Spark Plasma Sintering Samples Research Articles

Spark Plasma Sintering (SPS) to the Mullite –Zirconia Ceramics Development

This research is devoted on the investigation of effect of SPS (spark plasma sintering) by comparing with the conventional sintering on the mullite-ZrO2 ceramics development. Mullite-ZrO2 ceramics was produced from different time ball-milled powders. The effect of illite nanoparticles to one part of powder was investigated to promote the sintering process. The density and compressive strength as well as microstructure and crystalline phase development of ceramics were used to show the impact of SPS by comparing with conventional sintering. It was shown that sintered at 12500C by SPS samples achieved densities for up to 1.6-2.2 times times higher as by conventional sintering at 13000C. Together the compressive strength and Vicker's microhardness correlate with the changing of density. The microstructure of the SPS samples is dense and is formed from good textured mullite and cubic ZrO2 particles, but by conventionally sintered – there are formed mullite-corundum crystals with inclusion of tetragonal ZrO2. illite clay additive is effective on density and compressive strength increase only for conventionally sintered samples, but not for SPS. DOI: http://dx.doi.org/10.5755/j01.ms.21.1.5501

Solid State Recycling of Aluminium Sheet Scrap by Means of Spark Plasma Sintering

Various solid state or ‘meltless’ recycling techniques have recently been developed for light metal scrap in form of chips. Main objective of all approaches is to bypass the need for remelting in order to reduce the overall required energy, and to avoid the material losses that occur during this step. Within this paper, the use of Spark Plasma Sintering (SPS) is proposed as a novel solid state recycling/welding technique for sheet metal scrap. Aluminium 5182 alloy scrap, derived from sheet metal, was successfully consolidated into a fully dense billet via SPS. The use of pulsed electric current heating, in temperatures well below the alloy melting point, combined with mechanical pressure, enchased the densification process resulting into a void-less material. The recycled SPS sample was fully densified and microstructural investigation has been performed in order to confirm effective oxide film breakage. The results illustrate the effectiveness of SPS in aluminium scrap consolidation, also in form of sheet scrap, providing additional means in solid state recycling. The involved mechanisms that contribute to oxide film fracture and scrap consolidation in SPS are being discussed.Keywords: Aluminium, recycling, spark plasma sintering (SPS)

Effects of Prior Annealing on the Spark Plasma Sintering of Nanostructured Y2O3 Powders

The effects produced by annealing Y2O3 nanopowders on their spark plasma sintering (SPS) behavior are systematically investigated in this work. It is found that the annealed powders display higher sinterability with respect to the as‐received ones. Indeed, the maximum densification level reached from pristine powders is about 97.5%, whereas density decreases when further increasing either the sintering temperature or the dwell time. In contrast, the density of SPS products obtained from pretreated powder monotonically increases with temperature and processing time, thus leading to fully dense materials in 30 min at 1050°C and 60 MPa. Correspondingly, it is found that the annealing treatment markedly inhibits grain coarsening during SPS. Thus, dense translucent samples with grain size below 100 nm can be attained from annealed powders. On the other hand, white‐opaque specimens with significantly coarser microstructures (up to 1‐μm‐sized grains) are obtained when pristine powders are directly processed under the same sintering conditions. Furthermore, it is observed that the annealing treatment of SPS samples in air allows for graphite contamination removal, whereas no improvement in term of light transmittance is produced.

Evidences of grain boundary capacitance effect on the colossal dielectric permittivity in (Nb + In) co-doped TiO2 ceramics.

The (Nb + In) co-doped TiO2 ceramics were synthesized by conventional solid-state sintering (CSSS) and spark plasma sintering (SPS) methods. The phases and microstructures were studied by X-ray diffraction, Raman spectra, field-emission scanning electron microscopy and transmission electron microscopy, indicating that both samples were in pure rutile phase while showing significant difference in grain size. The dielectric and I–V behaviors of SPS and CSSS samples were investigated. Though both possess colossal permittivity (CP), the SPS samples exhibited much higher dielectric permittivity/loss factor and lower breakdown electric field when compared to their CSSS counterparts. To further explore the origin of CP in co-doped TiO2 ceramics, the I–V behavior was studied on single grain and grain boundary in CSSS sample. The nearly ohmic I–V behavior was observed in single grain, while GBs showed nonlinear behavior and much higher resistance. The higher dielectric permittivity and lower breakdown electric field in SPS samples, thus, were thought to be associated with the feature of SPS, by which reduced space charges and/or impurity segregation can be achieved at grain boundaries. The present results support that the grain boundary capacitance effect plays an important role in the CP and nonlinear I–V behavior of (Nb + In) co-doped TiO2 ceramics.

Synthesis of M-type SrFe12O19 by mechanosynthesis assisted by spark plasma sintering

We present a comparative study of synthesis of M-type strontium hexaferrite from strontium and iron single oxides mechanically activated by high-energy ball milling for 5h, and assisted by two different methods: (a) conventional heat treatment and (b) Spark Plasma Sintering (SPS), both at relative low temperatures (⩽900°C). Although it was found that both methods promoted the complete structural transformation of precursors to Sr-hexaferrite phase (S.G. Pmc21) for temperatures above 700°C, higher saturation magnetization was found for SPS samples. X-ray diffraction analysis revealed that the structural transformation undergoes formation of an intermediate metastable structure (Fe2Sr2O5) in both methods, but with slight different kinetics. Maximum specific magnetization of 67emu/g at 18kOe and coercivity of 3.7kOe were recorded from powder mixtures milled for 5h, which were subsequently SPS-ed at 700°C. By contrast, although magnetization values for the same milled samples after being annealed at 700°C slightly decreased, it leads to a significant increase in the coercive field reaching 5.4kOe. These results are explained on the basis of factors such as the complete formation of hexaferrite and the attained density of consolidated powders, in addition to particle and grain sizes also reported.

Microstructures of Melt-Processed and Spark Plasma Sintered Ceramic Waste Forms

Hollandite-rich ceramic waste forms have been demonstrated to exhibit high durability while simultaneously accommodating a wide range of radionuclides in their matrices. This paper presents preliminary results on the preparation and characterization of ceramic waste forms prepared by two different methods—melt processing and spark plasma sintering (SPS). Both processes resulted in similar phase assemblages but exhibited different microstructures depending on processing method. The SPS samples exhibited fine-grained (<1 µm) and dispersed phases, whereas the melt-processed sample contained larger grains (10–20 µm) of specific phases. Additional data will need to be collected on the aqueous leaching durability and radiation resistance to evaluate each processing method for waste form performance.

Spark plasma sintering of an Al-based powder blend

Abstract The present study investigates the sintering response of a hypereutectic aluminum–silicon alloy, specifically Alumix-231 (Al–14Si–2.5Cu–0.5Mg), using Spark Plasma Sintering (SPS). The focus was to link the influence of the sintering parameters to the sintered density, microstructure, hardness and flexural properties in T1 and T6 conditions. The results were compared to those obtained using the conventional press and sinter route, a method typically used for this alloy. The samples consolidated using SPS attained densities between 95% and 100%TD compared to those obtained by conventional sintering which were approximately 94%TD. Before and after the aging heat treatment, microhardness and mechanical properties were assessed for both SPS and press-sinter samples. After sintering, in the T1 condition, a hardness of 89±7 HV, a flexural strength of 702±28 MPa and a flexural strain of 6.2±1.2% were obtained for the SPS samples. In the T6 condition, a hardness of 157±5 HV, a flexural strength of 654±38 MPa and a flexural strain of 4.4±0.2% were obtained in the ideal SPS conditions. On the other hand, a hardness of 137±8 HV, a flexural strength of 315±69 MPa, and a flexural strain of 3.4±0.1% were achieved for the conventional sintering process after the T6 heat treatment. The hardness in the T1 condition was lower than what had been reported in the literature, however after a T6 heat treatment, hardness values obtained in this study were similar to previous work. On the other hand, the difference in flexural properties was unlike previous studies.

Synthesis and thermoelectric performance of a p-type Bi0.4Sb1.6Te3 material developed via mechanical alloying

A p-type Bi0.4Sb1.6Te3 thermoelectric compound was fabricated via mechanical alloying of bismuth, antimony and tellurium elemental powders as starting materials. The mechanically alloyed compositions were sintered through a spark-plasma sintering (SPS) process. The effect of the milling time was investigated. In order to characterize the powders obtained via mechanical alloying, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analysis were used. The morphological evolution was studied by scanning electron microscopy (SEM). Results showed that the p-type Bi0.4Sb1.6Te3 compound was formed after 2h of milling. Further, the variation of milling time showed that the synthesized phase was stable. All the powders exhibit the same morphology albeit with slight differences. Measurements of the electrical resistivity, Seebeck coefficient and thermal conductivity were performed in the temperature range 300–520K for the SPS samples. The resulting thermoelectric figure of merit ZT reaches a maximum of 1.2 at 360K for the p-type bulk material with a 5h milling time. This study demonstrates the possibility of preparing thermoelectric materials of high performance and short processing time.

The spatial distribution of temperature and oxygen deficiency in spark-plasma sintered superconducting Bi-based materials

Pre-reacted powders of (Bi–Pb)2Sr2Ca2Cu3O10+δ (Bi-2223) were consolidated by using the spark plasma sintering (SPS) technique under vacuum and at different consolidate temperatures TD. X-ray diffraction patterns revealed that the dominant phase in all SPS samples is the Bi-2223 phase, but traces of the Bi2Sr2CaCu2O10+x (Bi-2212) phase were identified. We have found that the transport properties of SPS samples depend on their oxygen content because the SPS process is performed under vacuum. Simulations by using the finite element method (FEM) were performed for determining the actual temperature in which powders are consolidated. From these results we have inferred that SPS samples are oxygen deficient and such a deficiency is more marked near the grain boundaries, suggesting the occurrence of grains with core–shell morphology. We also argued that the width of the shell depends on the consolidation temperature, a feature corroborated by the FEM simulations.

Spark plasma sintering consolidation of Bi1.65Pb0.35Sr2Ca2Cu3010+δ ceramic samples

Pre-reacted powders of (Bi-Pb)2Sr2Ca2Cu3O10+δ (Bi-2223) were consolidated by using the spark plasma sintering (SPS) technique under vacuum and at different consolidated temperatures TD. We have found that the transport properties of SPS samples depend on their oxygen content because the SPS process is performed under vacuum. From these results we have inferred that SPS samples are oxygen deficient and that a post-annealing treatment, performed in air for a brief time interval, is needed to restore such a deficiency. Measurements of temperature dependence of the thermal conductivity, κ(T), were performed in all SPS samples including a reference sample sintered at 845 °C in air. The results indicate that the electronic contribution to the thermal conductivity in all samples is very low. Values of κ were found to be higher in SPS samples mostly because of their high volume density of ~ 5.7 g/cm3. We have also performed measurements of the critical current density as a function of the applied magnetic field, Jc(Ba). All samples display a clear Josephson-like behavior and low values of the superconducting critical current density at zero applied magnetic field. The experimental results indicate that the oxygen deficiency in SPS samples is marked near the grain boundaries. This suggests the occurrence of grains with core-shell morphology, where the width of the shell is consolidation temperature dependent.

Powder metallurgy routes toward aluminum boron nitride nanotube composites, their morphologies, structures and mechanical properties

Aluminum/boron nitride nanotube (BNNT) composites with up to 5wt% (i.e., 9.7vol%) nanotube fractions were prepared via spark plasma sintering (SPS) and high-pressure torsion (HPT) methods. Various microscopy techniques, X-ray diffraction, and energy dispersive X-ray analysis confirmed the integration of the two phases into decently dense and compact composites. No other phases, like Al borides or nitrides, formed in the Al–BNNTs macrocomposites of the two series. The BNNTs were found to be preferentially located along Al grain boundaries in SPS samples (grain size was 10–20μm) creating micro-discontinuities and pores which were found to be detrimental for the sample hardness, whereas in HPT samples, the tubes were rather evenly distributed within a fine-grained Al matrix (grain size of several hundred nm). Therefore, the hardness of HPT samples was drastically increased with increasing BNNTs content in Al pellets. The value for Al–BNNT 3.0wt% sample was more than doubled (190MPa) compared to a pure Al–HPT compact (90MPa). And the room temperature ultimate tensile strength of Al–BNNTs HPT samples containing 3.0wt% BNNT (~300MPa) became ~1.5 times larger than that of a BNNT-free HPT–Al compact (~200MPa).

キャリアドープしたCuFeS&lt;sub&gt;2 &lt;/sub&gt;焼結体の作成と熱電特性

Carrier-doped CuFeS2 samples have been synthesized by a solid-state reaction (SSR) and the spark-plasma sintering (SPS) methods, and the relation between the synthesis conditions and the thermoelectric properties is shown. Samples prepared by SSR have a density less than 80 %, while SPS samples show a high density close to 100 %. This results in the great improvement to reduce the electrical resistivity (ρ) in the SPS samples. ρ is further reduced by the subsequent annealing. Seebeck coefficient S shows large negative values for all the samples. S of Fe – and Zn – doped CuFeS2 samples are almost independent to the sample preparation methods. As a result, power factor S2/ρ of the SPS and annealed samples have almost doubled from that of the SSR samples, reaching 1 mW/K2m at 400 K.

기계적합금화에 의한 α-Fe/Al2O3자성 나노복합재료의 제조 및 치밀화

본 연구에서는 <TEX>$Fe_2O_3-Al$</TEX>계 나노복합재료를 제조하기 위하여 실온 기계적 합금화법(MA)을 적용하였다. <TEX>$Fe_2O_3$</TEX>와 순금속 Al의 혼합분말을 5시간 동안 MA 처리한 결과 <TEX>${\alpha}-Fe$</TEX> 기지에 <TEX>$Al_2O_3$</TEX>가 미세하게 분산된 <TEX>${\alpha}-Fe/Al_2O_3$</TEX> 나노복합분말을 얻을 수 있었다. 또한 MA 분말의 자화값 및 보자력 측정을 통하여 볼밀처리 중 순금속 Al에 의한 헤마타이트의 고상환원 과정을 자세히 관찰할 수 있었다. MA 분말시료의 벌크화를 위하여 소결온도 <TEX>$1000^{\circ}C$</TEX> 및 <TEX>$1100^{\circ}C$</TEX>, 압력 60 MPa 에서 SPS 소결을 실시하였다. SPS 과정에서 MA 5시간 시료의 수축은 소결 개시 후 <TEX>$700^{\circ}C$</TEX> 이상에서 크며 소결온도 <TEX>$1100^{\circ}C$</TEX>까지 비교적 단조롭게 수축함을 알 수 있었다. X선 회절 결과로부터, MA 분말을 <TEX>$1100^{\circ}C$</TEX>에서 SPS 소결시킨 <TEX>${\alpha}-Fe/Al_2O_3$</TEX> 나노복합재료의 경우 <TEX>${\alpha}-Fe$</TEX>상 평균 결정립 크기가 180 nm임을 알 수 있었다. 또한 MA 분말을 <TEX>$1000^{\circ}C$</TEX>에서 SPS 소결시킨 시료의 보자력이 88 Oe로 여전히 높은 값을 보이는 사실로부터 소결과정 중 자성상 <TEX>${\alpha}-Fe$</TEX>의 결정립 성장이 크게 억제된 것으로 판단된다. Fabrication of nanocomposite material for the <TEX>$Fe_2O_3-Al$</TEX> system by mechanical alloying (MA) has been investigated at room temperature. It is found that <TEX>${\alpha}-Fe/Al_2O_3$</TEX> nanocomposite powders in which <TEX>$Al_2O_3$</TEX> is dispersed in <TEX>${\alpha}-Fe$</TEX> matrix are obtained by mechanical alloying of <TEX>$Fe_2O_3$</TEX> with Al for 5 hours. The change in magnetization and coercivity also reflects the details of the solid state reduction process of hematite by pure metal of Al during mechanical alloying. Densification of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies at <TEX>$1000^{\circ}C$</TEX> and <TEX>$1100^{\circ}C$</TEX> under 60 MPa. Shrinkage change after SPS of MA'ed sample for 5 hrs was significant above <TEX>$700^{\circ}C$</TEX> and gradually increased with increasing temperature up to <TEX>$1100^{\circ}C$</TEX>. X-ray diffraction result shows that the average grain size of <TEX>${\alpha}-Fe$</TEX> in <TEX>${\alpha}-Fe/Al_2O_3$</TEX> nanocomposite sintered at <TEX>$1100^{\circ}C$</TEX> is in the range of 180 nm. It can be also seen that the coercivity (Hc) of SPS sample sintered at <TEX>$1000^{\circ}C$</TEX> is still high value of 88 Oe, suggesting that the grain growth of magnetic <TEX>${\alpha}-Fe$</TEX> phase during SPS process tend to be suppressed.

Enhancement of photovoltaic properties with Nb modified (Bi, Na) TiO3–BaTiO3 ferroelectric ceramics

In this study, the photovoltaic properties of Nb doped (Bi0.5Na0.5) TiO3–xBaTiO3 (BNBT) ferroelectric ceramics prepared by spark plasma sintering (SPS) were investigated and compared with the conventionally sintered (CS) samples. Much higher photocurrent and photovoltage were observed in SPS samples due to their more compact and homogenous microstructures with smaller grain size. Nb doping increases the photovoltaic response of SPS BNBT ceramics significantly. Optimal photovoltaic response is obtained for 1 at.% Nb doped SPS BNBT ceramics with a large above-bandgap photovoltage, showing the potential in future photovoltaic application.

The Effect of Microstructure on the Thermoelectric Properties of Polycrystalline Higher Manganese Silicides

In order to obtain single-phase higher manganese silicides (HMS) and investigate the effect of sintering conditions on the thermoelectric properties of a HMS system, HMS compounds were synthesized using a vacuum induction melting method and sintered using spark plasma sintering (SPS) and hot pressing methods. Single-phase HMS with a small amount of second phases was obtained in all of the HMS samples produced. Changes in the electrical conductivity and Seebeck coefficient of the HMS were observed when the sintering temperature was changed, which can be attributed to the presence of the second phases. Similar changes of thermoelectric properties were observed in both the SPS and hot-pressed samples. However, the electrical conductivity and Seebeck coefficient of HMS samples (SPS-HMS) were higher than those of the hot-pressed samples, which can be attributed to SPS's shorter holding time and its ability to control the diffusion rate. The SPS-HMS sample sintered at 1123 K (1123 K SPS-HMS) had a higher figure of merit than any other sample although the sample had a lower power factor. The high value of the figure of merit of the sample can be attributed to its low thermal conductivity. The highest figure of merit value of 0.41 was measured at 850 K in the 1123 K SPS-HMS, which is comparable to the results reported earlier. The results of the present study can be used to optimize the fabrication process of HMS thermoelectric materials.

Effects of mechanical alloying process and sintering methods on the microstructure and thermoelectric properties of bulk Bi0.5Sb1.5Te3 alloy

In this study, bulk ternary alloy Bi0.5Sb1.5Te3 was fabricated by mechanical alloying combined with hot pressing (HP) and spark plasma sintering process (SPS), respectively. Differences in microstructure and thermoelectric properties between HP samples and SPS specimens with different ball milling parameters were investigated visually. The results showed that certain improvement in rotate speed of ball milling process can reduce the formation time of Bi0.5Sb1.5Te3 powders from 60 h to 12 h. Mechanical alloying process had an important effect on the Seebeck coefficient of Bi0.5Sb1.5Te3 alloy, negative Seebeck coefficient may even appear due to the deformation induced donor-like effect. The study also discovered that laminated structure was much more familiar in hot pressed Bi0.5Sb1.5Te3 alloy compared with that in the spark plasma sintering samples. The figure of merit of Bi0.5Sb1.5Te3 alloy was enhanced with the increasing hot pressing temperature and reached 0.0027 K−1 around room temperature. Spark plasma sintered samples with 450 rpm + 60 h + 20:1 ball milling parameters have the maximum figure of merit 0.0048 K−1 which is about 1.5 times higher than the maximum TE value of HP samples.

Dielectric and ferroelectric properties of Ba1−xSrxTiO3 ceramics: effects of grain size and ferroelectric domain

Ba1−xSrxTiO3 (x = 0·3, 0·35) ceramics are fabricated by spark plasma sintering (SPS) technique and conventional sintering (CS) route. Their microstructures are determined together with the dielectric and ferroelectric properties. The grain sizes of SPS samples are smaller than those of CS. The differential scanning calorimetry data show that almost no latent heat associated with phase transition can be detected in the SPS sintered ceramics. Raman spectroscopy analysis suggests that the tetragonal phase increases with increasing the grain size, and the higher content of tetragonal phase leads the superior dielectric properties in CS sintered samples. Transmission electron microscopy analysis indicates that domain structure heavily relies on the grain size of ceramics. The poor ferroelectric properties of SPS ceramics are originated from the incomplete development of the tetragonal structure.

Consolidation of mechanically alloyed Cu–Ni–Fe material by spark plasma sintering and evaluation as inert anode for aluminum electrolysis

A Cu65Ni20Fe15 powder was prepared by mechanical alloying at semi-pilot scale to form a face-centered-cubic (fcc) phase (γ-phase). This powder was then consolidated by spark plasma sintering (SPS) to form a pellet. The crystallographic structure of the material is not affected by the consolidation process, although there is a slight increase of the crystallite size from 20 to 32 nm, and a slight decrease of the lattice strain from 0.5% to 0.2%. The relative density of the SPS consolidated sample is 95%. Thermogravimetric analysis confirms the good oxidation resistance of the SPS sample with a mass gain of only 0.4% after 20 h of oxidation at 700 °C, which is attributed to the rapid formation of a protective NiFe2O4 layer. The SPS sample was then evaluated as inert anode for Al electrolysis in low-temperature (700 °C) KF–AlF3 electrolyte. After 20 h of electrolysis at an anode current density of 0.5 A cm−2, the cell voltage reaches an unstable value of 5.0 V. The purity of the produced aluminum is 99.4% and the wear rate of the electrode is estimated at 1.8 cm year−1.

Investigation of thermoelectric properties of Cu2GaxSn1 − xSe3 diamond-like compounds by hot pressing and spark plasma sintering

P-type compounds Cu2GaxSn1−xSe3 (x=0.025, 0.05, 0.075) with a diamond-like structure were consolidated using hot pressing sintering (HP) and spark plasma sintering (SPS) techniques. High-temperature thermoelectric properties as well as low-temperature Hall data are reported. Microstructural analysis shows that the distribution of Ga is homogeneous in the samples sintered by HP but inhomogeneous in the samples sintered by SPS, even with an electrically isolating and thermally conducting BN layer during the sintering. The Seebeck coefficients of the samples sintered by HP and SPS show similar dependence on the carrier concentration and are insensitive to the composition inhomogeneity. In contrast, the composition inhomogeneity results in lower carrier mobility and thus lower electrical conductivity in the samples sintered by SPS than those sintered by HP. Lattice thermal conductivity is further reduced through Ga doping; however, this effect is weakened by the inhomogeneous distribution of Ga. Due to their larger carrier mobility and lower lattice thermal conductivity, the samples sintered by HP exhibit 15–35% higher thermoelectric figure of merits (ZT) than those SPS samples with a high Ga doping level and without the coated BN layer, in which the composition homogeneity is worse. A ZT value of 0.43 is obtained for the HP Cu2Ga0.075Sn0.925Se3 sample at 700K.

Tribological performance of Al–12Si coatings created via Electrospark Deposition and Spark Plasma Sintering

Spark plasma sintering (SPS) and Electrospark deposition (ESD) were used as two manufacturing routes to create Al–12Si coatings with refined microstructures. The tribological performance of these coatings was tested using a linear reciprocating tribometer. The SPS and ESD coatings were compared to a conventionally as-cast ingot (AC) of Al–12Si to observe the effect of particle refinement and morphology changes. The ESD sample exhibited lower wear rates than the AC and SPS samples. During the wear process, the Al and Si phases on the surface of the AC sample were significantly refined. This refinement led to a microstructure near the surface that was similar to the sample generated by SPS, and therefore showed similar wear rates at the end of testing. Subsurface cracking, which is known to occur in the wear of powder metallurgy alloys, was observed for sample SPS. The refinement of the Al and Si phases achieved for ESD process was much greater than what resulted during the wear of the AC and SPS surfaces and as a result the ESD sample showed lower wear rates throughout the entire test.