High-pressure Spark Plasma Sintering Research Articles

Dense nanocrystalline UO2+x fuel pellets synthesized by high pressure spark plasma sintering

AbstractNanocrystalline UO2+x powders are prepared by high‐energy ball milling and subsequently consolidated into dense fuel pellets (>95% of theoretical density) under high pressure (750 MPa) by spark plasma sintering at low sintering temperatures (600°C‐700°C). The grain size achieved in the dense nano‐ceramic pellets varies within 60‐160 nm as controlled by sintering temperature and duration. The sintered fuel pellets are single phase UO2+x with hyper‐stoichiometric compositions as derived by X‐ray diffraction, and micro‐Raman measurements indicate that random oxygen interstitials and Willis clusters dominate the single phase nano‐sized oxide pellets of UO2.03 and UO2.11, respectively. The thermal conductivities of the densified nano‐sized oxide fuel pellets are measured by laser flash, and the fuel stoichiometry displays a dominant effect in controlling thermal transport properties. A reduction in thermal conductivity is also observed for the dense nano‐sized pellets as compared with micron‐sized counterparts reported in the literature. The correlation among the SPS sintering parameters—microstructure control—properties is established, and the nano‐sized UO2+x pellets with controlled microstructure can serve as the model systems for fundamental understandings of fuel behaviors and obtaining critical experimental data for multi‐physics MARMOT model validation.

High-pressure spark plasma sintering of silicon nitride with LiF additive

High-pressure spark plasma sintering of Si3N4 with Y2O3, Al2O3 and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% α-Si3N4 phase and 8% β-Si3N4 phase. The relatively high pressure applied (up to 650MPa) had a substantial effect on densification by enhancing particle rearrangement, making it possible to obtain dense Si3N4 at a significantly lower sintering temperature (1350°C). Consequently, virtually no α to β phase transformation transpired during the liquid phase sintering process. The LiF additive had an indispensable influence on the densification process by lowering the viscous glass formation temperature, which also contributed to enhanced particle rearrangement. The nearly fully dense samples (theoretical density ≥99%) obtained displayed a good combination of mechanical properties, namely elastic modulus (304–316GPa), hardness (1720–1780 HV2) and fracture toughness (6.0MPam1/2).

Influence of pulsed current during high pressure sintering on crystallite size and phase composition of diamond with Ti[sbnd]B bonding phase

Diamond production by sintering and synthesis is known from the mid-fifties of the twentieth century. The sintering technology of PCD materials has not significantly changed even to this day. Nonetheless, there are publications providing a lot of success in the field of synthesis of diamond composite materials sintering using SPS technology, yet these studies are limited to pressures mainly up to 140MPa. In this work HP SPS (High Pressure Spark Plasma Sintering) device, was used to sinter diamond composite with titanium‑boron bonding phase. Thus obtained material was then compared with HP-HT (High Pressure High Temperature) sintered sample, to investigate the influence of the heating by means of direct pulsed current on the crystallite size and phase composition of diamond with 10wt% of Ti+2B bonding phase. Both HP SPS and HP-HT processes were carried out at 8.0±0.2GPa and at the temperature of 1650±50°C during 20s of holding time. The XRD analysis carried out showed that the different type of current used for heating has significant influence on kinetics of the sintering, which is visible in differences in phase composition and level of lattice strain, when comparing standard HP-HT with HP SPS methods.

The Optimization of Sintering Conditions for the Preparation of Ti-Al-Si Alloys

This work deals with optimization of Spark Plasma Sintering conditions for the preparation of Ti-Al-Si alloys. Ti-Al-Si alloys are appropriate material for high-temperature applications, especially for aerospace and automotive industry. They are characterized by low density, good mechanical properties and resistance against oxidation. Ti-Al-Si alloys were prepared by powder metallurgy using reactive sintering, followed by milling and Spark Plasma Sintering. Preparation of intermetallic compounds is complicated due to high melting points of intermediate phases, the exothermic reaction during their formation and high reactivity of the melt with melting crucible. TiAl10Si20 alloy was prepared by Spark Plasma Sintering under a pressure of 48 MPa and by High Pressure Spark Plasma Sintering under pressure of 6 GPa. The temperature was chosen between 1100 °C and 1204 °C. The porosity of TiAl10Si20 alloy decreases with pressure and with the high pressure also increases the hardness by 200 HV 5. Abrasive wear resistance of TiAl10Si20 alloy is very good.

Sodium doped polycrystalline SnSe: High pressure synthesis and thermoelectric properties

p-type NaxSn1-xSe bulks were prepared by combining high pressure synthesis and spark plasma sintering. The final samples were characterized with XRD and SEM, and demonstrated strong structural anisotropy with the b-c planes in the grains preferentially orientated perpendicular to the SPS compressing direction. High pressure prompted Na doping process and increased the hole concentration up to 3.08 × 1019 cm−3 in Na0.02Sn0.98Se, leading to significant increased electrical conductivity and power factor. Meanwhile, the thermal conductivity was substantially suppressed after Na doping. The highest ZT of 0.87 was achieved in the optimal Na0.02Sn0.98Se at 798 K measured along the direction perpendicular to the SPS compressing.

An inverse Hall-Petch relation in nanocrystalline MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Inverse Hall-Petch behavior was observed in a transparent nanocrystalline magnesium aluminate spinel ceramic with grain size lower than 30nm fabricated by high pressure spark plasma sintering (HPSPS). The ceramic was consolidated from 12nm powder, synthesized by a solution combustion method and displayed an optical transmission of 80% at 550nm wavelength. The hardness values of the specimens followed the Hall-Petch relation down to a grain size of about 30nm, reaching a maximal hardness of 20GPa. The hardness value decreased from 20 to 17.6GPa when grain size decreased to 17nm.

High-Pressure Spark Plasma Sintering (HP SPS): A Promising and Reliable Method for Preparing Ti-Al-Si Alloys.

Ti–Al–Si alloys are prospective material for high-temperature applications. Due to low density, good mechanical properties, and oxidation resistance, these intermetallic alloys can be used in the aerospace and automobile industries. Ti–Al–Si alloys were prepared by powder metallurgy using reactive sintering, milling, and spark plasma sintering. One of the novel SPS techniques is high-pressure spark plasma sintering (HP SPS), which was tested in this work and applied to a Ti–10Al–20Si intermetallic alloy using a pressure of 6 GPa and temperatures ranging from 1318 K (1045 °C) to 1597 K (1324 °C). The low-porosity consolidated samples consist of Ti5Si3 silicides in an aluminide (TiAl) matrix. The hardness varied between 720 and 892 HV 5.

Ferromagnetic L10 Phase Formation in the Mn–Al–C Alloys Induced by High-Pressure Spark Plasma Sintering

Structural and magnetic characterization of Mn–Al–C permanent magnets obtained by spark plasma sintering (SPS) is reported. The transformation from the parent $\epsilon $ -phase to the ferromagnetic $\tau $ -phase occurs simultaneously with the process of sintering. The use of a tungsten carbide crucible enabled us to decrease the sintering temperature, which hampered the precipitation of secondary phases. As a result, sintered samples show higher coercivity as compared with annealed powder. However, the finely milled powder, ensuring better densification, turned out to be more prone to phase decomposition during sintering. The phase constitution of the samples was determined by X-ray diffraction, and magnetic hysteresis loops were recorded using a vibrating sample magnetometer. Scanning electron microscopy and electron backscatter diffraction were employed to study the microstructure and orientation distribution of grains after SPS. The dependence of coercivity and magnetization on the preparation conditions and sintering temperature is analyzed. To combine good magnetic properties with proper density, further optimization of the production parameters is necessary.

Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400MPa pressure at 1200°C was about 170nm, while for samples fabricated under 1000MPa at 1000°C the average grain size was remarkably smaller (about 50nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.

Improved transparency and hardness in α-alumina ceramics fabricated by high-pressure SPS of nanopowders

Nanocrystalline alumina powder with an average crystallite size of ≤50nm has been consolidated by spark plasma sintering (SPS) and hot pressing (HP) with a view to achieving dense, fine grained alumina bodies that display transparency. When as-synthesised powder was densified directly, excessive grain growth resulted from both the SPS and HP techniques and hence a large final grain size was observed. Attempts to improve the uniformity of the green microstructure prior to densification were unsuccessful when spray freeze dried granules were used, whether pre-pressed into a compact or not. The use of 53% dense slip cast green compacts, however, enabled final density of ~99.96% and a mean grain size of ~0.32µm to be achieved when SPS conditions of 1200°C and 500MPa were applied for 20min. These samples offered in-line transmittance values of up to ~80% and microhardness values of 22GPa.

All-Solid-State Lithium-Ion Battery Assembled By Low-Temperature / High-Pressure Spark Plasma Sintering

Oxides are generally non-flammable, durable and non-toxic materials; high safety and reliability in a battery system is assured by the use of oxide as an electrolyte instead of the highly-reactive non-aqueous liquid. To develop an oxide-based all-solid-state lithium-ion battery (ASS-LIB) for applications such as electric vehicles, the formation of strong interfacial contact between the electrolyte and the electrode powder is desirable, which can be achieved through powder technology. In oxide-based ASS-LIB, the good contact interfaces should be prepared by a simple powder sintering process, and also produces little ion-blocking impurities.[1] A Li2O-Al2O3-TiO2-P2O5 (LATP) solid electrolyte with NASICON-type structure possesses high bulk conductivity of over 10-4 S cm-1 at R.T. and high electrochemical stability (applicable potential range: 2.6−6 V vs Li/Li+)), which is a suitable candidate for assembling ASS-LIB with high safety and chemical stability. However, the poor interfacial contact between electrode and solid electrolyte is one of the general problems for showing the electrochemical activity of ASS-LIB. Spark plasma sintering (SPS) would be a useful tool for designing the ASS-LIB since dense ceramics can be sintered at shorter time with suppressing the formation of by-products at the interface. Actually, Aboulaich et al. have successfully measured the charge-discharge profiles of Li3V2(PO4)3 and LiFePO4 electrodes at ASS-LIB assembled with Ge-based Li1.5Al0.5Ge1.5(PO4)3 electrolyte by this SPS technique.[2] However, most electrode materials produce impurities by contact with the solid electrolytes containing Ti4+ ions as LATP after sintering at high temperature (900oC).[3] In this presentation, the ASS-LIBs using LATP electrolyte were assembled by SPS with tungsten carbide die, instead of conventional carbon die, which could be applied high pressure over 200 MPa and processed at low temperature below 300 °C, resulting in suppressing the formation of by-products at the interface. Samples for AC impedance measurement were prepared by SPS of Au/LATP powder (average particle size: 0.2 μm)/Au. The sintering temperature, applied pressure and time at SPS were 300 °C, 600 MPa and 1 min for the densification. During the SPS process, the shrink of the LATP pellet was observed at around 200 – 250 °C, which was obviously lower than the densification temperature of LATP at conventional sintering in furnace. The increase of neck region at the grain-boundary of LATP particles could be also observed by cross-sectional SEM image of LATP pellet. As a result, total Li-ion conductivities for LATP pellet was 2.2 x 10-5 S cm-1 at 30 °C. Composite electrode powder was prepared from a mixture of 50 wt% carbon-coated LiFePO4 and 50 wt% LATP electrolyte. Au/composite electrode powder/ LATP powder was assembled by SPS process at the same condition for the sample of AC impedance measurement. Lithium foil was used as a reference/counter electrode. A poly(ethylene oxide)-based polymer electrolyte film was inserted between the lithium foil and the LATP electrolyte separator to prevent the reduction of Ti4+ ion in LATP by contacting with the lithium metal. Electrochemical charge-discharge test was performed at a constant current of 10 μA cm-2. The ASS-LIB shows initial charge-discharge profile which was similar to the liquid electrolyte case, and the discharge capacity was 75 mAh g-1 at 28 °C and 131 mAh g-1 at 60°C, respectively. No impurity peak was observed in the powder XRD pattern of the LiFePO4-LATP composite electrode after SPS process, due to the low sintering temperature. Therefore, the low-temperature and high-pressure SPS process with tungsten carbide die would be one of the superior techniques for assembling ASS-LIBs with suppressing the formation of by-products and reducing grain-boundary resistance at electrode/electrolyte and electrolyte/electrolyte interfaces.

Al–Cr–Fe quasicrystals as novel reinforcements in Ti based composites consolidated using high pressure spark plasma sintering

Ti based metal matrix composites (MMCs) reinforced with Al–Cr–Fe quasicrystals have been prepared using high pressure spark plasma sintering (HP-SPS) with a high heating/cooling rate of 200°C/min between 550°C and 750°C and without any isothermal process. As the sintering pressure increases, the compositional variation of Al–Cr–Fe reinforcements diminishes. It is found that under a sintering pressure of 250MPa, the phases in Al–Cr–Fe powders, including two quasicrystals, decagonal Al–Cr–Fe and icosahedral Al–Cr–Fe, and two of their approximants, Al9(Cr,Fe)4 and Al8(Cr,Fe)5, can be preserved. An interfacial layer forms between Ti matrix and Al–Cr–Fe reinforcements in the sintering process. This interfacial layer consists of Al3Ti and AlTi. The incorporation of 20wt.% Al–Cr–Fe particles into Ti matrix was found to improve the microhardness to Hv 480 and reduce the wear rate by around 70%. This implies that Al–Cr–Fe is a promising reinforcement candidate for Ti.

Microstructure and mechanical behavior of NS/UFG aluminum prepared by cryomilling and spark plasma sintering

We report on a study of the microstructure and mechanical behavior of a bulk nanostructured (NS)/ultra-fine grained (UFG) aluminum fabricated by cryomilling and high-pressure spark plasma sintering (SPS). The compressive yield stress of the consolidated material is determined to be 380 MPa, which is significantly higher than that of commercial strain hardened aluminum (124 MPa). Microstructural studies reveal that the nanometric grains are embedded inside a matrix of ultra-fine grains in the bulk material. The corresponding average grain size is approximately 125 nm and with a distribution of grains that are smaller than 500 nm. Moreover, chemical analysis of the powder particles and the consolidated samples indicates that carbon and oxygen levels remain unchanged, and that there is a slight decrease in the nitrogen level and a significant reduction in the hydrogen level after spark plasma sintering. Semi-quantitative analysis suggests that the mechanisms that contribute to the strength of the consolidated material include: grain boundary strengthening, second phase strengthening and dislocation strengthening.

Spark plasma sintering of Ti1−xAlxN nano-powders synthesized by high-energy ball milling

The present study focused on the fabrication of bulk materials from Ti1−xAlxN nano-powders using a spark plasma sintering (SPS) apparatus. Super-saturated Ti1−xAlxN solid solutions containing differing fractions of AlN (10, 20, 30 and 50mol%) were synthesized by high-energy ball milling (HEBM) of pure nitrides. The complete dissolution of AlN in TiN was achieved after 100h of milling. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-filtered transmission electron microscopy spectra imaging and energy dispersive X-ray spectroscopy. The crystalline size of the mechanically alloyed powders after 100h of milling was about 12–14nm. Ti1−xAlxN powders of various compositions were sintered by SPS under pressure of 63MPa at 1673K. Maximal hardness and bending strength values (610MPa and 18.6GPa, respectively) were obtained for composites containing 20mol%AlN. Powder with 20% mole%AlN was consolidated under pressure of 500MPa in the 1273–1423K temperature range by high pressure SPS (HPSPS). A fully dense nano-structured specimen, processed at 1423K, displayed a Young modulus of 420GPa, hardness of 20.5GPa, bending strength of 670MPa and fracture toughness of 7.1MPam0.5.

Functional Properties of Nd:YAG Polycrystalline Ceramics Processed by High‐Pressure Spark Plasma Sintering (HPSPS)

High‐pressure spark plasma sintering (HPSPS) was employed to fabricate polycrystalline Nd:YAG specimens with desired functional properties. Specimens fabricated under a uniaxial pressure of 300 MPa at 1300°C at a heating rate of 50°C/min and holding time of 60 min displayed submicrometer microstructure and elevated mechanical properties, including resistance to thermal shock. Optical properties (i.e., spectral transmittance, fluorescence emission spectra and fluorescence lifetime) of the HPSPS‐processed specimens were close to those obtained with specimens fabricated by conventional sintering procedure. Specifically, remarkable differences in threshold power and laser slope efficiency were found and attributed to the variance in Nd concentration in the specimens tested. The results of this study indicate that the low cost and timesaving HPSPS process allows for the fabrication of polycrystalline Nd:YAG specimens with optical properties suitable for laser applications.

High-pressure spark plasma sintering (SPS) of transparent polycrystalline magnesium aluminate spinel (PMAS)

Abstract A high pressure SPS (spark plasma sintering) process was applied for consolidation of un-doped polycrystalline magnesium aluminate spinel. This approach allows fabricating a fully dense transparent ceramic with submicron grain size and high hardness values at a relatively low temperature (1200 °C). The light transmittance of the specimens increases with increasing applied pressure, while the hardness gradually decreases. The optimal combination of properties was achieved after sintering at 1200 °C at a heating rate of 5°/min, a holding time of 15 min and an applied pressure of 350–400 MPa. The specimens display the level of transmittance in the visible wavelengths and hardness values comparable with the best results reported in the literature for the two-stage fabrication process (pressureless sintering and hot isostatic pressing).

Tough and dense boron carbide obtained by high-pressure (300 MPa) and low-temperature (1600°C) spark plasma sintering

Dense boron carbide (above 95%) was achieved through high pressure (300 MPa) and low temperature (1600°C) Spark Plasma Sintering (SPS). This approach resulted in improvement of fracture toughness and of dynamic toughness when compared to corresponding toughness values of the sample sintered by conventional SPS (2100°C, 50 MPa). Dynamic toughness was extracted from Split Hopkinson Pressure Bar measurements. Results are understood based on microstructure and on very different behaviour of the samples in respect to residual B2O3 and carbon available in the raw B4C powder.

Low‐Temperature Spark Plasma Sintering of Pure Nano WC Powder

For the first time we have demonstrated the densification of high‐purity nanostructured (davg ≈ 60 nm) tungsten carbide by High Pressure Spark Plasma Sintering (HPSPS) in the unusually low temperature range of 1200°C–1400°C. The high‐pressure sintering (i.e., 300 MPa) produced dense material at a temperature as low as 1400°C. In comparison with more conventional sintering techniques, such as SPS (80 MPa) or hot isostatic pressing, HPSPS lowered the temperature required for full densification by 400°C–500°C. High Pressure Spark Plasma Sintering, even in absence of any sintering aid or grain growth inhibitor, retained a very fine microstructure resulting in a significant improvement in both hardness (2721 HV10) and fracture toughness (7.2 MPa m1/2).

Dynamic pore coalescence in nanoceramic consolidated by two-step sintering procedure

Abstract Two-step sintering (TSS) concept was adopted in the consolidation of 3 mol% yttria doped zirconia nanopowder. Partially densified bodies with 87% theoretical density (TD) were firstly prepared using high-pressure spark plasma sintering (SPS) technique and followed by second-step pressureless sintering. The samples achieved only 96% TD final density after 30 h soaking. It was found that the densification process was impeded by dynamic pore coalescence with a pore growth factor of 10. The phenomenon was explained by the coalescence of interconnected small pores generated by differential sintering of nanoceramic green compacts. Such pore coalescence was accompanied with particle movement, which resulted in sintering state deviating from the frozen state. Present results indicated that the active range for particle rearrangement was greatly extended during nanoceramic sintering and the efficiency of TSS approach was greatly dependent on the homogeneity of green bodies.

Transparent ceramics: Processing, materials and applications

Abstract Transparent polycrystalline ceramics have found various applications, such as laser hosts, infrared (IR) windows/domes, lamp envelopes and transparent armors, instead of their single crystal counterparts, due mainly to their processing flexibility in fabricating items with large sizes and complex shapes and more importantly cost-effectiveness. High optical transparent ceramics require high purity and high density. To achieve high purity final products, it is necessary to use high purity precursor powders. To get high density, various sintering technologies have been employed, such as high-pressure (HP) sintering, high isostatic pressure (HIP) sintering, vacuum sintering and spark plasma sintering (SPS). At the same time, various wet chemical synthesis routes have been used to produce precursor powders at submicron or even nanometer scales, with significantly improved sinterability. Transparent ceramics for armor and IR window/dome applications should have superior mechanical and thermal properties. Therefore, nanosized ceramics and nanocomposite ceramics have attracted much attention more recently. This review was aimed to summarize the latest progress in processing, materials and applications of transparent ceramics. It is arranged by starting with a brief introduction, followed by a detailed description on various sintering technologies used to develop transparent ceramics. After that, potential applications of transparent ceramics, together with their optical, mechanical and thermal properties, will be discussed. It will be concluded with discussions on future trend and perspectives, as well as some important issues, of transparent ceramic materials.