Spark Plasma Sintering System Research Articles

Effect of n-type Dopants on CoSb3 Skutterudite Thermoelectrics Sintered by Spark Plasma Sintering

<TEX>$CoSb_3$</TEX> Skutterudites materials have high potential for thermoelectric application at mid-temperature range because of their superior thermoelectric properties via control of charge carrier density and substitution of foreign atoms. Improvement of thermoelectric properties is expected for the ternary solid solution developed by substitution of foreign atoms having different valances into the <TEX>$CoSb_3$</TEX> matrix. In this study, ternary solid solutions with a stoichiometry of <TEX>$Co_{1-x}Ni_xSb_3$</TEX> x = 0.01, 0.05, 0.1, 0.2, <TEX>$CoSb_{3-y}Te_y$</TEX>, y = 0.1, 0.2, 0.3 were prepared by the Spark Plasma Sintering (SPS) system. Before the SPS synthesis, the ingots were synthesized by vacuum induction melting and followed by annealing. For phase analysis X-ray powder diffraction patterns were checked. All the samples were confirmed as single phase; however, with samples that were more doped than the solubility limit some secondary phases were detected. All the samples doped with Ni and Te atoms showed a negative Seebeck coefficient and their electrical conductivities increased with the doping amount up to the solubility limit. For the samples prepared by SPS the maximum value for dimensionless figure of merit reached 0.26, 0.42 for <TEX>$Co_{0.9}Ni_{0.1}Sb_3$</TEX>, <TEX>$CoSb_{2.8}Te_{0.2}$</TEX> at 690 K, respectively. These results show that the SPS method is effective in this system and Ni/Te dopants are also effective for increasing thermoelectric properties of this system.

Microstructure and Compression Properties of Al-Si Alloy Foams by Spark Plasma Sintering Technique

Porous aluminum alloy has been developed by powder metallurgy route using Spark plasma sintering (SPS) technique. Sintered material was produced by SPS system after getting the mixture of Al-12Si alloy and titanium hydride powders. Porous materials are prepared under various process conditions, and the pore morphology was investigated. Compression test is performed at crosshead speed of 1mm/min, 10mm/min and 100mm/min with no lubricant. The compression strength, σC i.e. plateau stress was estimated 12MPa at the density of porous materials, 0.7 Mg/m3. Densification strain εD from compression curve is around 0.6. These properties depend on pore morphology of porous materials, and it is possible to control the morphology under specific condition with this process. Plateau stress and absorbed energy of heat treated porous Al-Si alloy were estimated by measurement of a first peak stress and calculated an area up to 0.5 strain from compressive stress-starin curves. Young’s modulus is measured by starin gauge method under compression test. Porous aluminum alloy filled mold die is also produced successfully.

Spark Plasma Sintering and Microstructural Characterization of Additive-Free Polycrystalline β-SiC

Additive-free -SiC powders were sintered by means of Spark Plasma Sintering System. Experiments were performed in the temperature range from 1650°C to 2200°C, 3 to 10 min holding time and pressure from 50 until 150 MPa. In order to favour sinterization, the starting powder was mechanically activated: defect concentration was increased by centrifugal ball milling. Applied temperature, holding time and/or pressure were varied to analyze their effect on the densification and grain growth kinetics. The full sinterization of the material was obtained for temperatures as high as 1900°C and over. The relative density of the obtained material was high, up to 97.0  0.6 % the theoretical density for 2200°C sintering temperature. An intense grain growth took place while sintering. The final microstructure exhibited a grain size distribution range from 1.0 to 2.5 m, depending on the sintering conditions. Such grain growth strongly depends on the sintering time, not so much on the sintering temperature.

Effects of WC particle size on densification and properties of spark plasma sintered WC–Co cermet

Abstract The WC–Co cermet bulks were prepared by spark plasma sintering (SPS) using powder mixtures with different-scaled WC particles. The SPS densification process was studied by calculating the current distribution between the powder sample and the die in the SPS system. The microstructures were characterized and compared for different samples by the WC grain size, Co mean free path and contiguity of WC grains. In spite of a weak effect of WC particle size on the SPS densification stages, the WC particle size plays a significant role in the homogeneity of the cermet microstructure. Good mechanical properties of the SPSed cermet were obtained with an optimized WC and Co particle-size combination. The effects of scale combination of WC and Co particles on the microstructure hence the properties of the SPSed cermet were discussed.

Preparation and characterization of lanthanum carbide encapsulated carbon nanocapsule/lanthanum hexaboride nanocomposites

LaC 2@CNC/LaB 6 nanocomposites were prepared using a spark plasma sintering system, and their mechanical properties and the intensity of characteristic La X-rays from the composites were characterized for application as X-ray target material. Using LaB 6 as a binder, we succeeded in producing LaC 2@CNC/LaB 6 nanocomposites with a bulk density (2.91 g/cm 3) and specific electric resistance (3.12 × 10 − 4 Ω cm) through solidification at 2123 K. The fracture bending strength of LaC 2@CNC/LaB 6 nanocomposites (224 MPa) was 1.5 times larger than that observed for graphite/LaB 6 composites. The most emitted characteristic La Lα X-ray from LaC 2@CNC/LaB 6 nanocomposites was 7743 counts/mm 2/s in comparison with 6372 counts/mm 2/s for graphite/LaB 6 composites.

Moving finite-element mesh model for aiding spark plasma sintering in current control mode of pure ultrafine WC powder

The intricate bulk and contact multiphysics of spark plasma sintering (SPS) together with the involved non-linear materials’ response make the process optimization very difficult both experimentally and computationally. The present work proposes an integrated experimental/numerical methodology, which simultaneously permits the developed SPS model to be reliably tested against experiments and to self-consistently estimate the overall set of unknown SPS contact resistances. Unique features of the proposed methodology are: (a) simulations and experiments are conducted in current control mode (SPS-CCm); (b) the SPS model couples electrothermal and displacement fields; (c) the contact multiphysics at the sliding punch/die interface is modeled during powder sintering using a moving mesh/moving boundary technique; (d) calibration and validation procedures employ both graphite compact and conductive WC powder samples. The unknown contact resistances are estimated iteratively by minimizing the deviation between predictions and on-line measurements (i.e., voltage, die surface temperature, and punch displacement) for three imposed currents (i.e., 1,900, 2,100, 2,700 A) and 20 MPa applied pressure. An excellent agreement is found between model predictions and measurements. The results show that the SPS bulk and contact multiphysics can be accurately reproduced during densification of ultrafine binderless WC powder. The results can be used to benchmark contact resistances in SPS systems applicable to graphite and conductive (WC) powder samples. The SPS bulk and contact multiphysics phenomena arising during sintering of ultrafine binderless WC powders are finally discussed. A direct correlation between sintering microstructure, sintering temperature, and heating rate is established. The developed self-consistent SPS model can be effective used as an aiding tool to design optimum SPS experiments, predict sintering microstructure, or benchmark SPS system hardware or performances.

A novel rapid route for synthesizing WC–Co bulk by in situ reactions in spark plasma sintering

A novel rapid route for preparing WC–Co cemented carbides bulk, which integrated the synthesis of the composite powder by in situ reactions and subsequent consolidation in the spark plasma sintering (SPS) system, was proposed. The phase configurations both in the synthesized composite powder and in the sintered bulk were analyzed in details. It was shown that the obtained WC–Co bulk has a homogeneous and fine-grained structure and good combined mechanical properties. As compared with the conventional methods, the present preparation route has been remarkably simplified. Particularly, with the greatly reduced temperature and time in the preparation procedures, it is effective to control the grain coarsening during the processes. By taking into account the characteristics of the SPS technology, the formation mechanism of the WC–Co composite by in situ reduction and carbonization reactions was proposed.

Preparation and characterization of a novel hydroxyapatite/carbon nanotubes composite and its interaction with osteoblast-like cells

Hydroxyapatite (HA) and its based biomaterials could chemically bond directly to bone when implanted, resulting in the formation of a strong bone-implant interface. Carbon nanotubes (CNT) are believed to be very promising in the enhancement ceramic matrix and played an important role as reinforcement for imparting strength and toughness to brittle HA bioceramic. Here we demonstrate the potential use in reinforcing biomaterials through an attempt to use CNT strengthen brittle HA bioceramic. This work aims to study the optimum sintering conditions of CNT modified HA to get CNT–HA composite with improved mechanical properties using a rapid spark plasma sintering system. The microstructure, phases, chemical compositions and mechanical properties of the ceramic samples were characterized by various advanced methods. Though no obvious chemical reaction between CNT and HA has been detected, the physical crosslink caused by the presence of CNT resulted in that a CNT–HA composite with a relatively high value of modulus (~ 131.1 GPa) and hardness (~ 6.86 GPa) achieved at the sintering temperature of 1100 °C. In vitro cellular responses to the composites were assessed to investigate the proliferation and morphology of a human osteoblast cell line cultured on the various composites.

Spark plasma sintering-fabricated one-dimensional nanoscale “crystalline-amorphous” carbon heterojunction

One-dimensional (1D) nanoscale “crystalline-amorphous” carbon heterojunction is fabricated by post-treatment of an amorphous carbon nanofiber in a spark plasma sintering (SPS) system. It is proposed that the unique SPS process is responsible for the heterojunction formation. Studies of the electrical transport property show that the nanoscale heterojunction exhibits a typical rectification behavior. The heterojunctions may have broad potential applications in nanoelectronics and optoelectronics and the SPS technique could be a distinctive approach to construct 1D functional nanomaterials with high throughput.

Change in relative density of WC-Co cemented carbides in spark plasma sintering process

The relative density of WC-Co cemented carbides during spark plasma sintering (SPS) was analyzed. Based on the change in displacement of the ram in the SPS system, the relative densities in the sintering process can be achieved at different temperatures. The results indicated that densification of the samples started at near 900°C, the density rapidly reached its maximum at the increasing temperature stage, in which the temperature was lower than the sintering temperature of 1200°C, and most of the densification took place in the stage. Besides, the theoretical values were consistent with the experimental results.

Compressive behavior of micro-metallic closed cellular materials fabricated by spark-plasma sintering

A method to fabricate the metallic closed cellular material has been developed. Powder particles of polymer coated with a nickel–phosphorus alloy layer using electro-less plating were pressed into pellets and sintered at high temperatures by a furnace and spark-plasma sintering system. A metallic closed cellular material containing different materials from that of cell walls was then fabricated. The results of the compressive tests show that this material has the different stress–strain curves among the specimens that have different thickness of the cell walls and the sintering temperatures of the specimens affect the compressive strength of each specimen. Also, it seems that the results of the compressive tests show that this material has high-energy absorption and Young's modulus of this material depends on the thickness of the cell walls and the sintering temperature. These obtained results emphasize that this material can be utilized as energy absorbing material.

Fabrication Method for Metallic Closed Cellular Materials Containing Different Materials from that of Cell Walls by SPS Method

Several methods of fabricating metallic closed cellular materials a spark plasma sintering (SPS) system have been developed. Powdered polymer particles coated with a nickel-phosphorus alloy layer using electro-less plating were pressed into pellets and sintered at high temperatures in a furnace using SPS system. Metallic closed cellular materials containing different materials from the cell walls were then fabricated. The physical, mechanical and damping properties of these materials were measured. The results of the compressive tests show that this material has different stressstrain curves among the specimens that have different cell wall thicknesses and the sintering temperatures of the specimens affect the compressive strength of each specimen. Also, it seems that the results of the compressive tests show that this material has a high-energy absorption. The internal friction of this material was measured and the results show that this internal friction is the same as that of pure aluminum. These results suggest that this material can be utilized as an energy absorbing and passive damping material.

The cell size effect of closed cellular materials fabricated by pulse current assisted hot isostatic pressing on the compressive behavior

Abstract A hot isostatic pressing technique assisted by the pulse current supplied by a spark plasma sintering system was used to fabricate metallic closed cellular materials. Phenol resin spherical particles coated with a Ni–P alloy were sintered using this technique. Cellular nickel alloy samples with different cell sizes were obtained. Compressive tests showed that the compressive strength of this material with a smaller cell size was higher than that with a larger cell size. The addition of fine cells to a coarse cellular structure can improve the compressive strength of the specimen.

Fabrication of metallic closed cellular materials by sintering of metal coated particles

A metallic closed cellular material for smart materials has been fabricated by sintering of metal coated powder particles. Powder particles materials coated with nickel-phosphorus alloy layers using electro-less plating were pressed into pellets (green pellets) and sintered in vacuum at high temperature using an electric furnace or a spark plasma sintering system. Then a metallic closed cellular material containing organic material was then constructed. The cross-sections of the specimens were observed using the scanning electron microscope. The compressive and damping tests were carried out to measure the mechanical property of this material. The results of the compressive tests show that this metallic closed cellular material has a long plateau region and high-energy absorption. In addition, the specimens sintered at different conditions have different mechanical properties. The compressive strength would increase due to the increasing of cell wall thickness. The results of the damping tests show that the internal friction of this material is same as that of pure aluminum.

Effect of Aluminium on High-Energy Ball Milling and Spark Plasma Sintering of Silicon Nitride

A commercial silicon nitride powder with oxide sintering additives was ground with high-energy ball mill to obtain nano-sized powder. Metallic aluminium powder was added as a grinding additive. Effect of high-energy ball milling was evaluated by X-ray diffraction analysis. After milling, height of background increased and peak height of silicon nitride decreased in XRD chart, which suggested that vitrification and/or decrease in grain size of silicon nitride occurred. The milled powders were sintered by spark plasma sintering system. Aluminium nitride was formed during sintering by reaction of aluminium and atmospheric nitrogen. Dense nano-ceramics, which were composed of silicon nitride and sialon, were obtained by sintering at 1550 oC.

The Sintering Residual Stress of WC-Ni Cemented Carbide Alloy

Cemented carbide alloy is widely used for high strength and hardness. The tips of cemented carbide alloy are sintered from the WC powder and often brazed on the wide sliding surface. Although welding is better than brazing, the welding crack occurs sometimes because the cemented carbide alloy has low contents of the bonding metal. Functionally gradient composite material will be expected to improve the weldability. One of the factors of the crack is the residual stress existed in the material. This paper describes an X-ray measurement method of both residual stress in WC and Ni phase of the cemented carbide alloy. Six kinds of specimen with bonding material Ni from 10% to 60% weight contents were made by spark plasma sintering (SPS) system. The stresses of the specimen were measured by X-ray diffractometer. As a result, the average residual stress between WC and Ni phase was approximately 500 MPa in compression, while the stress was released considerably after annealing.

Spark-plasma sintering the 8-mol% yttria-stabilized zirconia electrolyte

The 8-mol% yttria-stabilized zirconia (YSZ) is a widely chosen electrolyte for solid oxide fuel cells (SOFC) and oxygen sensors because of its excellent hightemperature ionic conductivity [1–4]. The ionic conductivity and mechanical strength of the electrolyte are strongly related to the microstructure of the electrolyte [5]. Gibson et al. [6] investigated the relationship between YSZ sample density and its corresponding ionic conductivity. Tomonari Takeuchi et al. [7] reported that different sintering methods of the electrolyte could get distinct microstructure and induce variety of the ionic conductivity of the electrolyte. Spark plasma sintering (SPS) method enables the manufacture of dense materials in a short period at low sintering temperatures compared with current methods. SPS method is a manufacturing technology for materials, which utilizes the high energy of a high temperature plasma (discharge plasma) generated when pulseformed electric energy is applied to the space among fine particles [8]. SPS has been employed to study alloys [9], functionally gradient materials [10, 11] and several ceramics such as SiC [12], ZrO2 [13], Al2O3 [14], Zr2O/Al2O3 [15] and TiC/Al2O3 [16]. However, for SPS sintered ZrO2 ceramics, the past research was interesting mainly for its microstructural and mechanical properties, In this article, the electrical properties of the 8-mol% YSZ sintered by SPS was studied in comparison to conventional sintering. Commercial 8-mol% YSZ powder (TZ-8YS, Tosoh Corp., Tokyo, Japan) with an average particle size of ca. 0.1 μm was used as starting material. The YSZ powder was sintered to disks by SPS with Dr. Sinter LAB 515 System (Sumitomo Coal Mining, Kanagawa, Japan) or by conventional sintering in air. For SPS, the following sintering steps were taken: YSZ powder (∼2 g) was placed in a graphite die (15 mmφ), and the SPS system was evacuated to 5 Pa and uniaxial pressure of 28 Mpa was applied. During theses procedures, the sintering temperature was increased to 1350 ◦C at a rate of ca.160 ◦C min−1 and maintained at 1350 ◦C for 10 min. Then the sample was cooled to room temperature naturally (SPS pellet). The SPS pellet were annealed in air at 1000 ◦C for 2 hrs to remove carbon contamination. For conventional sintering (CS), the green pellet with a dimension of 15 mm in diameter and approximately 4 mm in thickness was formed by die compaction at similar uniaxial pressure of 140 MPa to ensure the comparable green density. The pellets were then sintered at 1450 ◦C for 4 hrs by electrical resistance furnace and cooled to room temperature naturally (CS pellet). The sintered densities of the samples were obtained using the conventional Archimedes technique and a digital micrometer measuring the exact diameters and thickness. The Jeol JSM-5900LV (Jeol Pte Ltd., Tokyo, Japan) scanning electron microscope (SEM) and X’Pert Pro MPD X-ray Diffraction System (Philips, Netherlands) were employed to study the microstructural characteristics of the sintered samples. The sintered electrolytes were coated with platinum paste on both sides as electrodes and fired at 1200 ◦C for 2 hrs. The ionic conductivity of the sintered samples was measured by AC impedance analyzer (HP4194A, USA). The measuring frequency range is from 100 Hz to 1 MHz with an applied voltage of 100 mV. Measurements were carried out between 300–850 ◦C at an increment 50 ◦C. The relative density of the SPS pellet and CS pellet are 99 and 97% respectively. The XRD patterns for the SPS pellet and CS pellet as shown in Fig. 1, indicated a cubic unit cell (Fm3m). Using the value of full width at half maximum (FWHM) of the peak (111). The calculated finer crystallite size of the SPS pellet is 154 nm and that of the CS pellet is more than 1 μm. Fig. 2 shows SEM micrographs of the surface of the SPS pellet and CS pellet, and shows that the average grain size of SPS is 0.8 μm and they are in close contact with each other. In contrast to the SPS pellet, the grain size of the CS pellet increased to more than 3 μm, as reported previously for dense 8YSZ ceramics sintered by conventional heating [17].

Radio frequency (rf) plasma spheroidized HA powders: powder characterization and spark plasma sintering behavior

The present study describes the synthesis of spheroidized hydroxyapatite (HA) powders using a radio frequency (rf) inductively coupled plasma (ICP) torch. The spheroidized powders were consolidated through a spark plasma sintering (SPS) system. The microstructure and crystallographic phases in the synthesized powders were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD) and Raman spectrometry. Results showed that the HA feedstock decomposed after rf plasma processing. Crystalline HA, α-tri-calcium phosphate (α-TCP), tetra-calcium phosphate (TTCP) and calcium oxide (CaO) were detected in the plasma-spheroidized powders. Raman spectra results indicated strong presence of amorphous calcium phosphate (ACP) in the spheroidized powders. The particle size distribution and specific surface area were influenced through the rf plasma working plate power levels. The sintering behavior of the rf plasma synthesized powders was analyzed through the SPS process and the results indicated that the spheroidized powders commence sintering at approximately 900°C and through to 1150°C. After sintering above 1100°C for 3min, the relative densities of the SPS compacts reached 96% of the theoretical value. The SPS compacts were immersed in simulated body fluids (SBF) for different durations and the results confirmed their bioactivities.

A Novel Pulse‐Current‐Assisted Sintering Method for Fabrication of Metallic Cellular Structures

A novel isostatic pressing method by spark plasma sintering system is presented for the fabrication of cellular metals. It is a faster and more flexible sintering method compared with traditional hot pressing. The authors show that cellular metal with fine and controllable structure can be fabricated by this method.

Fabrication of closed cellular nickel alloy containing polymer by sintering method

A cellular solid of a Ni–P alloy containing polymer for passive damping was fabricated by sintering methods using a spark plasma sintering (SPS) system. The mechanical and damping properties of this material were measured. The obtained results showed that this cellular structural solid of Ni–P alloy containing polymer has a high capacity for damping.