Spark Plasma Sintering Samples Research Articles

Effect of grain size on the dielectric and magnetic properties of YFeO3 ceramics densified by microwave sintering and spark plasma sintering from powders synthesized via reverse chemical co-precipitation

YFeO3 (YFO) ceramics were synthesized by reverse co-precipitation at different pH values followed by microwave sintering (MS) and spark plasma sintering (SPS) techniques. Also, the effect of the grain size on the magnetic and dielectric properties of the sintered ceramics was examined using various characterization methods. Moreover, FESEM micrographs showed that the particle size of the calcined samples has declined by increasing the pH value. Furthermore, the magnetization curves exhibited weak ferromagnetic behavior in the pre-sintered powder samples. Besides, the Ms value was enhanced from 1.26emu/g to 2.3emu/g by an increase in pH from 8.5 to 10.5 owing to reducing the particle size and subsequently raising the amount of uncompensated surface spins. Furthermore, the powder samples showed an increase in the Hc and Mr values from 15Oe to 28Oe and 0.127emu/g to 0.296emu/g, respectively, with a reduction in particle size from 0.632 μm to 0.272 μm. Additionally, the specimens processed through the sintering techniques demonstrated a remarkable reduction in magnetic characteristics. Therefore, a decrease in magnetic saturation values from 2.3emu/g to 0.43emu/g for the MS sample and to 0.17emu/g for the SPS specimen was attributed to a marked reduction in the amount of free surfaces and subsequently uncompensated surface spins. At last, the observed dielectric characteristics of all the sintered pellets had a common dielectric behavior. This signifies that by increasing the frequency the ε′ and tanδ decline at lower frequencies and become frequency-independent at higher frequencies based upon the Maxwell-Wagner polarization theory. On the other hand, the ε′ value for the SPS sample produced at pH = 10.5 underwent a decline of 2.5 times at the low frequency of 200 Hz compared to the MS one due to the significant decrement of the grain boundaries on account of the coarsening of grains.

Interfacial investigation of Al/Ni in new generation of composite (Metal-3D lattice metal): Annealing time effect

In the present research, an advanced Al composite reinforced with open-cell nickel foam (OCNiF) was produced using pre-compressed and Spark Plasma Sintering (SPS). For this purpose, open-cell nickel foam with 10 PPI was placed in a mold then the open-cell nickel foam was filled with pure Al powder and pre-compressed. The initial tablet was sintered at 420 °C under the load of 40 MPa. Afterward, the parts were annealed at 470 °C for two, four, and 8 h. Microstructural investigations was carried out by Field Emission Scanning Electron Microscope (FE-SEM) and X-ray diffraction (XRD). The results showed a complementary process for sintered particles, which became more complete with increasing time of annealing stemming from diffusion. Evaluations showed that the Al3Ni phase began to nucleate after 4 h of annealing at the Al/OCNiF interface. The mentioned phase stabilized and reached to 3 % by weight through annealing After 8 h. Annealing increased the hardness of the substrate by more complete sintering. The hardness of Al3Ni and Al3Ni2 phases increased to 503 and 647 Vickers, respectively. The polarization test showed that the corrosion resistance first increased and then decreased during annealing compared to the SPS sample. Ni affected the development of the protective layer to prevent the pitting corrosion. However, as the annealing time increased, the corrosion current decreased, which was due to the absence of Ni in the protective layer and the formation of corrosion microcells due to the formation and growth of Al3Ni and Al3Ni2 phases.

Evolution of phase stability and structural properties in CrFeNiTiV high-entropy alloy under high-temperature heat treatment conditions

A novel equiatomic CrFeNiTiV high entropy alloy (HEA) was prepared by mechanical alloying (MA) and following spark plasma sintering (SPS). Further, the effects of heat treatment temperature on the phase, microstructure, and mechanical properties of HEAs were investigated. The results indicated the formation of a single body-centered cubic (BCC) phase after 30 h of MA. After SPS, it is found that the separation of a single phase into BCC, Ni3Ti, and TiC-rich phases, owing to the large atomic radius and mixing enthalpy of Ti with other elements. Compared with sintered HEA, the initial phases did not change after heat treatment at elevated temperatures, but the coarsening of phases was observed. The microscopic results confirmed that all phases are homogeneously distributed with good interfacial bonding after heat treatment at 1000 °C. The hardness and elastic modulus of HEA increased with increasing heat treatment temperature up to 1000 °C, due to increased precipitation of Ni3Ti and TiC-rich phases and then slightly reduced for the sample heat treated at 1100 °C HEA owing to the coarsening of existing phases. The creep displacement was reduced for the heat-treated HEAs compared to the SPS sample, suggesting improved creep resistance after heat treatment. In addition, an artificial neural network was used to predict the hardness behavior of heat-treated HEAs, and it exhibited an excellent accuracy of about 93.4%. Therefore, the present study provides valuable insights into the phase stability and mechanical properties of HEAs suitable for high-temperature applications.

Transient liquid phase bonding assisted spark plasma sintering of La-Fe-Si magnetocaloric bulk materials

The poor mechanical properties of La-Fe-Si based alloys restrict the potential of these magnetocaloric materials. Hence, a novel transient liquid phase bonding (TLPB) processing technique was deployed during spark plasma sintering (SPS) of La-Fe-Si based bulk materials. A series of binder-free LaFe11.8Si1.2 bulk materials were prepared by SPS and hot-pressing sintering (HPS). Transient melting, which occurred during SPS, accelerated the decomposition of the 1:13 phase, resulting in low porosity and high strength. Compared to HPS samples, SPS samples have significantly larger maximum compressive strength (σbc)max of ∼637.9 MPa at a strain of 4.7%, good thermal conductivity of ∼7.6 W·m−1·K−1 at 300 K, and a large (−ΔSM)max value of ∼11.7 J·kg−1·K−1 at TC= 193 K under a magnetic field change of 0 − 5 T. Thus, binder-free La-Fe-Si based bulk magnetocaloric materials with an excellent property set have been successfully prepared by TLPB assisted SPS technology.

Duplex and graded austenitic-to-martensitic steels by powder metallurgy: Interface diffusion and strength composite effect

Versatility of powder metallurgy was used to design new duplex and compositionally graded steels. Rapid consolidation by Spark Plasma Sintering (SPS) and longer consolidation by Hot Isostatic Pressing (HIP) were applied on two austenitic 316L and martensitic Fe-9Cr powders, either homogeneously blended or uniaxially graded before sintering.The microstructural investigations by micro-hardness, SEM, EDX and EBSD showed a continuous crystallographic structure at the austenite–martensite interface and a similar grain size in SPS and HIP samples, whereas a larger interdiffusion length of Cr and Ni was observed in the HIP sample, as confirmed by diffusion calculations. The tensile behaviour of materials could not be described by a simple law of mixtures. To better understand this phenomenon, the obtained materials were described as a composite and iso-strain and iso-stress models were used and discussed. The microstructural characterizations show that during the consolidation, diffusion of the chemical species modifies the nature and the respective fractions of phases, which explains the discrepancy between the models and the experimental mechanical behaviour of the duplex alloys.

Effect of sintering and annealing on electrochemical and mechanical characteristics of Na3Zr2Si2PO12 solid electrolyte

AbstractNASICON (Sodium superionic conductor) type Na3Zr2Si2PO12 (NZSP) has received a lot of interest as the solid electrolyte for all‐solid‐state sodium‐ion batteries (ASSIBs). The electrolyte has superior interfacial characteristics, high thermal stability, and good ionic conductivity. Because of their higher energy density, improved mechanical stability, no liquid leakage problem, and higher operating voltages, All solid‐state batteries are expected to replace liquid electrolyte‐based batteries in many applications. The solid electrolyte also acts as a separator, and hence additional separator is not required for cell operations. Because of its 3D open architecture and continuous diffusion channels, NZSP is considered a better solid electrolyte. The NZSP solid electrolyte has been synthesized by spark plasma sintering (SPS) followed by annealing the sintered materials. The SPS method leads the material to have higher density and ionic conductivity. Conventional sintering of the materials requires a temperature as high as 1225°C; however, the temperature required for the SPS is as low as 1050°C. Moreover, conventional sintering yields samples of relative density up to 91%, while SPSed samples have achieved a maximum density of around 98%. The ionic conductivity of solid electrolyte SPSed at 1050°C for 10 min is found to be 3.5 × 10−4 S/cm with an activation energy of 0.27 eV. The annealing of the SPSed samples improves the ionic conductivity of the SPS1050‐20mins sample to roughly double the value obtained from the as‐prepared SPS sample because there are fewer secondary phases and a structural change from a rhombohedral to a monoclinic system. To ascertain the samples' crystal structure, particle shape, and ionic conductivity, materials were characterized using X‐ray diffraction, scanning electron microscopy, and electrochemical impedance spectroscopy. The samples' mechanical characteristics, for example, the hardness and fracture toughness of the samples, were also determined.

Microstructure and Raman spectroscopy analysis of LiNiZn ferrite ceramics sintered by spark plasma method

In this study, the effects of different parameters (sintering temperature and holding time) of spark plasma sintering (SPS) on the microstructure and magnetic properties of LiNiZn ferrite ceramics were systematically investigated, and the samples sintered by SPS (SPS samples) were compared with the samples sintered by traditional sintering (TS samples). X-ray diffraction analysis confirmed that all the samples were ferrites with spinel structure. The microstructure of the samples was analyzed by scanning electron microscopy. Compared with TS samples, SPS samples exhibited finer and more uniform grains and denser structure. At the sintering temperature of 1000 °C, SPS samples exhibited higher saturation magnetization (Ms, 79.7 emu⋅g−1) and lower ferromagnetic resonance linewidth (ΔH, 231 Oe) than TS samples (Ms: 69.3 emu⋅g−1, ΔH: 298 Oe). The phase structure and cation distribution of the samples were analyzed by Raman spectroscopy. Herein, it was speculated that the instantaneous high temperature generated by spark plasma redistributed the cations in the crystal, and the distribution mode was stabilized by rapid cooling. Based on the change of magnetic moment of ferrite molecules caused by cation distribution, a method of judging the specific Ms by fitting analysis of Lorentz curve of A1g Raman signal of ferrite ceramics was proposed. The SPS samples showed more compact and uniform microstructure and more reasonable cation distribution than TS samples, and these characteristics were mainly responsible for the remarkable improvement of magnetic properties. The results indicate that SPS method can effectively alleviate the contradiction between Ms and ΔH, and provide an alternative pathway to prepare high-performance soft magnetic ferrite.

Spark plasma sintering of 13Ni-400 maraging steel: Enhancement of mechanical properties through surface modification

The present work aimed to reduce the amount of gas-atomized spherical powder, cost reduction, and reduction in the use of inert gas (N2, Ar) used during the selective laser melting process but to have similar properties and functionalities through the spark plasma sintering (SPS) process. In this work, 13Ni-400 maraging steel disks were fabricated using Spark Plasma Sintering (SPS) using optimized process parameters. The SPS samples are then subjected to surface melting and layered deposition processes. A detailed microstructural characterization and mechanical testing (hardness and compression properties) were carried out on these samples. The results indicated that the surface melted specimen exhibited the best combination of mechanical properties and microstructure, with a highly refined and uniform microstructure. This was attributed to the unique processing conditions employed during surface melting allowing the material to rapidly solidify. The surface melted samples show the highest yield strength among the three conditions (SPS, SPS + surface melting, and SPS + layer deposition). However, both the surface melting and layer deposited samples have similar ultimate compressive strength. Overall, the study offers a promising approach for producing defect-free parts with maximum densification and improved mechanical properties for newly developed 13Ni-400 maraging steel.

Additive manufacturing of TiC-based cermets: A detailed comparison with spark plasma sintered samples

The present work is a comparative study on the TiC-430 L ferritic stainless steel (FSS) cermets manufactured via two powder metallurgical processes, namely, conventional spark plasma sintering (SPS) and metal additive manufacturing (AM) process (laser powder-bed fusion process (LPBF)/selective laser melting (SLM)). The rescanning strategy has been used to preheat and melt the powder bed with different laser parameters during the SLM process to suppress the presence of residual thermal stress leading to the fabrication of cermets without cracks. The as-fabricated SPS samples (95 %) show a relatively lower density than the SLM-built parts (~98 %). A study of their mechanical properties such as hardness, compressive strength, and fracture toughness was conducted and discussed in detail. Further, the corrosion behavior of the fabricated cermets parts was evaluated in 3.5 wt% NaCl. The SLM-prepared specimens reveal finer microstructures and better mechanical properties (compressive strength and fracture toughness) due to the presence of fine microstructure. Furthermore, the corrosion current density of TiC-430 L fss-based cermets fabricated by SLM is approximately 270 times lower than that of cermets parts fabricated by SPS, indicating excellent corrosion resistance. On the other hand, the hardness shows an opposite trend, where the SPS samples show the maximum hardness as compared to the SLM counterparts due to the presence of hard and coarse TiC particles along with some metallic carbides formed during the SPS process. The results reveal that AM processes not only can fabricate cermets with intricate shapes but can also fabricate them with improved mechanical and corrosion properties.

SPS-Prepared High-Entropy (Bi0.2Na0.2Sr0.2Ba0.2Ca0.2)TiO3 Lead-Free Relaxor-Ferroelectric Ceramics with High Energy Storage Density

Compared to batteries and electrochemical capacitors, dielectric capacitors are widely studied because of their huge advantages in terms of charging/discharging speed and power density. In this work, high-entropy (Bi0.2Na0.2Sr0.2Ba0.2Ca0.2)TiO3 lead-free relaxor-ferroelectric ceramics were prepared by both conventional sintering (CS) and spark plasma sintering (SPS). The results showed that the ceramic prepared by SPS obtained a high energy storage density of 6.66 J/cm3 and a satisfied energy storage efficiency of 77.2% under an electric field of 430 kV/cm. This is directly related to the high density, fine grains, number of oxygen vacancies, and composition uniformity of the SPS samples. This study provides a new path for the preparation of high-entropy dielectric energy storage ceramics with high energy storage properties.

Low-density Fe40Mn19Ni15Al15Si10C1 high entropy steel processed by mechanical alloying and spark plasma sintering: Phase evolution, microstructure and mechanical properties

The present study deals with the synthesis of a novel Fe40Mn19Ni15Al15Si10C1 low-density high entropy steel (HES) processed through mechanical alloying (MA) and spark plasma sintering (SPS). The elemental powder was milled for 35 h, leading to the formation of multiphase structures i.e., BCC phase (a = 0.286 nm; cI2), γ-brass type (a = 0.872 nm; cI52) and partially ordered B2-type (a = 0.290 nm; cP2) phases with the trace amount of Si. The milled powder was found to be thermally stable up to ∼500 °C, however, the formation of Fe5Si3-type phase was evident at ∼520 °C. The spark plasma sintered (SPSed) sample was able to retain the BCC phase (a = 0.286 nm), γ-brass type (a = 0.872 nm), B2 (a = 0.289 nm) along with the formation of Fe5Si3 type silicide phase (a = b = 0.667 nm, c = 0.468 nm; hP16). These SPSed HES sample was found to have low density (∼6.49 ± 0.3 g cm−3), high microhardness (∼7.8 ± 0.3 GPa) and good compressive strength (∼2046 ± 160 MPa) with an appreciable ductility of ∼19%. The enhanced mechanical properties of the SPSed sample can be attributed to dual-phase microstructure i.e., BCC and B2 along with finer nano-sized silicide precipitates leading to dominant strengthening mechanisms (i.e., grain-boundary and dislocation strengthening). The experimental findings on phase evolution, thermal stability and mechanical properties of non-equiatomic HES were correlated with the various thermodynamic parameters, CALPHAD modelling and strengthening mechanisms. The significance of the present study is to design a high entropy steel containing the dual phase microstructures i.e., BCC as major phase and B2 as secondary phase, for giving rise to high mechanical strength and high temperature stability.

Microstructure and dielectric properties of 0.99(0.94Bi0.47Na0.47Ba0.06TiO3-0.06BiAlO3)-0.01NaNbO3 lead-free ceramics via different methods

In this work, 0.99(0.94Bi0.47Na0.47Ba0.06TiO3-0.06BiAlO3)-0.01NaNbO3 lead-free ceramics were synthesized by means of different methods including conventional solid state sintering (SSS), cold-sintering-assisted sintering (CSS), and spark plasma sintering (SPS) methods. Their microstructure and dielectric properties were studied comparatively. The ceramics via SPS and SSS exhibit pure structure, while a secondary phase exists in the ceramics via CSS. Compared to SSS, CSS and SPS can obviously reduce the processing temperatures for obtaining dense ceramics. The ceramics via SPS have the smallest mean grain size of 80 nm, while those via CSS and SSS exhibit increased grain sizes. The dielectric measurement demonstrates that temperature stability in dielectric constant at high temperatures is affected by sintering methods. The SPS sample shows the poorest temperature stability of permittivity. The ceramics post-annealed at 1050 °C via CSS exhibits the widest temperature range between 49 °C and 412 °C for temperature coefficient of capacitance ≤±15%, which can be expected to be used in the field of high-temperature capacitors.

Origin of the uniaxial magnetic anisotropy in cobalt ferrite induced by spark plasma sintering

Cobalt ferrite (CFO) is a promising candidate for magnetostrictive applications like actuators or sensors. We have recently shown that uniaxial magnetic anisotropy can be induced in CFO by reactive sintering using spark plasma sintering (SPS), which leads to an improvement of its magnetostrictive properties. However, the origin of the anisotropy and the formation mechanism remain unexplained so far. In this study, different SPS processes have been conducted to determine which parameter is responsible for the induced uniaxial anisotropy. We demonstrate that the magnetic anisotropy arises during the cooling step when done under SPS’s uniaxial compression. In addition, we also investigate the fundamental origin of the magnetic anisotropy induced during the SPS process. We show that the polycrystalline anisotropic cobalt ferrite obtained after SPS exhibits no texture. However, the SPS samples turn isotropic after being annealed in air at 400 °C/2 h, as shown by magnetic and magnetostrictive measurements. A change in ionic distribution after the annealing is also observed by Mössbauer spectroscopy. Our findings suggest that the induced magnetic anisotropy results from the ionic distribution of the Co2+ in the CFO’s spinel lattice, a mechanism previously observed in magnetic annealed CFO. This study advances the in-depth understanding of the relationship between SPS processing and magnetic properties of cobalt ferrite.

Investigation of the thermal shrink mechanism, thermal conductivity and compressive resistance of TaTiP3O12 ceramics

Dense TaTiP3O12 ceramics were synthesized by the solid-state method and spark plasma sintering (SPS) with 6 wt% V2O5 as a sintering aid, and their phase, microstructure, thermal conductivity, hardness, compressive strength, and expansion property and mechanism were investigated. Results show that the pure phase can be achieved by the two methods. In particular, the sample prepared by SPS possesses a relative density of 97.62% and a porosity of 3.07%, and has better properties than that prepared by the solid-state method. The SPS sample has a thermal conductivity at room temperature of 2.03 w/(m· °C), a Vickers hardness of 4.34 GPa and a compressive strength of 175.98 MPa, which are 0.95, 1.49 and 1.59 times greater than those of the sample prepared by the solid-state method, respectively. In addition, the TaTiP3O12 ceramic prepared by SPS exhibits a linear ultralow negative thermal expansion property with a coefficient of thermal expansion of −0.74 × 10−6 °C −1 (-100–400 °C). The negative thermal expansion in TaTiP3O12 is induced by the coupling effect of [Ta(Ti)O6] octahedron and [PO4] tetrahedron caused by the transverse vibration of bridging oxygen atoms.

Investigation of microstructure, mechanical properties, and wear behavior of aluminum A380 nanocomposite reinforced with graphene nanosheets produced by spark plasma sintering technique

The study reports the development of graphene nanosheets reinforced aluminum A380 matrix nanocomposites using powder metallurgy. Nanocomposites are fabricated by a mechanical alloying process and the bulk nanocomposites are prepared by spark plasma sintering (SPS) technique in a vacuum at 540 °C. 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt% of graphene nanosheets are used as a refinement for nanocomposites. Microstructural, mechanical properties, and wear behavior of the Al A380–graphene nanocomposites were investigated. Testing and analysis of fabricated composites revealed an improvement in hardness (59.96%) and wear rate (108.57%). Grain refinement action caused by graphene, high hardness, and graphene's self-lubricating nature is reasoned for these improvements. Optical microscopy and scanning electron microscopy (SEM) analysis were used to investigate microstructures of the nanoparticle distribution in the nanocomposites of the SPS samples. The wear resistance of resultant composites is reported at a higher side, decreasing friction coefficient owing to graphene's self-lubricating nature. Moreover, the incorporation of lubricant graphene nanosheets significantly reduced the wear rate and the friction coefficient in comparison with the Al A380 matrix composites. Wear morphology is characterized using SEM. In the end, the Al A380–0.5% graphene nanocomposite sample represents the greatest hardness and wear behavior, and good microstructure to compare with the other experimented samples.

Phase evolution characteristics, thermal stability, and strengthening processes of Fe–Ni based ODS invar steel produced by mechanical alloying and spark plasma sintering

In the present study, the base composition of Fe-42 wt% Ni invar was used to prepare three different oxide dispersion strengthened (ODS) steel compositions with addition of 2 wt% Y2O3 and a varying amount of Ti (0.3%, 1%, and 2% wt.%) by mechanical alloying (MA) using SPEX8000 M high energy ball mill. The mechanically alloyed samples were compacted first to make pellets, and batch-annealed at 400, 600, 800, and 1000 °C to verify its thermal stability. The hardness of the as-milled and annealed samples was estimated to apprehend the effect of Ti content and temperature on the mechanical properties. Phase evolution was studied by using the x-ray diffraction (XRD) technique of the as-milled and annealed samples of all the compositions. Then, all the milled samples were spark plasma sintered (SPSed) at 1000 °C for further investigation of sintering characteristic, microstructure, and mechanical properties. The SPSed samples were subjected to compression and Vickers hardness testing to study the change in mechanical properties of the different compositions after sintering. Yield strength values of the three compositions were theoretically calculated through strengthening mechanisms analysis using XRD data and microstructural information of the SPSed samples and correlated with the experimentally determined compressive yield strength. The microstructural study of the SPSed samples was performed using transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) analysis. The relationship between microstructure and mechanical properties has been examined in the context of various strengthening processes such as dislocation strengthening, solid solution strengthening, grain size strengthening, and precipitation/dispersoid strengthening in the SPSed alloys.

Accelerated property evaluation of Ni–Co materials libraries produced by multiple processing techniques

Ternary iron-cobalt-nickel (Fe–Co–Ni) alloys have many structural and magnetic industrial applications. The counterpart binary Ni–Co alloys, which are used in energy systems, have been much less studied than Fe–Co and Fe–Ni alloys. The properties of Ni–Co alloys depend sensitively on the processing methodology. However, a systematic evaluation of the effect of processing on the properties has not been carried out. Hence in this work, we studied multiple properties of NixCo1−x material libraries processed by three different techniques. Ni–Co powders were produced by low temperature chemical reduction, compositionally graded alloys were produced by spark plasma sintering (SPS) of powders, and bulk alloys were processed by arc melting. The structural, magnetic, mechanical, and electrical properties of the Ni–Co alloys produced by these techniques were determined via an accelerated methodology. The crystal structure, coercivity, resistivity and hardness of the alloys varied significantly with the processing technique. The powder samples exhibited highest Hc values, while the lowest Hc, highest microhardness and electrical resistivities were obtained in SPS samples. The broad change in properties with composition and processing techniques provides a rich opportunity to select both the composition and the processing techniques needed for specific applications.

Converting n-type Co4Ge6Te6 skutterudite into p-type and enhancing its thermoelectric properties through Fe substitution

This study is devoted to converting an n-type mixed anion Co4Ge6Te6 skutterudite into a p-type material through partial substitution of Fe for Co. Co4-xFexGe6Te6 (x = 0, 0.04 and 0.12) samples were prepared by solid-state methods and characterized by scanning electron microscopy and powder X-ray diffraction. Thermoelectric properties of the spark plasma sintering samples were investigated in the temperature range of 50–500 °C. The positive Seebeck and Hall coefficients of Co4-xFexGe6Te6 (x = 0.04 and 0.12) suggest that the n-type Co4Ge6Te6 was successfully converted into a p-type semiconductor via the Fe substitution. Also, an increased carrier concentration in Co4-xFexGe6Te6 (x = 0.04 and 0.12) leads to a significant increase in the electrical conductivity, which improves the thermoelectric performance of the mixed anion Co4-xFexGe6Te6 skutterudite.

The superior properties of spark plasma sintered La-Fe-Si magnetocaloric alloys

LaFe11.8Si1.2 bulk magnetocaloric materials were prepared by hot-pressing sintering (HPS) and spark plasma sintering (SPS) techniques. The effects of the particle size, phase content and porosity on the thermal conductivity, mechanical and magnetocaloric properties were studied. Our results demonstrated that the reduction of porosity in bulk magnetocaloric materials was an effective way to enhance the magnetocaloric effect. Compared to the HPS samples, the porosity of the SPS samples, with an initial powder size of 100−200 μm, reduced by 18%, which resulted in (−ΔSM)max and (−ΔSM)Vmax increasing by 15% and 19%, respectively. The SPS samples had a large (−ΔSM)max value in the range of 7.3‒12.8 J/kg·K (μ0ΔH=2T), good maximum compressive strength of 216‒636 MPa and thermal conductivity of 5.8‒7.6 W/m·K. Thus, SPS has been shown to be a promising processing technology to fabricate magnetocaloric materials (MCMs) with excellent performance.

Microstructure Optimization and Magnetic Properties Enhancement of Hot Deformed Nd-Fe-B Magnets via Grain Boundary Diffusion and Secondary Hot Deformation

Based on spark plasma sintered (SPS) Nd-Fe-B magnets with PrCu sintering aid, hot deformed (HD) magnets are prepared through hot deformation technique. After that, using PrCu as the diffusion source, grain boundary diffusion (GBD) is implemented on HD magnet. Eventually, GBD magnet is subjected to hot deformation to regulate the distribution of RE-rich phase and increase <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c$ </tex-math></inline-formula> -axis orientation, referred to as the secondary deformation (SD). The results show that hot deformation processes have an important influence on magnetic properties and microstructure, among which the deformation condition of 700 °C/0.5 h is the most favorable to obtain optimal magnetic properties. Through GBD, the coercivity, as high as 2117 kA/m, can be obtained, increasing by 7.5% comparing with that of the initial SPS sample. Moreover, it is found that the SD can effectively optimize the distribution of RE-rich phase, and improve <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$c$ </tex-math></inline-formula> -axis orientation of grains. Eventually, a good balance between coercivity and remanence is achieved. In addition, both GBD and SD can increase the thermal stability of HD magnets.