Porous NiAl Research Articles

Self-exothermic reaction behavior and pore structures of Ni-Al intermetallics with more than 70% porosity

Abstract In this study, high-porosity Ni-Al alloys with complete shape and controllable pore structures were prepared by combining the self-exothermic reaction with the space holder method. The observations show that porous Ni-Al sintered discs appeared to melt and deform after a violent reaction without adding space holder particles. The link between the volume content of urea particles and macroscopic morphology, combustion behavior, phase composition and pore structures of porous Ni-Al compounds were studied. The space holder pores acts as a heat absorber, greatly increasing the open porosity to 76.0% when adding 50 vol% urea particles. This leachable space holder method can be an effective strategy to regulate the sample shape and pore characteristics of porous Al-containing intermetallics.

Discrete element model for effective electrical conductivity of spark plasma sintered porous materials

AbstractThis paper aims to analyse electrical conduction in partially sintered porous materials using an original resistor network model within discrete element framework. The model is based on sintering geometry, where two particles are connected via neck. Particle-to-particle conductance depends on neck size in sintered materials. Therefore, accurate evaluation of neck size is essential to determine conductance. The neck size was determined using volume preservation criterion. Additionally, grain boundary correction factor was introduced to compensate for any non-physical overlaps between particles, particularly at higher densification. Furthermore, grain boundary resistance was added to account for the porosity within necks. For numerical analysis, the DEM sample was generated using real particle size distribution, ensuring a heterogeneous and realistic microstructure characterized by a maximum-to-minimum particle diameter ratio of 15. The DEM sample was subjected to hot press simulation to obtain geometries with different porosity levels. These representative geometries were used to simulate current flow and determine effective electrical conductivity as a function of porosity. The discrete element model (DEM) was validated using experimentally measured electrical conductivities of porous NiAl samples manufactured using spark plasma sintering (SPS). The numerical results were in close agreement with the experimental results, hence proving the accuracy of the model. The model can be used for microscopic analysis and can also be coupled with sintering models to evaluate effective properties during the sintering process.

Effect of Cold Compaction Pressure on Porous NiAl Articles Produced by Using Space Holder Urea via VCS

Porous NiAl samples were obtained by using 20, 40, 60 vol.% space holder urea particles. The urea particle size was in the range of 150-300 µm. Pressures of 50, 100, 200 and 300 MPa were applied by cold press to the mixtures formed by nickel, aluminum, preformed NiAl powders and urea particles. The dimensions, macrostructure, microstructure and compressive strength of the parts produced with different cold compaction pressures were analyzed after volume combustion synthesis (VCS). The aim of the study is to determine the cold compaction pressure which provides the closest dimensional changes after VCS, in samples having different urea amounts. These samples will be used for forming a multi-stack article in a further study. Since a high difference in dimensional change of different layers during VCS will lead to cracking and separation, similar dimensional change is required for different layers in a multi-stack sample design. The samples which were cold pressed with 300 MPa pressure sometimes contained cracks after they were taken out of the die after cold pressing or after VCS. Therefore, they could not be subjected to characterization. The closest dimensional change and higher compressive strength values after VCS were obtained in samples having different urea contents, when they were cold pressed at 200 MPa. The highest mean compressive strength (218.8±29.5 MPa) was attained in the sample which was prepared with 20% urea particles and which was cold compacted with 200 MPa.

Reaction Process and Pore Structure Control of Porous NiAl Intermetallic Compounds via Thermal Explosion

Reaction Process and Pore Structure Control of Porous NiAl Intermetallic Compounds via Thermal Explosion

Effect of Diluent Amount on Properties of Porous NiAl

Porous NiAl parts were formed by using Ni and Al elemental powders, preformed NiAl as diluent and NaCl particles as space holder (SH). The aim of utilizing preformed NiAl (30%-40%) as a diluent was to preserve the shape of the products. The amounts of the SH NaCl particles in NiAl were 25-50-75 vol.% and their sizes were in 300-500 µm range. Porous NiAl samples were prepared by volume combustion synthesis (VCS). The adiabatic temperatures of the Ni+Al mixtures having 30 and 40% diluent NiAl were calculated as 1638.9 and 1460.8°C, respectively. Formation of NiAl phase was verified by XRD analyses. In the green pellets, the total porosity amount was higher than the added NaCl amount. Also it was slightly higher in the product pellets than in the green pellets before VCS. Compressive strength and microhardness values of the samples which contained 30% diluent NiAl were higher than the samples which contained 40% diluent. Average compressive strength values of the products that were obtained by 25% NaCl and 30 and 40% diluent NiAl additions were 112.0±29.5 and 66.0±20.5 MPa, respectively.

Effect of the heating rate on the thermal explosion behavior and oxidation resistance of 3D-structure porous NiAl intermetallic

Porous NiAl intermetallic compounds demonstrate great potential in various applications by their high porosity and excellent oxidation resistance. However, to obtain a controllable NiAl intermetallic structure by tuning different process parameters remains unclear. In this work, porous NiAl intermetallic compounds were fabricated by economic and energy-saving thermal explosion (TE) reaction. The relationship between microstructure and process parameters was revealed using three-dimensional X-ray microscopy (3D-XRM) with high resolution and non-destructive characteristics. The geometrical features and quantitative statistics of the porous NiAl obtained at different heating rates (2, 10, 20 °C min−1) were compared. The result of the closed porosity calculation showed that a lower heating rate (2 °C min−1) promoted the Kirkendall reaction between Ni and Al, resulting in a high closed porosity (5.25%). However, at a higher heating rate (20 °C min−1), a homogeneous NiAl phase was observed using the threshold segmentation method, indicating uniform and complete TE reaction can be achieved at a high heating rate. The result of the 3D fluid simulation showed that the sample heated at 10 °C min−1 had the highest permeability (2434.6 md). In this study, we systematically investigated the relationship between the heating rates and properties of the porous NiAl intermetallic, including the phase composition, porosity, exothermic mechanism, oxidation resistance, and compression resistance. Our work provides constructive directions for designing and tailoring the performance of porous NiAl intermetallic compounds.

Constructing of 3D porous composite materials of NiAl/CNTs for highly efficient solar steam generation

Solar steam generation technology is considered as a facile and effective way to produce clean water from undrinkable water resources. Herein, a composite material with a novel 3D structure has been proposed for steam generation. By developing porous NiAl, which exhibiting high strength and corrosion resistance, and carbon nanotubes (CNTs), fabricated via self-propagating reaction (SHS) and chemical vapor deposition (CVD) methods, respectively, the composite materials of NiAl/CNTs and 3D structures have been created. The results shows that the 3D composite materials of NiAl/CNTs possess high porosity (≥72.8%), open-pore structure can provide abundant passage for water transportation and steam escape. In particular, the combination of the extraordinary light absorption properties of CNTs and the multiple scattering of light in the 3D porous NiAl pores grants the materials with an excellent broad-band light absorption rate (94.4%) and high evaporation rate (2.41 kg m−2 h−1) under 1 sunlight irradiation. Furthermore, the NiAl/CNTs composite materials with 3D structure shows good purification effect on heavy metal ion, such as Cr3+, Cd2+, Cu2+, Pb2+, and Zn2+ and organic pollutants of RhB, as well as have excellent stability in simulated seawater and sewage. The findings provide guidance for the design of new high-performance materials that perfectly combine traditional 3D porous materials and CNTs for seawater desalination and sewage treatment.

Thermal conductivity analysis of porous NiAl materials manufactured by spark plasma sintering: Experimental studies and modelling

This work presents a comprehensive analysis of heat transfer and thermal conductivity of porous materials manufactured by spark plasma sintering. Intermetallic nickel aluminide (NiAl) has been selected as the representative material. Due to the complexity of the studied material, the following investigation consists of experimental, theoretical and numerical sections. The samples were manufactured in different combinations of process parameters, namely sintering temperature, time and external pressure, and next tested using the laser flash method to determine the effective thermal conductivity. Microstructural characterisation was extensively examined by use of scanning electron microscopy and micro-computed tomography (micro-CT) with a special focus on the structure of cohesive bonds (necks) formed during the sintering process. The experimental results of thermal conductivity were compared with theoretical and numerical ones. Here, a finite element framework based on micro-CT imaging was employed to analyse the macroscopic (effective thermal conductivity, geometrical and thermal tortuosity) and microscopic parameters (magnitude and deviation angle of heat fluxes, local tortuosity). The comparison of different approaches toward effective thermal conductivity evaluation revealed the necessity of consideration of additional thermal resistance related to sintered necks. As micro-CT analysis cannot determine the particle contact boundaries, a special algorithm was implemented to identify the corresponding spots in the volume of finite element samples; these are treated as the resistance phase, marked by lower thermal conductivity. Multiple simulations with varying content of the resistance phase and different values of thermal conductivity of the resistance phase have been performed, to achieve consistency with experimental data. Finally, the Landauer relation has been modified to take into account the thermal resistance of necks and their thermal conductivity, depending on sample densification. Modified theoretical and finite element models have provided updated results covering a wide range of effective thermal conductivities; thus, it was possible to reconstruct experimental results with satisfactory accuracy.

Reaction mechanism and oxidation resistance at 700–900 °C of high porosity NiAl intermetallic

Porous NiAl intermetallic compound exhibit attractive properties. However, their simple and controllable preparation and understanding of oxidation mechanism are still incipient. Thermal explosion was employed to facilely fabricated highly porosity NiAl. The pore structure of the product has been meticulously characterized using 3D-XRM, and oxidation corrosion test was conducted in a fluid and static air at 700–900 °C for up to 120 h. Dynamic oxidation shows that 800 °C is the turning point of oxidation mechanism. The oxidation of porous NiAl follows the logarithmic oxidation law below 800 °C and the parabolic oxidation law above 800 °C.

Preparation of Porous NiAl Intermetallic with Controllable Shape and Pore Structure by Rapid Thermal Explosion with Space Holder

Due to the high exothermic characteristic of NiAl during the reaction synthesis process from Ni–Al elemental powders, the NiAl intermetallic melts frequently, and the specimens are difficult to maintain their original shape, which leads to the inhomogeneity of the pore size and morphology. To tackle this problem, porous NiAl intermetallic monolith with controllable shape and pore structure was prepared through thermal explosion (TE) using NaCl as space holder. The TE behavior was recorded, and the effect of the volume fraction of NaCl on the phase composition, macroscopic feature, pore morphology and open porosity were investigated. The results showed that NiAl was the main phase in the products, and the specimen was free from cracking or deforming when NaCl content reached 30 vol%. The interconnected channel and pore windows were formed, and the open porosity was improved greatly to 63% by adding 50 vol% NaCl. The leachable space holder route provides a simple way to control the shape, pore structure and open porosity of the synthesized porous NiAl intermetallic. The use of NaCl as space holder reduced the combustion temperature of NiAl porous intermetallic compounds and kept the macroscopic shape intact.

Development of a new infrared heater based on an annular cylindrical radiant burner for direct heating applications

Infrared heaters based on radiant burners are widely used in industry for thermal processing of materials such as rolled steel, textiles, food and other products. A new configuration of a gas-fired infrared heater is proposed and has been experimentally studied. The heater is constructed using an annular cylindrical radiant burner mounted inside a stainless steel conical reflector. The combustion of premixed natural gas with air is stabilized in the cavity of the cylindrical burner which is made of porous Ni-Al intermetallic. The reflector not only refocuses the omnidirectional radiant flux of the burner, but also participates in heat exchange with flue gases and emits an additional IR flux. The radiant heat output and NOX emission of the IR-heater are experimentally studied in two combustion modes, i.e. operation with and without preliminary heating of combustion air by recuperating flue gas heat. The operating conditions providing a 70–75% radiation efficiency and NOX emission of about 75 ppm in the power range of 700–5700 W are discussed. For evaluating the potential application of a new IR-heater, the obtained results in terms of the relative NOX emission per unit of produced infrared heat are compared with those of typical radiant tubes and electric IR heaters.

Coral-flake Co particles electrodeposited into the porous NiAl matrix

The synthesis of nanoscale arrays of magnetic particles, which may be used in magnetic recording devices, has attracted considerable interest recently. The coral-flake Co particles were electrodeposited into the pores of the porous NiAl matrix. Compared with the nonanodized samples, the anodized samples in H3PO4 solution formed thicker Al2O3 during the production of ordered Co particle arrays into the porous NiAl matrix. The deposition at a negative potential resulted in increased imax and decreased tmax. The diameter of Co particles increased with increasing deposition time. The interspace of Co was consistent with the interspace of pores distributed in the NiAl matrix. The coercivity (Hc) and the ratio of remanent magnetization to saturation magnetization (Mr/Ms) decreased with increased growth rate of the porous NiAl matrix at the same deposition time. Hc and Mr/Ms also decreased with the prolonged deposition time due to increasing Co size.

Synthesis of Porous Ni3Al Intermetallic Nano-compounds in a Molten LiCl with Assistance of CaH2 as a Structure-controlling Agent

Porous Ni3Al intermetallic nano-compound was successfully prepared by a chemical method at 600 °C in a LiCl-CaH2 system. The prepared powder had a crystal structure completely identical to Ni3Al phase with a porous network structure formed by interconnected nanoparticles. The BET surface areas were as high as 13.1–13.6 m2/g, corresponding to the estimated particle sizes of 59.0–61.3 nm. The surface areas were higher than previous reports (0.1–4.5 m2/g) obtained by conventional physical methods.

In situ construction of Ni enriched porous NiAl as long-lived electrode for hydrogen evolution at high current densities

Abstract The hydrogen evolution reaction (HER) is a key process in water electrolysis to produce hydrogen, but the development of highly active, stable and cost-effective monolith HER electrocatalysts by simple methods remains a great challenge. Here we report a compelling and powerful strategy to prepare a Ni enriched three-dimensional nanoporous NiAl catalyst which can be directly used as a highly active and long-lived HER electrode in acids. The electrode exhibits low onset overpotential of only −33 mV. Overpotentials of merely −157 mV and −226 mV are needed to reach current densites of −100 and −200 mA cm−2, respectively, superior to recently reported state-of-art Ni-based monolith HER catalysts. The high activity is likely due to in situ obtained nanoporous structure with high surface area and nanosized Ni. Under a medium overpotential of −335 mV, the electrode can sustain −350 mA cm−2 up to 60 h, showing good long-term stability at high current density. The favorable combination of the high activity, stability, facile preparation and low-cost makes the Ni enriched nanoporous NiAl a promising electrocatalyst for high-speed hydrogen production.

Microstructure and Mechanical property of Porous Nickel aluminides Fabricated by Reactive Synthesis with Space Holder Powder

Effect of Ni:Al blending ratio on porous structure of porous nickel aluminides fabricated through reactive synthesis with space holder particles were investigated. Fabricated porous nickel aluminides had large pores derived from NaCl space holder particles and small pores derived from reactions between Ni and Al. Porosity and size of the small pores increased with increasing Al content in the raw powder mixture. Compressive property of porous NiAl are also investigated. porous NiAl exhibited good energy-absorption properties with relatively high plateau stress, high plateau end strain, and relatively flat plateau stress. This study suggests the possibility of intermetallic-based porous materials as high-performance energy absorber.

Rapid Preparation of Porous Ni–Al Intermetallics by Thermal Explosion

ABSTRACTPorous Ni3Al, NiAl, and NiAl3 intermetallics have been synthesized through simple and rapid process of thermal explosion (TE) from 75 at.% Ni/25 at.% Al, 50 at.% Ni/50 at.% Al and 25 at.% Ni/75 at.% Al elemental powders. The effect of the three components on temperature profile, phase composition, pore structure, and oxidation resistance were investigated. The results showed that the temperature of the specimens increased rapidly from 638°C to 1381°C, 620°C to 1613°C and 642°C to 1172°C in a few seconds. X-ray diffraction patterns indicated that Ni3Al, NiAl, and NiAl3 were the main phase or single-phase compounds in the TE products. Porous Ni3Al, NiAl, and NiAl3 intermetallics showed variable pore structures. Moreover, all the products of three components exhibited good oxidation resistance, particularly the porous NiAl and NiAl3 intermetallics.

Influences of sintering parameters on shape-retention ability of porous Ni3Al intermetallic fabricated by powder metallurgy

Porous Ni3Al intermetallic is successfully fabricated through elemental powder metallurgy of Ni-25 at% Al samples. The influences of sintering parameters, such as heating rate, isothermal treatment on the sintering behaviors of the samples are investigated. When the samples are heated continuously from room temperature to 1000 °C at a relatively high rate (>5 °C/min), the reaction between Ni and Al happens in an intense way. The heat suddenly released by the reaction produces liquid phase in the material, which facilitates the reaction significantly. Therefore, a sudden expansion (∼6%) occurs, resulting in severe deformation of the samples. The shape retention ability of the sample can be improved by isothermally treating the sample at a proper temperature (470–490 °C) for prolonged time or heating it up at a low heating rate. Under these sintering conditions, the reaction between Ni and Al happens in a much more controllable solid-state way. Therefore, the abrupt expansion can be decreased to ∼0.5%. During the solid-state reaction, imbalanced diffusion between Al and Ni results in more Al diffusing into Ni than the other way around, therefore creating pores on the Al side. As a result, homogeneous porous Ni3Al intermetallic samples can be fabricated and the regular shape of the sample can be well preserved.

Fabrication of porous NiAl intermetallic compounds with a hierarchical open-cell structure by combustion synthesis reaction and space holder method

We have developed a process to fabricate porous NiAl intermetallic compounds with a bi-modal porous structure by combining the combustion synthesis reaction and the space holder method. The fabricated porous NiAl showed large pores of the order of several tens of micrometers derived from the NaCl space holder and small pores of the order of several micrometers derived from the combustion synthesis reaction. The NaCl space holder acted as a heat absorber and contributed to the formation of small pores by preventing the melting of NiAl. Although the reaction was difficult to complete for large volume fractions of NaCl, it was possible to obtain single-phase NiAl by controlling the sintering conditions. We could finally obtain highly porous (porosity: >80%) single-phase NiAl with a hierarchical open-cell structure.

Fabrication, Pore Structures and Mechanical Properties of (TiB2–Al2O3)/NiAl Porous Composites

Open-celled porous (TiB2–Al2O3)/NiAl composites were successfully fabricated by using spherical carbamide as space holders via self-propagating high-temperature synthesis (SHS). Effects of 10Al–3B2O3–3TiO2 contents (0–20 wt%) on the pore structures and the quasi-static compressive behaviors of the resultant materials were investigated. The porous (TiB2–Al2O3)/NiAl composites exhibit composite pore structure consisting of homogeneously distributed and interconnected millimeter pores and micropores. The millimeter pores virtually inherit the shape and size of carbamide particles, while the pore size of micropores increases with increasing the 10Al–3B2O3–3TiO2 content. Depending on the volume fraction of the carbamide, the porosity of the porous materials can be easily controlled in a range of 55%–85%. When the porosity is about 72%, the compressive strengths of porous NiAl and porous (TiB2–Al2O3)/NiAl composite with 15% 10Al–3B2O3–3TiO2 in green compact are 19 and 32 MPa, and the corresponding strains are 2.9% and 5.7%, respectively. Furthermore, the quasi-static compressive behavior of porous (TiB2–Al2O3)/NiAl composites can be estimated by Gibson–Ashby model.

Tungsten wires and porous NiAl prepared through directional solidification and selective dissolution

ABSTRACTThe combination of directional solidification and selective dissolution was applied to fabricate tungsten (W) wires and porous NiAl matrix. A NiAl–W pseudobinary eutectic alloy with 1.5 at.% tungsten was directionally solidified in a Bridgman-type oven at 1700°C. Results confirmed that the relationships of the growth rate with the interfibrous spacing and diameter of W fibrous phases in the directionally solidified samples are in accordance with the Jackson and Hunt (J−H) model. Afterward, the NiAl matrix was selectively dissolved in an HCl:H2O2 solution to reveal W wires, which present various three-dimensional (3D) morphologies at different growth rates. The W fibrous phases in the NiAl–W alloy samples were then selectively removed with a mixed etchant of ammonium acetate to form a porous NiAl matrix at a constant potential. Dynamic corrosion curves revealed that etching W from the NiAl matrix was inhibited after 2–3 h. The porous structures of NiAl after removing W phases are linked to the 3D morphologies of W fibrous phases embedded in the NiAl matrix. The aspect ratio of W wires and the structures of porous NiAl can be adjusted by selecting the process parameters of this combined technology.