p-type Ca2.7Bi0.3Co4O9 leg and an n-type Ca0.9Yb0.1MnO3 (GPR-bulk) leg was fabricated. Both oxide legs used for the measurement are 3.1–3.5 mm in both width and thickness and ∼5 mm in height. The device generated up to 0.14 W of power when the hot- and cold-side temperatures at the ends of the oxide legs were 1095 and 390 K, respectively.

Effect of spark plasma sintering temperature on the microstructure and mechanical properties of a Ti 2AlC/TiAl composite

The effect of spark plasma sintering (SPS) temperature on the microstructure and mechanical properties of a bulk ultrafine structured Ti 2AlC/TiAl composite prepared by SPS of a mixture of mechanically alloyed Ti–Al powder with a composition of Ti–50Al (at%) and 10 vol% carbon nanotubes (CNTS) has been investigated. X-ray diffraction analysis showed that the sintered bulk material mainly composed of γ-TiAl and Ti 2AlC phases. The microstructures of the samples were examined using scanning electron microscopy and transmission electron microscopy. It was found that when an SPS temperature of 950 °C was used, the bulk Ti 2AlC/TiAl composite had a high density of 98.3%, and a microstructure consisting of a continuous interpenetrating network of Ti 2AlC phase and equiaxed TiAl grains with an average size of 300 nm. The compressive yield strength and hardness of the sintered samples reached 2058 MPa and 6.12 GPa respectively. With increasing the SPS temperature up to 1150 °C, the TiAl grains became significantly coarsened, leading to clearly reduced mechanical properties.

Development of WC-Co Coatings Deposited by Warm Spray Process

The high-velocity oxy-fuel (HVOF) process is commonly used to deposit WC-Co coatings. There are some problems with this process; especially the decomposition and decarburization of WC during spraying make a coating brittle. To suppress such degradation, the warm spray (WS) process was applied to deposit WC-Co coatings, which is capable of controlling the flame temperature in the range of 500-2000 °C. The microstructure and phases of the deposited coatings were characterized by using SEM and XRD, and the mechanical properties such as hardness, fracture toughness, and wear properties were also investigated. WS process successfully suppressed the formation of the detrimental phases such as W2C and W, which are usually observed in HVOF coatings. The WS coatings showed the similar trend of the hardness variation for Co content with a sintered bulk material. Improvement of toughness and wear behavior was also observed in WS coatings.

Sintering and microstructure characterisation of metal oxide codoped yttria stabilised zirconia for thermal barrier coating applications

In this study, yttria stabilised zirconia (YSZ) was codoped with six metal oxides of different valence, atomic mass and ionic radius in order to investigate microstructural changes as a result of doping. A mechanical alloying process was used to produce the powder blends which were subsequently sintered at 1500°C for 120 h. A detailed microstructural study was conducted to characterise both the alloyed powders and sintered bulk materials. It was found that the occurrence of oxygen vacancies played a crucial role in stabilising the cubic and tetragonal phases and the presence of cations with larger ionic radius played a similar effect. Furthermore, the use of oxygen vacancy in the pseudo binary phase diagram was able to provide a general predication of phase transformation; however, an equivalent oxygen vacancy taking into consideration both the oxygen vacancies and dopant ionic size may give a more accurate prediction of microstructure.

Influence of disorder on the superconducting properties of polycrystalline MgB2

The influence of disorder introduced by neutron irradiation is investigated for sintered bulk material and for in-situ iron sheathed wires. We find strong enhancements of the upper critical field and a decrease of the upper critical field anisotropy. Accordingly, the irreversibility fields, defined as the fields, where the first continuous superconducting current path occurs and the resistivity disappears, are enhanced. The critical current densities increase after irradiation, especially at high magnetic fields. This enhancement is mainly caused by the changes of the reversible properties, but slight changes in flux pinnning are observed as well.

Characterization of the Microstructure and Electrical Conductivity of Plasma-Sprayed La<SUB>0.5</SUB>Sr<SUB>0.5</SUB>CoO<SUB>3</SUB> Coating

Lanthanum strontium cobalt oxide (LSCO) has been employed as cathode and intercollector materials for solid oxide fuel cells (SOFCs). In the present study, the La0:5Sr0:5CoO3 layer was deposited by atmospheric plasma spray (APS). The effect of heat treatment on microstructure and electrical conductivity of the LSCO deposits was examined. The electrical conductivity of the deposits along the lamellar direction was measured by a four-probe dc approach. The microstructure of the deposit was characterized by scanning electron microscopy and X-ray diffraction analysis. The results showed that the electrical conductivity of plasma-sprayed LSCO coating was lower than that of sintered bulk material with the same composition. The minor metastable phase in the as-sprayed coating reduced the electrical conductivity. The annealing treatment at 1000 � C led to the elimination of the metastable phase and improved the electrical conductivity. The lamellar structure features with the limited interface bonding limits the electrical conductivity. [doi:10.2320/matertrans.47.1654]