Spark Plasma Sintering (SPS)

Ultra-high temperature ceramics (UHTCs) are interesting materials for a large variety of applications under extreme conditions. This paper reports on the production and extensive characterization of highly dense, pure zirconium and tantalum diborides, with particular interest to their potential utilization in the thermal solar energy field. Monolithic bulk samples are produced by Spark Plasma Sintering starting from elemental reactants or using metal diboride powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). Microstructural and optical properties of products obtained by the two processing methods have been comparatively evaluated. We found that pure diborides show a good spectral selectivity, which is an appealing characteristic for solar absorber applications. No, or very small, differences in the optical properties have been evidenced when the two investigated processes adopted for the fabrication of dense TaB 2 and ZrB 2 , respectively, are compared.

Joule heating as a primary heating source mechanism was probed during spark plasma sintering (SPS) of pure metal powders (Fe, Ni and Cu). Resistance to electric path was estimated from voltage–current measurements obtained online during these experiments. Resistance was observed to saturate at the same value irrespective of the type of metal powder, after attaining a sintering temperature of ∼0.3Tm. This saturation in resistance is attributed primarily to the Joule heating that occurs at the graphite-foil and punch in an SPS system.

In this research, the microstructure and transport properties of p-type Bi 0.5 Sb 1.5 Te 3 thermoelectric materials were investigated as a function of milling time. The p-type Bi 0.5 Sb 1.5 Te 3 alloys were fabricated by mechanical alloying of elemental chunks of bismuth, antimony, and tellurium. This was followed by plasma spark sintering at 673 K. The micro-Vickers hardness (98.7 Hv) was considerably improved in the 90-min sample due to the presence of fine grains in the matrix that prevented crack propagation via grain-boundary hardening. The lowest lattice thermal conductivity (0.63 W/mK) was obtained for the 90-min sample, a value slightly lower than the minimum total thermal conductivity (0.872 ± 0.5 W/mK at 300 K) due to strong scattering of phonons and carriers owing to the completely randomness of the distribution of the fine-grain structure in the bulk samples. The maximum figure-of-merit (ZT = 0.98 ± 0.5 at 300 K) was obtained for the 90-min sample due to its superior power factor values.

Development of large scale high performance thermoelectric materials is one of the challenges in thermoelectric energy conversion. We have successfully fabricated large scale production (3–5 kg/min) of bismuth antimony telluride (Bi-Sb-Te) alloys by gas atomization (GA) and achieved a high figure of merit (ZT) value over unity (1) at 350 K. To get more performance from thermoelectric materials, we controlled the grain size of GA powders via mechanical milling (GA + MA) and consolidated the resultant nano powder by spark plasma sintering. The ZT values of GA + MA samples were greatly improved (15%) over those of GA bulk due to shifting control of their grain size from micron order to submicron order. The improved ZT values are due to the suppression of lattice thermal conductivity, which was drastically decreased due high scattering of phonons at numerous grain boundaries and twin interfaces. The mechanical properties were greatly improved (GA + MA sample hardness improved > 61% over the GA sample), which would provide impressive benefits for device fabrication and practical applications for thermoelectric energy conversion.

Soft magnets are extensively deployed in rotating electric machines. Accelerated development of new high Curie temperature, mechanically strong, magnetically soft magnets is urgently needed for next generation rotating electrical machines. The ternary Fe-Co-Ni alloy system was investigated. Spark plasma sintering (SPS) was employed to process compositionally graded (Ni-21Fe)-xCo samples. A combinatorial assessment of structural, magnetic, mechanical and electrical properties of sample sections was performed. Co-lean alloys exhibited a good combination of magnetic, mechanical, and electrical properties but low Curie temperatures. On the other hand, Co rich alloys exhibited elevated T c , high M s , and large hardness. An alloy library with a wide range of properties, M s : 100-150 emu/g, H c : 0.8-30 Oe, T c : 571-1108 °C, ρ: 9-12.9 μΩ-cm, nanohardness: 0.96-1.56 GPa was developed. Thus, SPS processed samples can be employed for the accelerated screening of magnetic materials.

Iron-based soft magnetic alloys (FeSiB, FeSiBNb, FeSiBCu, and FeSiBNbCu (Finemet)) have been fabricated via mechanical alloying followed by spark plasma sintering (SPS) process. FeSiB alloy powder was obtained by high energy ball milling of an elemental blend Fe, Si, and B powders. The effect of milling time on crystallite size and phase transformation was studied. Additionally, FeSiBCu, FeSiBNb, and FeSiBCuNb alloy powders were milled to study the effect of Cu and Nb on phase transformation, mechanical, and magnetic behavior. The mechanically alloyed powders were sintered via SPS process to achieve full densification. The microhardness and magnetic permeability of sintered FeSiB alloys were found to be increased monotonically with milling time primarily due to the smaller crystallite size and more uniform microstructure. Interestingly, the alloying of Cu or (and) Nb to FeSiB resulted in higher saturation magnetization and lower coercivity mainly due to large volume fraction of α-Fe 3 Si nanocrystals. Overall, these alloys exhibit reasonably good soft magnetic behavior along with excellent microhardness. Mechanical alloying followed by spark plasma sintering opens up a new avenue of processing amorphous-nanocrystalline alloys into bulk shape with good mechanical and magnetic properties.

The present work is focused on synthesis and heat treatment on non-equiatomic AlCoCrFeNiTi 0.5 high entropy alloy (HEA) with a composite structure reinforced by TiC nanoparticles. The initial alloy was prepared by mechanical alloying (MA) in a planetary ball mill, compacted by spark plasma sintering (SPS) and heat treated at different temperatures. Mechano-chemical reactions during the MA process as well as the microstructure and hardness of the SPS-ed compacts prior to and after the heat treatment were investigated. During MA, Cr-based supersaturated solid solution with the BCC structure was formed. After SPS at 1100 °C, the BCC solid solution decomposed into nano-grained microstructure consisting of FCC and ordered BCC solid solutions, s phase, and in-situ formed TiC nanoparticles. The high hardness of the alloy (762 HV) was retained after the subsequent heat treatment at 1100 °C (603 HV). It was shown that the fabrication of TiC reinforced nanocomposites from elemental powders without the use of expensive nanograined powders can be achieved.

A B S T R A C T Graphene was prepared using liquid phase exfoliation and dispersed in an alumina matrix using an ultrasonication and powder processing route. Al 2 O 3 –graphene composites with up to 5 vol% content were densified (>99%) using SPS. The fracture toughness of the material increased by 40% with the addition of only 0.8 vol% graphene. However for higher graphene contents the improvement in fracture toughness was limited. Graphene changed the

This work reports on the spark plasma sintering (SPS) of self-propagating high-temperature-synthesis (SHS)-derived Ni-W and Ni-W-2wt%hBN (4:1 molar ratio of metals) powders. The synthesis was carried out from a mixture of NiO and WO3 using Mg + C combined reducers through a thermo-kinetic coupling approach. Experiments performed in the thermodynamically optimal area demonstrated the high sensitivity of combustion parameters and product phase composition to the amount of reducers and hBN powder. The powder precursors with and without the addition of hBN were consolidated using SPS at a temperature and pressure of 1300 °C and 50 MPa, respectively, followed by a thorough phase and microstructural characterization of the obtained specimens. SHS-derived powders comprised the nano-sized agglomerates and were characterized by a high sinterability. The specimens of >95% density were subjected to ball-on-plate dry sliding wear tests at a sliding speed of 0.1 ms−1 and a distance of 1000 m utilizing an alumina ball of 10 mm in diameter under a 15 N normal load. The tests were performed at a temperature of 800 °C. A significant improvement in wear behavior was demonstrated for SHS-processed composites in comparison with their counterparts produced via conventional high-energy ball milling technique owing to the phenomena of ‘micro-polishing’, cyclic ‘self-healing’ and fatigue. However, the decisive effect of hBN addition in imparting lubrication during an HT wear test was not confirmed.

The influence of mechanical alloying (MA) and spark plasma sintering (SPS) processing parameters on the microstructure, magnetic and mechanical properties of Finemet (Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9) type alloys was investigated. Finemet alloy powder was obtained via mechanical alloying of elemental powders for 30 h, 60 h, 90 h, and 120 h with ball to powder ratio (BPR) of 10:1 and 15:1. The milled powders were consolidated using the SPS process. The XRD patterns confirm the presence of the α-Fe 3 Si phase in all Finemet alloys, also broadening of the (110) peak of α-Fe 3 Si was observed with increasing milling time. The microhardness of Finemet alloys increases with increase in milling time primarily due to a decrease in grain size. All samples show good saturation magnetization (M s), 120h sample exhibiting the highest M s of 166 emu/g, however coercivity increases with increasing milling time. Additionally, α-Fe 3 Si crystal size decreases with increasing the BPR primarily due to higher impact energy.

The Magnéli phase V6O11 was synthesized in gram amounts from a powder mixture of V6O11/V7O13 and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared V6O11 has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K)−1 while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m)−1, a Seebeck coefficient of α = −(35 ± 2) μV K–1, which leads to a power factor of PF = 4.9 ± 0.8 × 10–5W (m K2)−1 at ∼600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as V6O11 are involved. This study gives insight into the complications of SPS-assisted synthesis of complex oxide materials, provides new information about the thermal and electrical properties of vanadium oxides at high temperatures, and supports the concept of reducing the thermal conductivity of materials with structural building blocks such as crystallographic shear (CS) planes.

The preparation of uranium carbide (UC) by carbothermal reduction and its sintering into dense pellets by conventional means require high temperatures for long periods. We have developed a preparation route yielding fine UC powder with significantly increased sinterability. At first, a mixture of nanocrys-talline UO2 embedded in amorphous carbon (nano-UO2/C) was obtained by thermal decomposition of a gel containing solubilised uranyl nitrate and citric acid. Later, the nano-UO2/C powder was treated in a conventional furnace or in a modified spark plasma sintering facility at elevated temperatures (>1200 °C) in order to obtain uranium carbide powder. The effects of initial composition, temperature, gas/vacuum atmosphere and the overall reaction kinetics are reported.

Urea proved to be an appropriate precipitant for obtaining a core-shell alumina/yttria nanocomposite. Alumina/yttria nanocomposite powders with more appropriate morphology and highly sinterability. A fine-grained YAG ceramic was obtained by SPS of alumina-yttria nanocomposite. a b s t r a c t Alumina/yttria nanocomposite powder as an yttrium aluminum garnet (YAG) precursor was synthesized via partial wet route using urea and ammonium hydrogen carbonate (AHC) as precipitants, respectively. The products were characterized using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and energy dispersive spec-troscopy. The use of urea produced very tiny spherical Y-compounds with chemical composition of Y 2 (CO 3) 3 $nH 2 O, which were attracted to the surface of alumina nanoparticles and consequently, a core-shell structure was obtained. The use of ammonium hydrogen carbonate produced sheets of Y-compounds with chemical composition of Y(OH)CO 3 covering the alumina nanoparticles. A fine-grained YAG ceramic (about 500 nm), presenting a non-negligible transparency (45% RIT at IR range) was obtained by the spark plasma sintering (SPS) of alumina-yttria nanocomposite synthesized in the urea system. This amount of transmission was obtained by only the sintering of the powder specimen without any colloidal forming process before sintering or adding any sintering aids or dopant elements. However, by spark plasma sintering of alumina-yttria nanocomposite powder synthesized in AHC system, an opaque YAG ceramic with an average grain size of 1.2 mm was obtained.

Abstract— Highly dense barium titanate nanoceramics have been successfully prepared via a mechanical activation synthesis method and Spark Plasma sintering. Attractive electrical properties have been evidenced in these materials: a colossal permittivity, (3.5. 105) and low loss (0.07) at room temperature and 1 kHz, that are stable over a wide frequency range (from 40 Hz to 40 kHz). Surprisingly, the ferroelectric transition is still observed, for the first time to our knowledge, in these colossal permittivity materials.

The incorporation of ceramic nanoparticles in the bulk thermoelectric matrix is one of the new strategies to boost the Seebeck coefficient. In this research, different weight percentages of Y 2 O 3 (2, 4, and 6) nanoparticles (NPs) were incorporated into the pre-alloyed BiSbTe powder for making nanocomposites (NCs) by mechanical milling. The resultant NCs powders were subsequently consolidation by spark plasma sintering (SPS) at 450 C. The existence of Y 2 O 3 nano-inclusions was confirmed by x-ray diffraction and TEM-SAED analysis. The hardness of the nanocomposites was significantly improved (>49%) compared to that of pure BiSbTe, and this was attributed to grain-boundary hardening and to a dispersion strengthening mechanism. The electrical conductivity decreased while the Seebeck coefficient significantly improved (45%) at room temperature for the NCs to which 2 wt% Y 2 O 3 was added. This was due to the scattering of carriers through the energy filtering effect. The electronic component of the thermal conductivity greatly contributed to the reduction of total thermal conductivity (22%) in BiSbTe NCs to which 6 wt% Y 2 O 3 was added. A peak ZT of 1.24 was achieved for BiSbTe/(2 wt%) Y 2 O 3 NCs due to reduction in their thermal conductivity and improved Seebeck coefficient values.

The thermoelectric behavior and stability of Cu2SnS3 (CTS) has been investigated in relation to different preparations and sintering conditions, leading to different microstructures and porosities. The studied system is CTS in its cubic polymorph, produced in powder form via a bottom-up approach based on high-energy reactive milling. The as-milled powder was sintered in two batches with different synthesis conditions to produce bulk CTS samples: manual cold pressing followed by traditional sintering (TS), or open die pressing (ODP). Despite the significant differences in densities, ~75% and ~90% of the theoretical density for TS and ODP, respectively, we observed no significant difference in electrical transport. The stable, best performing TS samples reached zT ~0.45, above 700 K, whereas zT reached ~0.34 for the best performing ODP in the same conditions. The higher zT of the TS sintered sample is due to the ultra-low thermal conductivity (κ ~0.3–0.2 W/mK), three-fold lower than ODP in the entire measured temperature range. The effect of porosity and production conditions on the transport properties is highlighted, which could pave the way to produce high-performing TE materials.

Abstract Realistic microstructures of compacted powders formed by spark plasma sintering or field-activated sintering technology were modeled using the discrete finite-element method. Two key thermoelectric characteristics were studied:(1) the effect of the electric current pattern, ie, direct current (DC) and pulsed current, on temperature distributions in the compacted powders, and (2) the effect of compaction modes, ie, isostatic compaction and uniaxial compaction, on conductivity. Simulations showed that, for the same electric power ...

—In the present work, the properties of Al 2 O 3 nanocomposite prepared via spark-plasma sintering and reinforced with 0.5–2 wt % graphene are studied. Samples with different graphene contents are subjected to measurements of density, microhardness, coefficient of friction of composite–ruby, and frictional wear rate of composite. The fracture and wear track surface are inspected via fractography, and the composite as a whole is examined via X-ray diffraction. The graphene additive is established to increase the microhardness and to decrease the frictional wear rate by two orders of magnitude on account of absence of flaking of grains.

The remarkable damage-tolerance found in natural materials is developed through functional, multiscale architectures with structural gradients and graded interfaces. [1-6] One notable example is nacre, which comprises ceramic (mineral) plate-lets of polycrystalline aragonite (≈95% by volume) bonded by biopolymers in a "brick-and-mortar" structure. [1,3,7] The Many natural materials present an ideal "recipe" for the development of future damage-tolerant lightweight structural materials. One notable example is the brick-and-mortar structure of nacre, found in mollusk shells, which produces high-toughness, bioinspired ceramics using polymeric mortars as a compliant phase. Theoretical modeling has predicted that use of metallic mortars could lead to even higher damage-tolerance in these materials, although it is difficult to melt-infiltrate metals into ceramic scaffolds as they cannot readily wet ceramics. To avoid this problem, an alternative ("bottom-up") approach to synthesize "nacre-like" ceramics containing a small fraction of nickel mortar is developed. These materials are fabricated using nickel-coated alumina platelets that are aligned using slip-casting and rapidly sintered using spark-plasma sintering. Dewetting of the nickel mortar during sintering is prevented by using NiO-coated as well as Ni-coated platelets. As a result, a "nacre-like" alumina ceramic displaying a resistance-curve toughness up to ≈16 MPa m ½ with a flexural strength of ≈300 MPa is produced. Ceramic-Metal Composites mineral platelets in this structure impart high strength whereas the organic mortar acts as compliant layer to create ductility by permitting platelet sliding which dissipates locally high stresses. To maintain strength, such sliding is limited to a few micrometers by such mechanisms as frictional resistance from the surface roughness of the platelets, [8] the tensile/ shear resistance of the biopolymer inter-phase, [9] the presence of preexisting inter-layer bridges, [10,11] and in certain species the dovetail geometry of the platelets. [12] Toughening is generated extrinsically, [13] primarily by crack bridging leading to brick pull-out and crack-path deflection, [14] to an extent that the fracture toughness of nacre (in energy terms) is several thousand times higher than that of its constituents. There has been great interest in mimicking this nacre-like structure to generate high-toughness ceramics. Processing techniques such as freeze-casting (ice templating), [15-23] layer-by-layer alignment, [24,25] thermal spray processing, [26] sedimentation, [27] coextrusion, [28,29] magnetic platelet alignment, and vacuum filtration assisted alignment [24,30] have been used to recreate brick-and-mortar micro-structures. Using polymeric mortars, the nacre-like ceramics avoid sudden catastrophic failure by stabilizing slow crack growth in the form of crack-resistance (R-curve) behavior, although they cannot be used at elevated temperatures due to the presence of polymer phase. [20,24-27,31,32] Theoretical micromechanical modeling [33] of synthetic "brick-and-mortar" materials has indicated that metallic mortars may permit higher-temperature operation while further enhancing the strength and toughness properties due to their higher shear/ tensile strength (provided the mortar strength does not exceed the brick strength). [34] However, to realize the full effect of a metallic mortar, the bonding between the mortar and the bricks must be strong enough so that the vital interbrick displacements are within the mortar, and not along the brick-mortar interfaces. Moreover, processing such hybrid ceramics with a metallic compliant phase is difficult as metals do not generally wet ceramics and thus cannot be readily melt-infiltrated into a (e.g., freeze-cast) ceramic scaffold. [35-37] To avoid this issue, in this study we have developed a "bottom-up approach" to create fine-scale, high volume fraction (≈95%) alumina brick-and-mortar structures with an ≈5%

Results of uranium carbide sintering using a spark plasma sintering facility are presented here. The initial uranium carbide powder is produced from the thermal decomposition of a citric acid and uranium nitrate mixture. The study shows that spark plasma sintering is a very efficient compaction tool for uranium carbide material. Both onset of sintering and final density are strongly correlated to the UC synthesis conditions.

From serving as the primary host material from near-ideal neutron moderators to high-capacity discharge electrodes, graphite is a ubiquitous material for a number of advanced applications. The focus of this work, however, is on those applications requiring high-purity fine-grained densified graphite monoliths fabricated to scale with minimal effort. To date, graphite monoliths have been constrained to lower final part densities and purities due to the technical challenges associated with mainly hot pressing and extrusion, that ultimate limit these attributes. In this study, we report on the use of spark plasma sintering as an alternate method for fabrication at temperatures below 1,200°C and less than 300 MPa without the use of binders, additional resins, or post-thermal treatments. Formed part sizes from few to several millimeters in size, we report highly dense (2.095 g/cm 3), well-bonded, and low-defect-ridden graphite with uniform composition examined by detailed X-ray and electron-based microscopy. The results are an initial report on the use this technique to sinter graphite under moderate conditions that may support small to moderate-scale technical production needs that require low-cost, high-purity, and high-density graphite.