Modified Spark Plasma Sintering Research Articles

Engineering a Hierarchy of Disorder: A New Route to Synthesize High-Performance 3D Nanoporous All-Carbon Materials*.

A new nanoporous amorphous carbon (NAC) structure that achieves both ultrahigh strength and high electrical conductivity, which are usually incompatible in porous materials is reported. By using modified spark plasma sintering, three amorphous carbon phases with different atomic bonding configurations are created. The composite consisted of an amorphous sp2-carbon matrix mixed with amorphous sp3-carbon and amorphous graphitic motif. NAC structure has an isotropic electrical conductivity of up to 12000Sm-1, Young's modulus of up to ≈5GPa, and Vickers hardness of over 900MPa. These properties are superior to those of existing conductive nanoporous materials. Direct investigation of the multiscale structure of this material through transmission electron microscopy, electron energy loss spectroscopy, and machine learning-based electron tomography revealed that the origin of the remarkable material properties is the well-organized sp2/sp3 amorphous carbon phases with a core-shell-like architecture, where the sp3-rich carbon forms a resilient core surrounded by a conductive sp2-rich layer. This research not only introduces novel materials with exceptional properties but also opens new opportunities for exploring amorphous structures and designing high-performance materials.

High mechanical strength Si anode synthesis with interlayer bonded expanded graphite structure for lithium-ion batteries

Despite advancements in silicon-based anodes for high-capacity lithium-ion batteries, their widespread commercial adoption is still hindered by significant volume expansion during cycling, especially at high active mass loadings crucial for practical use. The root of these challenges lies in the mechanical instability of the material, which subsequently leads to the structural failure of the electrode. Here, we present a novel synthesis of a composite combining expanded graphite and silicon nanoparticles. This composite features a unique interlayer-bonded graphite structure, achieved through the application of a modified spark plasma sintering method. Notably, this innovative structure not only facilitates efficient ion and electron transport but also provides exceptional mechanical strength (Vickers hardness: up to 658 MPa, Young’s modulus: 11.6 GPa). This strength effectively accommodates silicon expansion, resulting in an impressive areal capacity of 2.9 mA h cm−2 (736 mA h g−1) and a steady cycle life (93% after 100 cycles). Such outstanding performance is paired with features appropriate for large-scale industrial production of silicon batteries, such as active mass loading of at least 3.9 mg cm−2, a high-tap density electrode material of 1.68 g cm−3 (secondary clusters: 1.12 g cm−3), and a production yield of up to 1 kg per day.

Mechanisms responsible for enhancing low‐temperature oxidation resistance of nonstoichiometric (Zr,Ti)C

AbstractA series of (Zr,Ti)Cx (x = 0.7–1.0) samples were fabricated by a modified spark plasma sintering apparatus to investigate the effects of carbon concentration and Ti substitutions on the oxidation behavior. Crushed powders of (Zr,Ti)Cx were oxidized in lab air (N2–20‐vol.% O2) from room temperature to 900°C. The results indicated that Zr0.8Ti0.2C0.8, with a nominal carbon concentration x = 0.8, displayed good oxidation resistance, which was attributed to the formation of dense t‐(Zr,Ti)O2 oxide solid solution. During the oxidation of (Zr,Ti)Cx, Ti substitutions for Zr enhanced the outward diffusion of carbon, enabling a uniform carbon layer and a Zr–Ti–C–O layer on the surface of carbides. The formed carbon layer improved the oxidation resistance of (Zr,Ti)Cx below 550°C, where carbon is relatively oxidation resistant. Increasing the Ti concentration was found to enhance the oxidation resistance of (Zr,Ti)Cx with an increased oxidation onset temperature (672 ± 2°C for Zr0.8Ti0.2C0.8).

Synthesis of YB2C2 by high‐energy ball milling and reactive spark plasma sintering

AbstractX‐ray pure yttrium diborodicarbide (YB2C2) was synthesized for the first time via modified spark plasma sintering (SPS) using Y2O3, B4C, and carbon as starting materials. Homogeneous intermixing of the raw powder by high energy ball milling (HEBM) promoted the formation of YB2C2 powder with layered structure by boro/carbo‐thermal reduction. The average particle size of the synthesized powder at an optimum synthesis temperature (1625℃) was 568 nm. Small amount of YB2C was formed by consuming YB4 at and above 1625°C. The metal basis purity of the synthesized YB2C2 powder was 99.84%. X‐ray pure YB2C2 was obtained by densifying the synthesized YB2C2 powder at 1900°C for 60 minutes at a pressure of 80 MPa using SPS. This study reports a simple method for the fabrication of X‐ray pure rare earth metal diborodicarbides (ReB2C2).

Spark plasma texturing: A strategy to enhance the electro-mechanical properties of lead-free potassium sodium niobate ceramics

Controlling the sintering and microstructure of lead-free potassium sodium niobate ((K1-xNax)NbO3, KNN) ceramics is of primary importance to optimize its piezoelectric / ferroelectric properties. However, sintering dense and monophasic KNN remains a challenge. Here, we prepare KNN ceramics using spark plasma texturing (SPT), a modified spark plasma sintering (SPS) technique, in which uniaxial pressure is applied in an edge-free configuration, allowing ceramics to deform in the radial direction. Densification at low temperatures (1000 °C) and for short times (20 min) is achieved by SPT accompanied by constrained grain growth (average grain size = 1.4 μm), resulting in enhanced piezoelectric properties (d33 = 108 pC N−1 and g33 = 21.2 × 10−3 Vm N−1). In addition, and of relevance, SPT KNN ceramics reveal a more homogeneous electrical microstructure postulated to be related with a reduced diffusion and local segregation of defects, resulting in grain cores and shells with more similar capacitances and conductivities. Our work brings new practical understanding to sintering of KNN and demonstrates the potential of alternative densification strategies for improved lead-free dielectrics.

Dispersion behavior of HfC‐based nanopowders in ethanol

AbstractThe dispersion behavior of two different types of ultrafine HfC‐based powders in ethanol was investigated using polyethyleneimine (PEI) as a dispersant. The first type was synthesized by carbothermal reduction using a modified spark plasma sintering technique (d50: 125 nm). For the 10 wt% HfC suspensions, the highest zeta potential value (67.7 mV), the least sediment after sedimentation test for one day, and finest particle sizes were obtained when the concentration of PEI was 2.0 wt%. The concentrated HfC slurries with a solid loading of 40 vol% were achieved using 1.0 wt% PEI. The second type was mixing the HfC powder with HfSi2‐C additives using a high‐energy ball milling. The concentrated HfC slurry containing 20 wt% of HfSi2‐C sintering additives was prepared up to 50 vol% solid loading using 0.50 wt% PEI. This is the first report for producing highly concentrated HfC‐based nano slurries, which were highly suitable for the wet process of ultrahigh‐temperature ceramic matrix composites (UTHCMCs).

Swelling negation during sintering of sterling silver: An experimental and theoretical approach

One of the main challenges of the sintering of sterling silver is the phenomenon of swelling causing de-densification and a considerable reduction of the sintering kinetics. This swelling phenomenon opposes sintering and it needs to be addressed by a well-controlled processing atmosphere. In the present study, the pressure-less sintering behavior of sterling silver is investigated in air, argon, and vacuum. A specially modified spark plasma sintering mold is designed to study the pressure-less sintering of sterling silver in a high vacuum environment. The conducted analysis is extended to the new constitutive equations of sintering enabling the prediction of the swelling phenomena and the identification of the internal equivalent pressure that opposes the sintering.

Continuous functionally graded material to improve the thermoelectric properties of ZnO

Functionally graded material (FGM) in terms of grain size gradation is fabricated from ZnO with a combination of modified Spark Plasma Sintering (SPS) graphite tooling, water sintering enhancements through transient liquid phase surface transport, and strategic SPS mechanical loading. The grain size gradation of the ZnO FGM spans from 180nm grains to 1.2 micrometers in a fully dense material. This is the first semiconductor or ceramic to be graded microstructurally to this extent. Predictions of the microstructure with a Master Sintering Curve (MSC) approach were done with a series of isothermal experiments on two different FGM conditions revealing a slight offset due to a constrained mechanism. The mechanical properties were tested with Vickers micro hardness across the sample, showing a gradient in hardness from 2.6GPa to 4.2GPa. In addition, the thermoelectric properties of the FGM were measured and show a zT of 2×10−5 at 100°C compared to uniform small- and large-grained samples of 1×10−6. This is an order of magnitude difference making a new path for improvements of bulk thermoelectric material.

Nano-sized zirconium carbide powder: Synthesis and densification using a spark plasma sintering apparatus

Nano-sized zirconium carbide powder was synthesized at 1600°C by the carbothermal reduction of ZrO2 using a modified spark plasma sintering (SPS) apparatus. The synthesized ZrC powder had a fine particle size of approximately 189nm and a low oxygen content of 0.88wt%. The metal basis purity of the synthesized powder was 99.87%. The low synthesis temperature, fast heating/cooling rate and the effect of current during the modified SPS process effectively suppressed the particle growth. Using the synthesized powder, monolithic ZrC ceramics with high relative density (97.14%) were obtained after the densification at 2100°C for 30min at a pressure of 80MPa by SPS. The average grain size of the densified ZrC ceramics was approximately 9.12μm.

Innovative preparation route for uranium carbide using citric acid as a carbon source

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 nanocrystalline 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.

Production of Sm–Fe–N bulk magnets by the spark plasma sintering method with dynamic compression

We report a modified spark plasma sintering method, called spark plasma sintering with dynamic compression (SPS-DC). In the present study, TbCu7-type Sm–Fe–N powder was consolidated into bulk materials of up to 823 K using this method. The Sm–Fe–N powder was successfully consolidated into bulk materials at low temperatures of 573–623 K. It was found that the Sm–Fe–N magnets produced at these low temperatures retained the TbCu7 phase without any appreciable decomposition of that phase. The magnetic properties of the Sm–Fe–N magnet produced at these low temperatures were comparable to those of the Sm–Fe–N powder. The Sm–Fe–N bulk magnet produced at 623 K exhibited a high remanence of 103 A m2/kg with a high coercivity of 0.73 MA/m.

Synthesis of a Fine (Ta0.8,Hf0.2)C Powder from Carbide or Oxide Powder Mixtures

Tantalum hafnium carbide ((Ta0.8,Hf0.2)C) powders were successfully synthesized using a modified spark plasma sintering (SPS) apparatus with TaC/HfC or Ta2O5/HfO2/C starting materials. The (Ta0.8,Hf0.2)C obtained from the carbides had a finer particle size of 220 nm, whereas those obtained from the oxides had less contamination during the milling process (0.35 wt%) than the other case. Particle coarsening of the solid‐solution phase was effectively suppressed by using a modified SPS apparatus because of the fast heating/cooling rate. High‐energy ball milling promoted a solid‐solution reaction for the formation of (Ta0.8,Hf0.2)C by refining the size and inducing the homogeneous mixing of the starting materials. By the combination of the fast heating and high‐energy ball milling, fine tantalum hafnium carbide powders with low contamination were successfully synthesized.

Synthesis and densification of nano-crystalline hafnium carbide powder

Ultra-fine hafnium carbide powder was synthesized by the carbothermal reduction of HfO2 at 1600°C using the modified spark plasma sintering (SPS) apparatus. The synthesized powder had a fine particle size of about 125nm and a low oxygen content of below 0.5wt%. The purity of the powder was >99.9% excluding Zr, and 99.6% when including Zr impurity. The intermediate reactions and particle growth were minimized due to the low synthesis temperature, fast heating/cooling rate and the effect of current during the modified SPS process. A relatively large amount of HfC powder could be synthesized by the new synthesis method (>175g/day). HfC ceramics with high relative density (>96%) were obtained after sintering the synthesized powder without sintering additives at 2300°C for 30min under 100MPa pressure by SPS. The average grain size of the obtained specimen was about 6μm.

Nano-TaC powder synthesized using modified spark plasma sintering apparatus and its densification

Using a modified spark plasma sintering apparatus, nano-tantalum carbide powder was synthesized at 1450°C by the carbothermal reduction of Ta2O5. The synthesized TaC powder had ultra-fine particle size of 62nm and a low oxygen content of about 0.5wt%. The metal basis purity of the synthesized powder was 99.94%. The particle growth during synthesis was effectively suppressed by using the modified spark plasma sintering process due to the low synthesis temperature and fast heating/cooling rate. Dense monolithic TaC ceramics were obtained by sintering the synthesized powder without sintering additives at 2200°C for 30min under 80MPa pressure. The average grain size of the dense TaC ceramics was 4μm.

Optimization of intergrain connection in high-temperature superconductor Bi2Sr2CaCu2Ox**Project supported by the National Basic Research Program of China (Grant No. 2011CBA00104), the National Natural Science Foundation of China (Grant No. 51472206), the ITER Project of China (Grant No. 2013GB110001), and the Program for Innovative Research Team in Shaanxi Province, China (Grant No. 2013KCT-07).

A modified spark plasma sintering (SPS) technique was developed for the fabrication of Bi2Sr2CaCu2Ox (Bi-2212) superconducting bulks with better intergrain connections. The influences of the modified SPS process on the microstructures, intergrain connections, and related superconducting properties were systematically analyzed. The modified SPS process can not only increase the final density of the bulk samples but also enhance the texture structures. Clean grain boundaries were obtained instead of the intergrain amorphous layers. Therefore the intergranular properties were obviously improved. Due to the better intergrain connections and the stronger flux pinning properties, the critical current densities of the Bi-2212 bulks obtained via the modified SPS process were greatly increased. The obtained improvements imply the possibility for the modified SPS technique to be used for enhancing the superconducting properties of the Bi-2212 tapes.

Sintering by intense thermal radiation (SITR): A study of temperature distribution by simulation and experiments

The modeling and a confirming experimental study of a sintering process by using intense thermal radiation (SITR) is demonstrated. Experiments were conducted in a modified spark plasma sintering set-up for the purpose of rapid consolidation of porous SiC ceramics. A finite element method based software package, COMSOL Multiphysics, was employed to simulate the temperature distributions with two different die geometries. Experimental verifications were performed by sintering of SiC foams and measuring the temperature differences between two fixed points on the die and samples at 1000–1800°C. SiC foams sintered under two geometries with 10min’ dwell resulted in the formation of grain necks. The compressive strengths are 19.2±0.7MPa (65.0±0.1vol% porosity) and 15.3±1.9MPa (69.0±0.3vol% porosity), respectively. The simulation and experimental results showed that the temperature distributions are strongly related to these geometries.

Preparation of nasal cavity-like SiC–Si3N4 foams with a hierarchical pore architecture

Rigid SiC–Si3N4 foams with hierarchical porosity were prepared through protein-based gel-casting followed by radiant sintering in a modified spark plasma sintering (SPS) set-up.

ZrB2–SiC Nano‐Powder Mixture Prepared Using ZrSi2 and Modified Spark Plasma Sintering

ZrB2–SiC nano‐powder mixture was synthesized using ZrSi2 source material and a modified spark plasma sintering apparatus. The particle size of ZrB2 and SiC was about 80 and 20 nm, respectively. The molecular‐level homogeneity of Zr/Si source and fast heating/cooling rate by SPS caused the formation of homogeneously intermixed nano‐powders. A strong exothermal reaction occurred at around 860°C, which caused strong agglomeration and growth of the synthesized powder mixture. The rapid reaction could be controlled by adding 20 wt% of NaCl, which acted as an inert filler.

Synthesis of NiTi by Low Electrothermal Loss Spark Plasma Sintering

Due to their excellent shape memory and superelasticity properties, NiTi compounds are considered among the most important shape memory alloys. The present work presents a synthesis method for obtaining dense NiTi from elemental powders using a modified spark plasma sintering (SPS) setup characterized by considerably higher current density through the sample at the same total current, as well as drastically decreased heat losses. This provides the possibility to maximize the use of the strong effect of current density on enhanced interdiffusion in metal-metal systems reported in the literature. In addition, the heat loss reduction results in significantly lower total power consumption for synthesis compared to the commonly used SPS setups. The new setup is based on a metallic die/plunger set, combined with a graphite heat generating foil and a thermoelectrical insulation layer. The effect of the processing time on the homogenization of NiTi and the evolution of the intermetallic compounds with time is studied and compared to conventional SPS. Blending for 5 minutes and processing for 30 minutes (with neither preheating, nor postheating, nor quench) were enough to get almost pure dense cubic NiTi (ρ = 6.10 gcm−3, dominant NiTi phase ≈ 97 pct). A small fraction of both Ti and NiTi2, but neither Ni nor Ni3Ti, was detected after 20 minutes of processing. Ni4Ti3 was detected but not quantified.