Uniform Phase Distribution Research Articles

Spark Plasma Sintering and Characterization of Al-TiB2 Composites

In this study, Al-TiB2 compacts fabricated by spark plasma sintering methods at different temperatures were characterized for densification, microstructural development, and mechanical properties. Sintering parameters used were temperatures of 500 °C and 550 °C under the pressure of 30 MPa. A very dense microstructure with uniform phase distribution and porosity was produced in the sample sintered at 550 °C with 2.5 wt% TiB2. The same sample exhibited excellent hardness value, and a high-tensile strength attributed to full metallurgical bonding, presence of sub-micron sized grains, and their uniform distribution. These results show that the TiB2 addition enhanced the composite’s hardness, sintered density, and tensile strength. In all the sintered samples, the fractographs revealed a mixed-mode fracture (ductile and brittle).

Chemically synthesizing exchange-coupled SmCo5/Sm2Co17 nanocomposites

A new strategy to chemically synthesize exchange-coupled SmCo5/Sm2Co17 nanocomposites by in situ decomposition of SmCox (5 < x < 8.5) is reported in this work. Our synthesis starts with the fabrication of Co/Sm2O3 (Sm to Co atomic ratio of Sm/Co = 1:4.2), which can be reduced into 40-nm SmCo5 single crystal nanoparticles by Ca under the protection of CaO, showing a high coercivity of 2.85 T and saturation magnetization (Ms) of 0.0671 A·m2·g−1. By changing the Sm/Co to 1:4.5, 1:4.8 and 1:5.2, SmCo5/Sm2Co17 nanocomposites with different proportions were acquired using the same process. Owing to the in situ decomposition of SmCox intermediate, the small size (both of their size less than 10 nm) and uniform phase distribution were achieved in our nanocomposites. Thus, the as-prepared nanocomposites display a strong exchange-coupling interaction. As a consequence, SmCo5/Sm2Co17 (Sm/Co = 1:5.2) exhibits a coercivity of 1.23 T and enhanced M7T (magnetization at 7 T) of 0.0812 A·m2·g−1, increasing by 21% than pure SmCo5. Our synthesis provides a new protocol to prepare exchange-coupled high-performance nanocomposites. The exchange-coupled SmCo5/Sm2Co17 nanocomposites with small size (both of their size less than 10 nm) have been chemically synthesized by in situ decomposition of SmCox (5 < x<8.5), which exhibits a coercivity of 1.23 T and enhanced Ms, increasing by 21% than pure SmCo5 .

Effect of double-step homogenization treatments on the microstructure and mechanical properties of Al–Cu–Li–Zr alloy

The Al3Zr particles formed in the Al–Cu–Li–Zr alloy during single and double-step homogenization treatments contributed to various microstructures and mechanical properties during the subsequent thermomechanical treatment. Compared with the single-step homogenization treatment (ST), the double-step homogenization treatment (DT) generated higher density and more uniform distribution of β′ (Al3Zr) phases, which makes the alloy retain more substructures and improve the initial strength of solid solution. During the ageing process, Al3Zr particles induced the T1 precipitation in grain interior and the width of PFZ get narrowed. Therefore, the strength and toughness of the DT sample improved in peak aged state compared with that of the ST sample. The peak hardness value of DT sample is 167.8 HV, which is higher than that of ST sample with 153.1 HV. Furthermore, the tensile strength and elongation of the DT sample at peak aged state (521 MPa, 556 MPa and 8.8 %, respectively) are higher than those of the STA sample (492 MPa, 535 MPa and 6.7 %, respectively), and the impact toughness of the DTA sample (13.2 J cm−2) is also higher than those of STA sample (9.1 J cm−2). Strength model showed that more uniform distribution of Al3Zr phases and substructures improve the initial strength of solid solution state, which increased by 16 MPa in the DT sample compared with the ST sample. The promotion of T1 precipitation inducing the strength increased by 18 MPa in the DT sample at the peak aged state compared with that of the ST sample. The increase of elongation and toughness contributed from the oriented grains with finer distribution and higher Schmid factor, less grain boundaries precipitates and narrower PFZ width.

Morphological Peculiarities of R–Fe–B (R = Nd, Pr) Alloys Formed upon Solidification by Strip-Casting

X-ray diffraction analysis, scanning electron microscopy, and atomic force microscopy are used to study the phase composition, texture, and peculiarities of the morphology of R–Fe–B (R = Nd, Pr, Dy) alloys prepared by strip-casting technology. Along with the traditional structural components of the alloys, namely, areas characterized by randomly oriented fine grains of the main magnetic phase with the Nd2Fe14B-type structure (which are formed near the contact surface of flakes) and coarse extended textured grains of the phase (in the volume of flakes), wide areas characterized by uniform distribution of rare-earth metal-rich phase with a period of 2–4 µm were also found. The formation of these areas takes place in the absence of coarse dendritic crystallites and, according to the suggested mechanism, is related to the solidification of the melt under conditions of its motion upon quenching on a cooled cylindrical surface. Data indicating the melt twisting upon solidification, which favors the formation of the structure with the aforementioned distribution of intergranular phase, are reported. The data obtained are discussed from the viewpoint of their practical utility for the improvement of the structure of alloys prepared by strip-casting and powders for manufacturing Nd–Fe–B permanent magnets.

A novel structured foam microreactor with controllable gas and liquid flow paths: Hydrodynamics and nitrobenzene conversion

This paper proposes a novel structured foam microreactor with controllable gas and liquid flow paths; in this microreactor, the liquid phase flows through a structured foam bed, and the gas phase flows through gas paths on both sides of the foam bed. A hydrodynamics analysis indicated that the proposed design ensured not only better residence time distribution and a longer residence time, but also uniform phase distribution and a stable gas–liquid interface. Accordingly, inefficient catalysts utilization and mass transport caused by the gas channeling in conventional foam-based microreactors was avoided. The results showed that the newly-developed microreactor exhibited higher nitrobenzene conversion because of the aforementioned merits. This study not only serves as a guideline for the optimization of the proposed microreactor, but also provides a new avenue for the design of multiphase catalytic reactors.

The Influence of Header Design on Two-Phase Flow Distribution in Plate-Fin Heat Exchangers

AbstractFluid flow maldistribution causes deterioration of heat transfer as well as pressure drop penalty in heat exchangers. A test bench was set up to investigate the effect of different header designs on air-water flow distribution in plate-fin heat exchangers (PFHX). Two-phase flow distribution was examined for air Reynolds numbers (ReG) of 28293542 and inlet qualities (x) of 46.3–64.0%. Two-phase flow distribution was seen to be more uneven in the heat exchanger in comparison with single-phase flow, the water distribution being more uneven than that of the air. For a fixed inlet quality, as the air flowrate was increased, the distribution of two-phase flow became increasingly nonuniform, showing a pattern similar to single-phase flow. Furthermore, the air distribution became more even, while the water flow became more unevenly distributed as the inlet quality increased. To mitigate the maldistribution, perforated plates were incorporated in the heat exchanger header. The improved headers significantly aided in distributing the two-phase flow more evenly. At ReG = 2829 and x = 46.3%, the heat exchanger effectiveness was expressed in terms of the unevenness in quality, Sx. The effectiveness decreased as the unevenness of the flow distribution was exacerbated, emphasizing the significance of uniform phase and flow distribution as a key element of heat exchanger design.

Vertical phase segregation suppression for efficient FA-based quasi-2D perovskite solar cells via HCl additive

Ruddlesden–Popper (RP) layered perovskites (with general formula of (RNH3)2An−1MnX3n+1) have drawn tremendous attention due to their excellent ambient stability while retaining promising device performance. However, quasi-2D perovskites with mixed-cation often suffer from severe phase segregation and vertically aligned multiple perovskite phases (with small n phases concentrated on the bottom and large n phases on the top) are usually formed. Here, we report a strategy to suppress vertical phase segregation by using hydrochloric acid (HCl) additive. As a result, homogenous quasi-2D perovskite films with uniform phase distribution and enhanced film quality are successfully obtained via this additive strategy, which is confirmed by PL measurements. With uniform distribution of different n value phases and high-quality film, FA-based quasi-2D perovskite solar cells with an efficiency of 13.16% and excellent stability was fabricated. This strategy paves a way to develop stable and efficient quasi-2D perovskite solar cells by suppressing phase segregation.

Effect of aluminum addition on microstructure and properties of a novel nickel–silicon-containing brass

Abstract

Electroless tin coated hydroxyapatite reinforced Mg–Sn alloy composite for enhanced bio corrosion resistance and bioactivity

The present study focuses on the influence of tin coated hydroxyapatite (HA) particles reinforced in Mg-5 wt.% Sn matrix on corrosion resistance. Electroless coating process has been used to coat hydroxyapatite particles with tin. From the results, it was understood that the electroless coating is a potential technique to successfully coat Sn on HA particles which helps in refinement of grains and uniform distribution of phases. It also prevents the dissociation of HA, thereby formation of unwanted phases such as CaSn which could be detrimental for corrosion properties due to galvanic coupling. The biodegradation was reduced by the addition of Sn coated HA in comparison with reinforcement of uncoated HA particles.

Slip-casting process of MgO–Y2O3 nanocomposite: Investigation of powder synthesis method

In this paper, yttria-magnesia composite nano-powders with a chloride and nitrate-acetate precursors were synthesized using the sol-gel method. The green body of MgO–Y2O3 nanocomposite was fabricated by the slip-casting method and the rheological properties were improved by adding Dolapix CE64 to the slurries as a dispersant agent. The Y2O3–MgO nanocomposite slurry with a solid loading of 60 wt% and the pH of 12 was obtained using 1 wt% Dolapix CE64.The X-ray diffraction (XRD), Field Emission Scanning Microscope (FESEM) and Brunaure-Emmett-Teller (BET) tests were used.Various slip/slurry characterization techniques such as sedimentation, viscosity measurements and porosimetry were used to optimize the slip casting parameters. The relative densities of the green and sintered body (1400 °C/2 h) were 50% and 86%, respectively. The FESEM results of the sintered yttria-magnesia composite with the chloride precursors showed the more uniform distribution of MgO and Y2O3 phases than the nitrate precursors.

Controlling the Microstructure and Texture Using Multidirectional Forging (MDF) to Develop a Low Corrosion Rate Mg Alloy

A new Mg alloy was cast, then extruded, and finally multidirectionally forged (MDF) at 180°C (MDF180) and 300°C (MDF300). The corrosion behavior was evaluated using electrochemical and immersion techniques. The mechanical property was assessed using tension and compression tests. The microstructures were analyzed using optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction (XRD), and thermodynamic calculations, while texture was studied by XRD and electron backscatter diffraction. Results indicated that due to grain refinement and good distribution of second phase after MDF, yield strength as well as elongation were improved. The corrosion rates were reduced for MDF180 due to the uniform distribution of the second phase along with uniform distribution of grains. Corrosion rate was more reduced for MDF300 due to dissolution of second phase and elimination of worked grains.

Modification of the microstructure by rotating magnetic field during the solidification of Al-7 wt.% Si alloy under microgravity

Abstract Directional solidifications of Al-7 wt.% Si alloy were carried out under microgravity on board the International Space Station to investigate the impact of a rotating magnetic field (RMF) on the solidified microstructure. It has been found that the RMF significantly influences the solidified microstructure in the conditions corresponding to the lowest growth rate, whereas it has negligible influence on the microstructure under the highest growth rate, indicating that the RMF intensity and the forced liquid flow are too weak, compared to the solidification front velocity, to impact the microstructure. For the lowest growth rate applied, the RMF application results in a more uniform eutectic phase distribution and a smaller dendrite arm spacing. This is ascribed to the RMF induced forced liquid flow that makes more uniform Si concentration in the bulk liquid above the solid-liquid interface. Additionally, the RMF application possibly modifies the columnar dendritic network by changing dendrite growth directions. This observation can be attributed to the combined effect of the RMF induced forced liquid flow and of the thermoelectric magnetic force on the dendrites resulting from the application of the RMF as well.

Thermally conductive SiC as support of aluminum fluoride for the catalytic dehydrofluorination reaction

Aluminum fluoride (AlF3) is a typical catalyst for dehydrofluorination of hydrofluorocarbons (HFCs) to fluoroolefins with high heat of reaction. Consequently, heat supply and sintering are the key challenges for AlF3-based catalysts. Herein, SiC with high thermal conductivity and resistance to HF corrosion is suggested as a candidate support of AlF3 catalyst. The interaction between AlF3 and SiC leads to uniform distribution of the catalytic phase, and as a result of this, AlF3/SiC exhibits high catalytic activity and stability for the dehydrofluorination of 1,1-difluoroethane to vinyl fluoride. This study proposes a novel catalyst support (SiC) for strong endothermic catalytic reactions involving HFCs for the first time to the best of our knowledge.

Microstructure and texture evolution of the β-Mg17A12 phase in a Mg alloy with an ultra-high Al content

In the present study, the effects of equal channel angular pressing (ECAP) on the microstructure and mechanical property of the Mg-20Al alloy were systematically investigated. For the first time, the texture of Mg17Al12 phase and its evolution with ECAP conditions were reported. The results show that increasing the processing temperature and passes generates more uniform distribution and finer size of β-Mg17Al12 phases. The large pieces of β-Mg17Al12 phases are composed of many fine grains with different crystallographic orientations. For the β-Mg17Al12 phase, a preferred distribution of (001) appears at 523 K and 573 K, and hardly varies with temperature. Nevertheless, a random texture is observed at 623 K. The (0002) poles exhibit a preferred distribution at 473 K, but this preferred distribution varies with temperature. A random distribution of (0002) poles is observed when processed at 623 K. Many types of crystallographic planar relationship between β-Mg17Al12 phase and α-Mg matrix are observed and the relationships of {11 2- 3}//{100} or {110} or {111} and {1 2- 11}//{100} or //{110} or {111} have a relatively higher frequency than others. The texture of α-Mg matrix is much different from that of the ECAPed Mg alloys with a relative low Al content, in which a texture with basal poles inclining approximately 45° away from the extrusion direction often develops. The mechanical properties of Mg-20Al alloy are closely related to the temperature and passes of ECAP. A higher temperature often decreases the yield strength, but hardly alters the maximum strength. There is a low plasticity for all the samples and increasing processing temperature slightly enhances the plasticity. The corresponding mechanisms were deeply discussed.

Sol-gel route: An original strategy to chemically stabilize proton exchange membranes for fuel cell

We present the elaboration, via a Sol-Gel (SG) route, of a new generation of hybrid membranes for PEMFC applications, in order to improve their performances and durability. The strategy was to create, within a commercial sPEEK membrane, a reactive SG phase able to reduce the oxidative species generated during FC operation and to improve the proton conductivity. The SG content is adjusted by tuning the SG precursors/sPEEK ratio used for the impregnation step. Raman analyses show that a uniform distribution of the SG phase in the membrane section is obtained when NMR analyses demonstrate its high extent of condensation, and reveal the structuring effect of the sPEEK membrane on the SG phase at the nanoscale. The membranes exhibit a better liquid water uptake and proton conductivity than pristine sPEEK membrane up to a SG uptake of 18%, with a gas permeability slightly higher than sPEEK but still lower than the benchmark Nafion. H2O2 accelerated aging tests evidenced the ability of the SG phase to prevent the oxidative degradation of sPEEK. Finally, the FC operability tests showed better and more durable performances for the hybrid membranes, without any increase of the gas permeability during operation.

Numerical Estimation of the Heat and Mass Transfer Efficiency Considering Nonuniformity in Water and Air Distribution

Using a system of differential equations of heat and mass transfer written in a cylindrical coordinate system with volumetric interphase sources of heat and mass transfer, a mathematical model of heat and mass transfer in film blocks of cooling tower irrigators has been developed. Expressions describing the interphase sources of heat and mass transfer and correlations for calculation of their parameters are presented. The parameters of the sources, i.e., the coefficients of heat and mass transfer, are related with the hydraulic resistance of irrigator blocks and can be calculated with both uniform and nonuniform distribution of air and water. The calculated volumetric coefficients of mass transfer were calculated and compared with the experimental data for regular mesh packing made of polyethylene and sheet-type metal packing. The numerical solution of the system of differential equations yielded the distribution of water, air, and air humidity content along the height of a packing block. The thermal efficiency of water cooling was calculated. It agrees satisfactory with the experimental data for spraying blocks made of polyethylene mesh. The causes of nonuniform distribution of phases among the packing blocks, which decreased the thermal efficiency of the cooling tower, were examined. The numerical investigations revealed that nonuniform air supply has the most significant influence decreasing the thermal efficiency of water cooling by as much as 35% or more compared with uniform phase distribution. The presented mathematical model can be used for estimating the cooling capacity of existing cooling towers at thermal power stations (TPSs) and industrial plants as well as in upgrading or developing high-efficient spraying blocks for film-type apparatuses.

On the corrosion behaviour of as-cast and heat-treated Mg-RE alloys in 0.9% NaCl solution

The microstructure and corrosion behaviour of as-cast and heat-treated Mg-1.44Nd and Mg-1.43Ce (wt.%) alloys in 0.9% NaCl (wt.%) solution were investigated using electrochemical tests, X-ray diffraction (XRD) and scanning electron microscopy, combined with energy-dispersive X-ray spectroscopy (SEM-EDS). The as-cast microstructure of both alloys revealed the presence of second phases. Heat treatment at 535°C for 6 h led to a more uniform distribution of the second phases in Mg-1.44Nd alloy and their dissolution along the grains boundaries in the Mg-1.43Ce alloy. As a result, the corrosion resistance was improved in the heat-treated alloys. Accordingly, the corrosion resistance values for the heat-treated alloys were much higher than those of the as-cast alloys, indicating that the heat-treated alloys were less susceptible to the corrosion. Also, the heat-treated Mg-1.43Ce alloy seems to have very good corrosion resistance (26890 Ω cm2) compared to the Mg-1.44Nd alloy (6156 Ω·cm2) by preventing pitting corrosion along the grains boundaries. The corrosion product was made up mainly of magnesium hydroxide Mg(OH)2 and magnesium oxide MgO and more uniform corrosion morphology were found in the heat-treated alloys.

Reinforced Al-Matrix Composites with Ni-Aluminides, Processed by Powders

The paper addresses issues of interest related to the uniform distribution of the reinforcement phase and the continuity of the metal matrix, for two ways of obtaining the powder composites, the hot extrusion and the spark plasma sintering (SPS) respectively. The reinforcement phase is the nickel aluminides obtained by mechanical alloying in ball mills, and the metal matrix is made of aluminum powders. Prior to extrusion, the mixture of aluminum powders and nickel aluminides was pressed and subsequently sintered in environments with high carbon potential, so that in the final product, nickel aluminides, carbide and aluminum oxides were found as reinforcement phases. The obtained results confirmed the hypothesis that, from a blank product with a random distribution of the reinforcement phase, a product with an ordered distribution of the reinforcement phase is obtained after extrusion and that for the same initial proportion of nickel aluminides as reinforcement phase, the composite hardness obtained by hot extrusion is higher compared to that obtained by SPS, the difference being determined by the increase of the proportion of the reinforcement phase by the appearance of aluminum carbide and aluminum oxides during the sintering operation in the high carbon environment.

Properties and DEFC tests of Nafion - Functionalized titanate nanotubes composite membranes prepared by melt-extrusion

Nafion based composites are promising materials to improve the performance of direct ethanol fuel cells. In this work, composite membranes of Nafion and titanate nanotubes functionalized with sulfonic acid groups were prepared by melt-extrusion and tested in a direct ethanol fuel cell. Far and mid infrared spectroscopies evidenced the formation of ionic bridges between the sulfonic acid groups of both functionalized nanoparticles and the ionomer. Small angle X-ray scattering measurements revealed that the melt-extrusion method leads to an uniform distribution of the inorganic phase in the ionomer matrix. Such structural analysis indicated that the improved the proton conduction properties of the composites, even with the addition of a high concentration of functionalized nanoparticles, are an outcome of the synergistic ionic network due to the hydrid organic/inorganic proton conducting phases. However, an improvement of the fuel cell performance is observed for 2.5 wt% of functionalized titanate nanotubes, which is a result of the lower ethanol crossover and the plasticizing effect of the aliphatic segments of the organic moieties grafted at the surface of the titanate nanoparticles.

Selective Laser Melting of In Situ TiB/Ti6Al4V Composites: Formability, Microstructure Evolution and Mechanical Performance

In the present study, a series of in situ TiB/Ti6Al4V composites were fabricated using selective laser melting. The formability, microstructure evolution and mechanical properties of the as-built samples added with different contents of TiB2 were studied. It is found that the densification level is related to both the content of TiB2 and laser energy density. The added TiB2 reinforcement particle can spontaneously react with titanium and then form the TiB phase. The needle-like TiB phase tends to transform into dot-like particles with the decrease in energy density. Additionally, with the increase in TiB2 content, the TiB phase is coarsened due to the increased nucleation rate and more reactions. The grain morphology is found to largely depend on the translational speed of solid–fluid interface determined by the temperature gradient and cooling rate. Also, the microhardness of the as-built TiB/Ti6Al4V composites is obviously improved. More interestingly, as the energy density increases, the microhardness of the as-built TiB/Ti6Al4V composites firstly increases and then decreases due to the synergy of grain size and different morphologies and distribution of TiB phases. The wear resistance of TiB/Ti6Al4V composites is far superior to that of Ti6Al4V alloy owing to the increased microhardness resulted from the uniform distribution of the hard TiB phase in the matrix.