Spark Plasma Sintering Conditions Research Articles

Plasma formation during flash sintering of boron carbide – Part I: Plasma characteristics

In so-called Field Assisted Sintering Techniques such as Spark Plasma Sintering (SPS) or Flash Sintering (FS), ceramic powder compacts are densified at high sample temperature in the presence of an electrical field. The formation of a plasma is often discussed without actual evidence for its appearance. In this work, which is the first of two parts, the mechanisms of plasma formation as well as its properties under FS conditions were investigated. Field induced breakdown of various atmospheres was conducted by a DC-field (up to 350 V/cm) at temperatures between 1000 and 1500 °C. Experiments were conducted both with and without a sample mounted between the electrodes. Alumina, boron carbide and silicon carbide in green or sintered form were used as the samples. The electrical and optical properties of the plasma were characterised, and the early stages were visualised by high-speed camera recordings. The results showed that the breakdown is caused by a flashover mechanism and the conductance of the plasma under the present conditions was of the same order as the conductivity of a boron or silicon carbide green body. Plasma formation did not lead to extensive sintering but the plasma was able to infiltrate the green body and cause transfer of matter from the interior to the surface and beyond. Based on the present results, it seems unlikely that a gas plasma is formed under the normal conditions of SPS or FS.

Downscaling grain size toward the nanometre range – A key-factor for tuning the crystalline structure, phase transitions, dielectric and ferroelectric behaviour in Ba0·8Sr0·2TiO3 ceramics

The present study aims to describe the role of the grain size on the properties of submicron- and nano-structured Ba0·8Sr0·2TiO3 (BST) ceramics. Dense (1 − 2% porosity) ceramics with average grain sizes in the range of (77 − 234) nm were consolidated under different spark plasma sintering conditions starting from nanopowders with a mean particle size of 70 nm, synthesized via the acetate variant of the sol-gel method. The structural analysis based on XRD data revealed a mixture of cubic and tetragonal modifications at room temperature for the precursor powders and for all the investigated ceramics. The structural heterogeneity of the individual ceramic grains with coexistence of cubic and tetragonal polymorphs was confirmed by HR-TEM investigations. Accordingly, a “brick-wall" model with cubic grain boundary regions and tetragonal grain cores is proposed. By increasing the grain size, from 77 to 234 nm, a decrease of the phase transitions diffuseness accompanied by an increase of the permittivity maxima (from 650 to 4500) and dielectric losses (from 5 to 7.5%, at 100 Hz), was detected by broadband dielectric spectroscopy. No variation of the Curie temperature in the investigated Ba0·8Sr0·2TiO3 ceramics was detected, unlike typically reported for BaTiO3 ceramics with similar grain sizes. The Curie-Weiss temperature and the Curie constant decrease when grain size is diminished, indicating an overall reduction of the ferroelectric active volume, as a scaling effect. The ferroelectric switching was demonstrated for all the selected fine-grained BST ceramics, either at nanoscale or macroscopically, with an increased ferroelectric character for the coarser submicron-structured ceramics, with respect to the nanocrystalline one. The observed properties of the fine-grained Ba0·8Sr0·2TiO3 ceramics are explained in the frame of multiphase coexistence and ferroelectricity “dilution” due to the increasing role of non-ferroelectric grain boundaries when reducing grain size and complete the knowledge on the scale-dependent properties of dense fine-grained BaTiO3-based ceramics.

Turning of high entropy alloy into high entropy carbides by modifying carbon content during reactive sintering

High entropy alloys containing equimolar multi-element composition enhance physical properties over traditional alloys. The concept was applied also for high entropy ceramics, but the solid-state reaction is necessary. In this work, high entropy alloy VNbMoTaW reacted with carbon to form high entropy carbide. Direct reaction in Spark Plasma Sintering (SPS) conditions leads to high entropy or entropy aided carbides depending on the amount of present carbon. The entropy aided carbide formed at low carbon concentrations below 0.8 of equimolar content contained all metallic elements, but two separated phases were identified: Nb, Ta or W, Mo rich. The microstructure of the entropy aided carbides is lamellar and refined compared to high entropy carbide (V0.2Nb0.2Mo0.2Ta0.2W0.2)C0.8. The presence of entropy aided carbide decreased the Vickers hardness from 23 GPa to 19 GPa, however, the unique microstructure might enhance other physical properties.

The influence of spark plasma sintering on multiscale mechanical properties of nickel-based composite materials

The paper presents a comprehensive investigation of the influence of the main process parameters of spark plasma sintering on the mechanical and microstructural properties of nickel-silicon carbide composites at various scales. Microstructure analysis performed by scanning and transmission electron microscopy revealed a significant interfacial reaction between nickel and silicon carbide due to the decomposition of silicon carbide. The chemical interaction of the matrix and reinforcement results in the formation of a multicomponent interphase zone formed by silicides (Ni31Si12 or/and Ni3Si) and graphite precipitates. Furthermore, several types of structure defects were observed (mainly nano/micropores at the phase boundaries). These significantly influenced the mechanical response of nickel-silicon carbide composites at different levels. At the macroscopic scale, uniaxial tensile tests confirmed that applying a 1000 °C sintering temperature ensured that the manufactured composite was characterised by satisfactory tensile strength, however, with a considerable reduction of material elongation compared to pure nickel. Moreover, the fractography study allowed us to identify a significant difference in the damage mode for certain nickel-silicon carbide samples. Secondly, the interface of the nickel matrix and silicate interphase was tested by bending with microcantilevers to evaluate its deformation behaviour, strength, and fracture characteristics. It was confirmed that a diffusive kind of interface, such as Ni-NiSi, demonstrates unexpected bonding properties with a relatively large range of plastic deformation. Finally, the nanoindentation of three main components of the nickel-silicon carbide composite was executed to evaluate the evolution of nanohardness, Young's modulus, and elastic recovery due to the application of various spark plasma sintering conditions.

Synthesis and analysis of the magnetic properties of YIG by means of sol–gel and spark plasma sintering

We presented a study on the structure and magnetic properties of yttrium iron garnet (YIG), Y3Fe5O12, which was synthesized by means of sol–gel method and spark plasma sintering (SPS). The YIG nanoparticles were prepared by sol–gel method followed by 800 °C annealing for 3 h. SPS procedures with different temperatures (700 °C∼900 °C) and holding times (30 min ∼ 90 min) were performed to study the influence of SPS conditions on the structure and magnetic properties of YIG. The X-ray diffraction pattern and Raman spectrum confirmed the presence of YIG at all studied conditions. All samples were pure garnet phase except for the samples with too high temperatures (900 °C) and too long holding time (90 min). The impurities were inferred to be due to the hypoxic environment caused by the vacuum sintering condition of SPS. The magnetic properties of YIG samples were significantly affected by SPS conditions. The YIG sample became multi-domain at the temperature as low as 750 °C and grew to 89.1 nm at 850 °C for 30 min holding time benefiting from to the special sintering effect of SPS. Large particles with loose texture were formed by the formation of sintering neck between the adjacent particles when the SPS temperature was higher than 800 °C. It are confirmed that the sample with extended holding time at a moderate temperature (800℃/60 min) show the best soft-magnetic properties, whose Ms, Mr and Hc are 27.5 emu/g, 2.4 emu/g and 19.2 Oe respectively. The results show that this method has good potential in preparing YIG with good magnetic properties.

Spark plasma sintering of HUST-1 lunar regolith simulant and its thermal shock resistance properties

The construction of lunar bases has become a new target for lunar exploration by many space powers worldwide. Sintered lunar regolith is one of the most promising building materials for in situ resource utilization (ISRU). Spark plasma sintering (SPS) technology has the advantageous features of a fast sintering speed and high density. This study explored the feasibility of sintering a HUST-1 lunar regolith simulant using SPS technology. The physical, mechanical, and thermal properties, as well as the microstructure and phase composition of the sintered samples were investigated at multiple scales. In addition, the effects of the SPS conditions on the sintering results were studied, including the sintering temperature, heating rate, and applied pressure. The test results indicated that the sintering conditions significantly affected the sintered products. Finally, the thermal shock resistances of the sintered samples were investigated at simulated lunar temperatures. The samples were treated at two different temperature ranges, one from −60 to 60 °C (±60 °C) and another from −120 to 120 °C (±120 °C). The results showed that the sintered samples exhibited excellent thermal shock resistance in the extreme temperature environment of the lunar surface. After 100 thermal test cycles at ± 60 °C and ± 120 °C, the compressive strength increased by 16.0 % and 33.4 %, respectively. The reason for the increase in strength remains unclear. The Brunauer-Emmett-Teller (BET) test results showed that this may be caused by the gradual disappearance of micropores smaller than 10 nm during thermal cycling.

Study of Structural, Compression, and Soft Magnetic Properties of Fe65Ni28Mn7 Alloy Prepared by Arc Melting, Mechanical Alloying, and Spark Plasma Sintering.

Soft magnetic Fe65Ni28Mn7 (at. %) alloy was successfully synthesized by mechanical alloying and spark plasma sintering (SPS) and, in parallel, the same composition was prepared by arc melting (AM) for comparison. Several SPS conditions were tested. X-ray diffraction and scanning electron microscopy were used to investigate the structure, phase composition, and morphology of the samples. It was found that mechanical alloying produced BCC and FCC supersaturated solid solution after 130 h of milling, with a fine microstructure (i.e., crystallite size of 10 nm). Spark plasma sintering performed at 750 °C and 1000 °C under two pressures of 50 MPa and 75 MPa revealed stable FCC phases. A single FCC phase was observed after the arc melting synthesis. The magnetic properties of milled powders and solids obtained by AM and SPS were investigated. The specimen consolidated by SPS at 1000 °C under the pressure of 50 MPa exhibits soft magnetic behavior (coercivity 0.07 Oe), whereas the mechanically alloyed sample revealed hard magnetic behavior. The specimen consolidated at 750 °C under a pressure of 75 MPa showed a higher compressive strength of 1700 MPa and a Vickers hardness of 425 ± 18 HV. As a result, sintering at 750 °C/75 MPa can be utilized to enhance the mechanical properties, while those sintered at 1000 °C/50 MPa increase magnetic softness.

Rapid preparation of traditional and high-entropy garnet ceramics by SPS at low temperature with ultrafast densification

Spark plasma sintering (SPS) is a fast and efficient method for preparing functional ceramics. This article investigates the phase evolution, thermal expansion and chemical stability of traditional (Y1.2Nd1.8Fe5O12) and high-entropy ((Y, Gd, Sm, Eu, Dy)3Fe5O12) garnet ceramics. Prepared under conventional SPS (50 MPa) conditions at 900–1100 °C. The results indicate that using SPS sintering method can significantly reduce the sintering temperature (1100 °C) and time (3 min) of garnet ceramics. (Y, Gd, Sm, Eu, Dy)3Fe5O12 (1.6 × 10−6 K−1) exhibits superior thermal expansion stability compared to Y1.2Nd1.8Fe5O12 (5.4 × 10−6 K−1). The strong REE-O bond energy and severe lattice distortion of high entropy garnet ceramics hinder the migration and diffusion of cations, resulting in higher leaching resistance of high-entropy garnet ceramics (∼10−7 g m−2 d−1-10−8 g m−2 d−1) compared to traditional garnet ceramics (∼10−5 g m−2 d−1-10−6 g m−2 d−1). Therefore, SPS has broad application prospects in the low-temperature preparation of high-entropy garnet ceramics for immobilization of actinide elements.

An SPS-RS Technique for the Fabrication of SrMoO4 Powellite Mineral-like Ceramics for 90Sr Immobilization.

This paper reports a method for the fabrication of mineral-like SrMoO4 ceramics with a powellite structure, which is promising for the immobilization of the high-energy 90Sr radioisotope. The reported method is based on the solid-phase "in situ" interaction between SrO and MoO3 oxides initiated under spark plasma sintering (SPS) conditions. Dilatometry, XRD, SEM, and EDX methods were used to investigate the consolidation dynamics, phase formation, and structural changes in the reactive powder blend and sintered ceramics. The temperature conditions for SrMoO4 formation under SPS were determined, yielding ceramics with a relative density of 84.0-96.3%, Vickers microhardness of 157-295 HV, and compressive strength of 54-331 MPa. Ceramic samples demonstrate a low Sr leaching rate of 10-6 g/cm2·day, indicating a rather high hydrolytic stability and meeting the requirements of GOST R 50926-96 imposed on solid radioactive wastes. The results presented here show a wide range of prospects for the application of ceramic matrixes with the mineral-like composition studied here to radioactive waste processing and radioisotope manufacturing.

Effects of spark plasma sintering parameters on the microstructure of U3Si2 pellets

In this study, U3Si2 pellets were fabricated by spark plasma sintering (SPS) of U3Si2 clinkers generated by a high-temperature solid reaction between U and Si powders. The influences of SPS operating parameters (dwell temperature from 1000 to 1300 °C and pressure from 30 to 90 MPa) on the microstructure (porosity, normal and distorted U3Si2 phase, grain size and shape) of U3Si2 pellets were analyzed. The density of U3Si2 pellets enhanced with increasing temperature and pressure and it reached of the 97.9% theoretical density (TD) at 1300 °C/60 MPa. The residual stress caused the generation of a distorted U3Si2 phase in the pellets, and low-temperature annealing at 300 °C effectively released the residual stress and eliminate the lattice distortion of U3Si2. The U3Si2 grain size increased with increasing dwell temperature and showed an opposite trend with increasing pressure. The grain growth mechanisms of the SPS-sintered U3Si2 pellets involved temperature-dependent surface diffusion and pressure-induced grain growth inhibition. The evolution of the U3Si2 grain morphology from equiaxed to columnar grains was due to plastic strain under SPS conditions.

Investigation of the composition, structure and magnetic properties of the Nd2Fe14B ceramics dependence on the initial powder characteristics and spark plasma sintering modes

High phase and structural sensitivity of magnetic properties strongly impacts the fabrication quality of the permanent magnets, therefore revealing effects of synthesis conditions on physico-chemical characteristics of magnetic materials is of paramount importance for optimization of magnet production especially in case when non-conventional techniques are involved. Here we report for the influence of pulse current frequency on structure, composition, and magnetic properties of an industrially-important Nd2Fe14B magnetic ceramics on the initial powder characteristics and the modes of its spark plasma sintering (SPS) in a vacuum environment. The influence of initial powder mechanical grinding as well as the number of generated electric current pulses (in the range from 3.3 to 326.7 ms) in SPS conditions on the structure (defects and pores formation, grain size), phase and element composition (formation of secondary nonmagnetic phases and oxides, impurities) of Nd2Fe14B alloy was studied by SEM, TEM, EDS and XRD methods. The regularity of changes in the magnetization saturation value (σ) and coercive force (Нс) depending on the morphology, structure and composition of the initial Nd2Fe14B powder and the alloy obtained by mechanical milling and subsequent sintering at different modes of pulse current has been established.

Solidification of cesium containing magnetic zeolite sorbent by spark plasma sintering

The possibility of effective solidification of spent sorbents containing radioactive cesium by spark plasma sintering (SPS) technology for their safe and long-term disposal was studied. The kinetics of consolidation spent sorbent based on magnetic zeolite after adsorption saturation with cesium (∼23 wt %) under SPS conditions in the range of 700–950 °C was carried out. The structural, elemental and phase composition changes during high-rate (minutes) heating of zeolite powder saturated with Cs+ ions was studied by SEM, EDX and XRD methods. As a result, ceramic samples with mineral-like structure of Fe-substituted pollucite CsFeSi2O6 were successfully obtained. It was determined that the optimal temperature of SPS consolidation was 900 °C, at which the formation of CsFeSi2O6 ceramics with a relative density of 91.2%, compressive strength of 497.6 MPa, Vickers microhardness of 1019 HV, rate of cesium leaching of 2.33 × 10−8 g/cm2 × day was reached. It was found that the mechanism of cesium diffusion from the volume of ceramics corresponded to flushing from the available surface of the sample, the lowest diffusion coefficient was 2.71 × 10−16 cm2/s, as well as the lowest penetration depth of the solution into the volume of ceramics was 1.04 × 10−5 cm. The adsorbents prepared by SPS up to 900 °C retained the magnetization, and at higher temperature the magnetization decreased, which was caused by phase and structural changes of ceramics. The characteristics of ceramic obtained from spent adsorbent complied with the requirements of GOST R 50926–96 and ISO 6961:1982 for solidified Cs-containing radioactive waste.

Effect of preliminary high-energy ball milling on the structural-phase state and microhardness of Ni3Al samples obtained by spark plasma sintering

The study of the influence of the duration of preliminary high-energy ball milling on the features of the structural-phase state and the level of microhardness of consolidated Ni3Al samples obtained by the method of spark plasma sintering has been carried out. It was found that the inhomogeneous state of the precursor from the 3Ni-Al powder mixture in the case of preliminary ball milling of a short duration (1 min) is a cause of the formation of an inhomogeneous structural-phase state of the consolidated Ni3Al sample. An increase in the duration of high-energy ball milling provides a homogeneous phase composition, promotes the refinement of the grain structure and an increase in the microhardness values of the obtained Ni3Al samples. The main factors determining the processes of structural-phase transformation during the formation of Ni3Al under the conditions of spark plasma sintering, depending on the preliminary high-energy ball milling, are revealed. It is shown that grain boundary strengthening is the one of the effective mechanisms for increasing the strength of the material under study.

Boosting the thermoelectric performance of GeTe via vacancy control and engineering sintering parameters

Pristine GeTe is a p-type degenerate semiconductor with very low thermoelectric performance due to its large carrier concentration induced by large Ge vacancy. In this work, Ge vacancy was effectively decreased through the melt quenching process and also non-equilibrium reaction caused by the spark plasma sintering (SPS) consolidation. The effects of excess Ge doping were studied on the polycrystalline and single-crystal specimens to understand the excess doping effect on the carrier concentration. The p-type carrier concentration for the Ge 1.03 Te polycrystalline specimen was significantly reduced to 2.43 ×10 20 cm -3 from 8.9 ×10 20 cm -3 of pristine GeTe. This work realized that the zT of the single-crystal specimen was lower than that of pellets prepared by the spark plasma sintering due to its high carrier concentration. Meanwhile, polycrystalline specimen sintered by spark plasma sintering exhibits lower thermal conductivity due to the reduction of carrier concentration and phonon scattering centers such as Ge precipitates and nanostructured defects. We also systematically studied the different SPS sintering conditions on thermoelectric properties of the Ge 1.03 Te polycrystalline specimen. Eventually, superior thermoelectric performance zT =1.8 at 700 K was obtained in 3% Ge excess Ge 1.03 Te polycrystalline specimen sintered at 773 K with 30 MPa pressure. The zT enhancement results from the synergistic contribution of the enhanced Seebeck coefficient by excess Ge doping and reduced thermal conductivity through SPS sintering pressure. This work demonstrates that the zT value of GeTe can be effectively enhanced by adding excess Ge with optimal SPS conditions. Our results provide an enlightening strategy to improve thermoelectric material performance. • Enhanced power factor in Ge excess doping sample. • Reduced the thermal conductivity through microstructure effects as well as Ge precipitates. • Understanding effect of different sintering pressures on thermoelectric properties of GeTe. • A record high ZT of 1.8 at 700 K were successfully realized in Ge 1.03 Te polycrystalline sample. • Comparing the single crystal and polycrystalline samples and studied their thermoelectric properties.

Immobilization mechanism of Gd2Zr2O7 ceramic for nuclear waste treatment

Nuclear waste generated from the nuclear industry should be safely treated because of its harmful effects on the environment and humans. In this study, the mechanism of the as-prepared Gd2Zr2O7 for nuclear waste immobilization was investigated as an example in the simulation of waste immobilization under spark plasma sintering (SPS) conditions. The solid solubility limit of the waste in the sintered matrices was 45 mol% at 1725 °C and 1800 °C. The phase transformation from fluorite to pyrochlore could be detected owing to the increase in the ionic radius ratio when the waste was immobilized in the matrix structure. A high sintering temperature promotes transformation by increasing the orderliness of the matrix structure and destroying the crystalline structures to form an amorphous structure, results in a decrease in structural orderliness. Moreover, the leaching rates of Nd, at approximately 3.5 × 10−8 g·m−2·d−1 at 42 d, demonstrated high chemical stability in the matrices doping with 45 mol% simulated nuclear waste. A mechanism is proposed in this study for treating radioactive waste using Gd2Zr2O7 ceramic under SPS conditions.

The Benefit of the Glassy State of Reinforcing Particles for the Densification of Aluminum Matrix Composites

In metallic glass-reinforced metal matrix composites, the glassy phase can serve a dual purpose: (i) it can behave as soft binder and porosity remover during consolidation; and (ii) it can act as the hard reinforcing phase after densification. The present work aimed to demonstrate the benefit of the glassy reinforcing particles for the densification of aluminum matrix composites. The consolidation behavior of Al–50 vol.% Fe-based alloy mixtures prepared using a glassy Fe66Cr10Nb5B19 alloy powder (Tg = 521 °C, Tx = 573 °C) or a crystalline Fe62Cr10Nb12B16 alloy powder was studied under spark plasma sintering (SPS) and hot pressing (HP) conditions. The powders were consolidated by heating above the glass transition temperature of the glassy alloy (up to 540 °C in SPS and 570 °C in HP). When the coarse aluminum powder was used, the reinforcing particles formed chains within the microstructure. In composites formed from the fine Al powder, the particles of the Fe-based alloy were separated from each other by the metallic matrix, and the tendency to form agglomerates was reduced. The glassy state of the alloy was shown to be beneficial for densification, as the metallic glass acted as a soft binder. The densification enhancement effect was more pronounced in the case of reinforcing particles forming chains. The hardness of the Al–50 vol.% glassy Fe66Cr10Nb5B19 composites obtained by SPS was twice the hardness of the unreinforced sintered aluminum (110 HV1 versus 45 HV1).

Investigation of the Densification Behavior of Alumina during Spark Plasma Sintering.

The article presents the results of the investigation of the mechanism of the densification behavior of alumina-based ceramics during spark plasma sintering. The role of the heating rates and additives were investigated. The first (initial) stage of sintering was investigated by the Young–Cutler model. The second (intermediate) stage of sintering was investigated as a process of plastic deformation of a porous body under external pressure. It was shown that, at the initial stage, the formation of necks between the particles is controlled by grain boundary diffusion (the activation energy is Qb ≈ 20 kTm). At this stage, accommodation of the shape of the alumina particles is also occurring (an increase in the packing density). The accommodation process facilitates the shrinkage of the powder, which is reflected in a decrease in the effective activation energy of shrinkage at low heating rates (10 °C/min) to Qb ≈ 17 kTm. At heating rates exceeding 10 °C/min, the intensity of the processes of accommodation of alumina particles turns out to be much slower than the existing diffusion processes of growth of necks between the alumina particles. It was shown that the grain boundary sliding mechanism that occurs in the second stage of sintering can play a decisive role under conditions of spark plasma sintering with a high heating rate. The found value of the activation energy at the second stage of sintering is also close to the activation energy of grain–boundary diffusion of alumina (Qb ≈ 20 kTm). The influences of the second phase particles of MgO, TiO2, and ZrO2 on densification behavior of alumina-based ceramics were investigated. Since at the first stage of sintering the densification relates with the formation of necks between the particles of alumina, the additives (0.5% vol) have no noticeable effect on this process. It was also shown that the second phase particles which are located at the grain boundaries of alumina are not involved in the slip process during the second sintering stage. Analysis shows that additives act only in the final (third) stage of spark plasma sintering of alumina.

Preparation of nanostructural oxide dispersion strengthened (ODS) Mo alloy by mechanical alloying and spark plasma sintering, and its characterization

The nanostructural oxide dispersion strengthened (ODS) Mo alloy was successfully fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). In this study, the MA and SPS conditions were systematically studied, and the characteristics of oxide particles were investigated. The results show that the MA process can refine the powder and promote the dissolution of interstitial atoms, but introduce the ZrO2 and excess O impurities. To recycle ZrO2 impurity as a source of active element Zr for alloy preparation, the content of Zr and O with milling time was quantitatively evaluated. In the SPS process, the precipitation of oxide particles retard the densification of as-MA Mo powders. Based on the calculated linear shrinkage, the densification behavior is controlled by sintering temperature, which is driven by grain boundary or dislocation climbing. A high relative density of 99.28% in ODS Mo alloy is achieved using the optimized MA (250 rpm/18 h) and SPS (1400 °C/5 min) parameters. Additionally, the ultrafine Mo grain structure (300 nm) and uniformly dispersed nanoscale Y-Zr-O oxide particles (20.32 nm) inside grains greatly improve the mechanical properties (569.63 ± 28.08 HV0.3) of ODS Mo alloys. Non-stoichiometric Y-Zr-O particles, with different Y/Zr ratios (Y/Zr <1), possess a special fluorite crystal structure.

Al matrix composites reinforced by in situ synthesized graphene–Cu hybrid layers: interface control by spark plasma sintering conditions

Tremendous impacts are usually made by the synthesis method and consolidation technique on microstructure and interface of graphene/Al composites. In the present work, an in situ gel-precursor decomposition route is proposed for the one-step synthesis of graphene nanosheet (GNS) decorated with Cu nanoparticles in the form of hybrid layers encapsulating Al grains (designated as GNS–Cu/Al). Consolidation is performed by spark plasma sintering (SPS) using markedly different sets of maximum temperature and maximum uniaxial pressure (400 °C/400 MPa or 500 °C/100 MPa). The powder and dense samples are investigated by several techniques including thermal analysis, X-ray diffraction and electron microscopy. The microhardness and elastic modulus of selected GNS–Cu/Al composites are investigated and related to the microstructure and preparation conditions. Results demonstrate that the interface structure is primarily determined by the roles of GNS–Cu hybrid layers and finely controlled by SPS conditions. This work paves a novel way to elucidate the evolutions of metal-decorated graphene hybrids in Al matrix composites.Graphical Abstract

Effect of sintering conditions on optical and mechanical properties of MgAl2O4/Al2O3 laminated transparent composite fabricated by spark-plasma-sintering (SPS) processing

To realize a high hardness in transparent MgAl2O4, the MgAl2O4/Al2O3 laminated composite was fabricated by a one-step spark-plasma-sintering (SPS) method. By sintering at a temperature of 1225 °C for 10 min and at a heating rate of ≤ 10 °C/min under a pressure of 300 MPa, the MgAl2O4/Al2O3 laminated composites can attain a high hardness with maintaining the wide band transparency. The in-line and IR transmission were ∼50 % at the visible wavelength of 500 nm and >77 % at the wavelength of 4 μm, respectively. The Vickers hardness measured on the surface of the Al2O3 layer perpendicular to the MgAl2O4/Al2O3 stacking exhibited 29 GPa, which is higher than those of the monolithic Al2O3 (26.6 GPa) and MgAl2O4 (17.2 GPa). The wide band transparency and mechanical properties can be realized by simultaneously attaining smaller grain sizes and higher densities of both the MgAl2O4 and Al2O3 phases in the laminated composite by optimizing the SPS conditions.