Influence of processing parameters on thermal conductivity of uranium dioxide pellets prepared by spark plasma sintering

High density uranium dioxide (UO₂) pellets with grain sizes between 0.9 μm and 9 μm were produced by spark plasma sintering (SPS). A systematic study was performed by varying the sintering temperature between 750 °C and 1450 °C and hold time between 0.5 min and 20 min to obtain UO₂ pellets with a range of theoretical densities (TD) and grain sizes. The microstructure development in terms of grain size, density and porosity distribution was investigated. The oxygen/uranium (O/U) ratio of the resulting pellets was found to decrease after SPS. The thermal conductivity of UO₂ pellets increased with the theoretical density but the grain size in the investigated range had no significant influence. The measured thermal conductivity values up to 900 °C were consistent with the reported literature for conventionally sintered UO₂ pellets. The benefits of using SPS over the conventional sintering of UO₂ are summarized.     

Densification,microstructure and properties of ultra-high temperature HfB2 ceramics by the spark plasma sintering without any additives

Ultra-high temperature HfB₂ ceramic with nearly full densification is achieved by using gradient sintering process of SPS without any additives. The effect of the sintering temperature on the densification behavior, relative density, microstructure, mechanical and thermionic properties is systematically investigated. The results show that the fast densification of HfB₂ ceramic occurs at the heating stage, and the highest relative density of 96.75% is obtained at T =1950 °C, P = 60 MPa and t =10min. As the temperature is increased from 1800 to 1950 °C, the grain size of HfB₂ increases from 6.12 ±1.33 to 10.99 ± 2.25 μm, and refined microstructure gives the excellently mechanical properties. The highest hardness of 26.34 ±2.1GPa, fracture toughness of 7.12 ± 1.33 MPa m^1/2 and bending strength of 501 ±10MPa belong to the HfB₂ ceramic obtained at T =1950°C. Moreover, both the Vickers hardness and fracture toughness obey the normal indentation size effect. HfB₂ ceramic also exhibits the thermionic emission characterization with the highest current density of 6.12 A/cm² and the lowest work function of 2.92 eV.     

High field ZnO varistors prepared by spark plasma sintering

Abstract

ZnO varistors with submicrometre and nanoscaled microstructures and enhanced electrical properties were prepared by spark plasma sintering (SPS). The densification, grain size and switch field of the varistors were compared with those of hot pressed material. The switching field increased with decreasing grain size, and very rapidly below 500 nm. Switching fields up to 180 kV cm^?1 were obtained for ceramics with submicrometre grain sizes (380 nm). This is nearly two orders of magnitude higher than those currently reported for commercial ZnO varistors. A nano powder, prepared by high energy milling, was sintered to a high density at much lower temperatures compared with the submicron powders and had a nanoscale grain size (45 nm). The nanoceramic broke down dielectrically under very high fields (>260 kV cm^?1) before a varistive response was apparent.     

Bulk boron carbide nanostructured ceramics by reactive spark plasma sintering

Bulk boron carbide (B₄C) ceramics was fabricated from a boron and carbon mixture by use of one-step reactive spark plasma sintering (RSPS). It was also demonstrated that preliminary high-energy ball milling (HEBM) of the B+C powder mixture leads to the formation of B/C composite particles with enhanced reactivity. Using these reactive composites in RSPS permits tuning of synthesized B₄C ceramic microstructure. Optimization of HEBM + RSPS conditions allows rapid (less than 30 min of SPS) fabrication of B₄C ceramics with porosity less than 2%, hardness of ~35 GPa and fracture toughness of ~ 4.5 MPa m ^1/2     

Effect of LaB6 additions on densification,microstructure, and creep with oxide scale formation in ZrB2-SiC composites sintered by spark plasma sintering

A comparative study has been carried out on densification, microstructure, and creep with oxide-scale formation in ZrB₂-20 vol.% SiC-(7, 10 or 14 vol.%) LaB₆ composite containing B₄C and C as additives, and prepared by spark plasma sintering at 1800 °C under 70 MPa ram pressure. Addition of LaB₆ has promoted densification of composites by scavenging oxygen impurity, thereby increasing their hardness. Constant load compressive creep tests at 1300 °C under 47 and 78 MPa stresses have shown the lowest creep rate in the 10 vol.% LaB₆ composite. The stress exponents obtained for composites having 10 vol.% LaB₆ (～1.3 ± 0.1) and 14 vol.% LaB₆ (～2.6 ± 0.2) suggest respectively, grain boundary diffusion with intergranular glassy phase formation and dislocation glide as operating mechanisms. Intergranular cracking caused by grain boundary sliding appears as the damage mechanism. Oxide scales formed during creep exhibit greater thickness and defect concentration than those by isothermal exposure at 1300 °C within similar duration.     

Improvement in microstructure and mechanical properties of Ti(C,N) cermet prepared by two-step spark plasma sintering

A fine grained Ti(C, N) cermet tool material was prepared by two-step spark plasma sintering. Microstructure evolution and densification mechanisms of Ti(C, N) during spark plasma sintering were studied. Effect of two-step sintering process and Ni content on microstructure and mechanical properties were also investigated. The critical activated densification temperature of Ti(C, N) is about 1300?℃, and the rapidest densification rate takes place at 1300?℃~1400?℃. Grains are in the size of 1?µm when the Ti(C, N) cermet was prepared by two-step spark plasma sintering. The optimal flexural strength, fracture toughness and Vickers hardness are 1094?±?42?MPa, 7.2?±?0.5?MPa?m^1/2 and 18.3?±?0.4?GPa, respectively. The Ti(C, N) cermets containing more content of Ni have higher toughness, which is due to the remarkable toughening effect of crack bridging by large grains.     

Low-temperature reactive spark plasma sintering of dense SiC-Ti3SiC2 ceramics

SiC-based ceramics are of great interest for various advanced applications. However, its fabrication requires high-temperature treatment at ～2000 – 2100 °С. In this study, we developed an approach based on low-temperature reactive spark plasma sintering to produce dense SiC-based ceramics with superior mechanical properties. It was found that an SPS temperature of 1600 °C and introduction of 10 – 15 wt% of mechanically activated non-oxide Ti–Si–C additive is required to manufacture ceramics with a theoretical density of higher than 90%. Nonetheless, employing 5 – 15 wt% of the additive mixture and an SPS temperature of 1700 °C, the maximum density of ～ 98% was achieved. The controlled formation and decomposition of the in-situ Ti₃SiC₂ MAX phase enables the fabrication of the engineering ceramics with enhanced compressive strength (550 MPa), elastic modulus (485 GPa), and microhardness (32 GPa), which are comparable to the best-reported SiC ceramics. The study has a significant potential for practical application in the production of advanced SiC-based ceramics for various purposes and could be used for further understanding and development of the high-temperature sintering methods.     

Micromechanical properties and microstructure evolution of magnesia partially stabilized zirconia prepared by spark plasma sintering

Magnesia partially stabilized zirconia (Mg-PSZ) is a widely used engineering ceramic owing to its high hardness and exceptional toughness. It is usually processed by conventional firing followed by subeutectoid aging. In this work, Mg-PSZ was prepared by spark plasma sintering (SPS) followed by sequential subeutectoid aging to fine-tune its mechanical properties. Mg-PSZ prepared by SPS with the rapid heating capability presents much smaller grains than conventionally prepared counterparts. After aging, a significant fraction of the matrix cubic phase transforms into tetragonal, orthorhombic, and monoclinic zirconia. Microindentation and in-situ microcompression tests reveal that aging Mg-PSZ for 4 h leads to maximum fracture toughness and fracture strain due to the tetragonal-to-monoclinic transformation toughening. Post compression TEM analyses show dominant monoclinic ZrO₂ decorated by a high density of twin boundaries and stacking faults formed to accommodate the shear deformation. Preparation of Mg-PSZ by SPS offers rapid and effective approaches in finetuning the phases and mechanical properties.     

放电等离子烧结合成单相MgAlON材料

以氮化铝、富铝镁铝尖晶石和氧化铝为原料 ,用放电等离子烧结 (SPS)技术合成了单相MgAlON,研究了其显微结构 ,并与用传统的无压烧结 (PLS)技术制备的单相MgAlON材料在显微结构和断裂行为上做了比较。结果表明 :用SPS法在 170 0℃保温 1min的条件下合成出的单相MgAlON材料 ,显微结构比用PLS法合成的更加均匀致密 ,且晶粒细小 ;前者的断裂模式主要是穿晶断裂 ,后者的断裂模式则主要是沿晶断裂。     

Microstructure and electrochemistry performance of the composite electrode prepared by spark plasma sintering

The 30%LiFePO₄-55%Li_1.5Al_0.5Ge_1.5(PO₄)₃-15%C composite electrodes were successfully prepared by spark plasma sintering, and the microstructure and electrochemistry performance were investigated. As sintered at 550 °C, the phase structure of the composite electrode remained unchanged. With the sintering temperature increasing, the Li_1.5Al_0.5Ge_1.5(PO₄)₃ were decomposed and transformed into nanocrystals and impurity phases of AlPO₄ and GeO₂. The 3D-microstructure showed that abundant pores existed in the composite electrodes. The volume fraction and connectivity of these pores decreased with the sintering temperature. The initial discharge capacity of the composite electrode was 138 mAh·g^?1 as sintered at 550 ℃, while it decreased monotonously with the sintering temperature. The decrease was related with the formation of impurities and closed pores. Moreover, the composited electrode sintered at 550 ℃ had severely capacity fading during electrochemistry cycles. By the EIS and postmortem analyses, it was determined that the capacity fading was due to the ion transport failure caused by the cracking during cycles.     

Synthesis,microstructure and property characterization of Mo4Y2Al3B6 ceramic fabricated by spark plasma sintering

Polycrystalline Mo₄Y₂Al₃B₆ ceramic (92.84 wt% Mo₄Y₂Al₃B₆ and 7.16 wt% MoB) was prepared by spark plasma sintering at 1250 ℃ under 30 MPa using Mo, Y, Al, and B as starting materials. The dense sample obtained has a high relative density of 96.6 %. The average thermal expansion coefficient is 8.38 × 10^?6 K^?1 in the range of 25–1000 ℃. The thermal diffusivity decreases from 6.50 mm²/s at 25 °C to 4.33 mm²/s at 800 °C. The heat capacity, thermal conductivity, and electrical conductivity are 0.30 J·g^?1·K^?1, 11.73 W·m^?1·K^?1, and 0.66 × 10⁶ Ω^?1·m^?1 at 25 °C, respectively. Vickers hardness with increasing load in the range of 10–300 N at room temperature decreases from 10.82 to 9.49 GPa, and the fracture toughness, compressive strength, and flexural strength are 5.14 MPa·m^1/2, 1255.14 MPa, and 384.82 MPa, respectively, showing the promising applications as structural-functional ceramics.     

Characterisation of transparent hydroxyapatite nanoceramics prepared by spark plasma sintering

Abstract

Hydroxyapatites (HA) have good biocompatibility and are used as bioceramics for artificial bones. The application areas can be extended further if transparent and dense HA ceramics can be prepared. The preparation of dense and transparent HA ceramics were attempted using a spark plasma sintering technique at relatively low temperatures (900–1000°C) under a pressure of 80 MPa for a short time of 10 min. The sintered body was almost fully dense (>99%) and transparent with a transmittance >70%. The microstructure was examined by SEM, TEM, STEM and EDX. The HA ceramics exhibited a microstructure with grains, approximately 100 nm size. A number of intragranular voids, 5–10 nm in size, with flat boundaries were also observed. The voids were believed to have been generated by evaporation during spark plasma sintering and were stabilised during cooling. The grain boundaries were clean without a glassy phase.     

Effects of 211 and 413 ordering on the corrosion behavior of V-Al-C MAX phases prepared by spark plasma sintering

In the present study, two V-Al-C based MAX phases, i.e., V₂AlC and V₄AlC₃ having two types of ordering were successfully manufactured by spark plasma sintering and the corrosion behavior of sintered samples was evaluated. Al, V and C metal powders were mixed with the desired molar ratios by a mixer mill, and sintered at 1300 °C. The relative density calculation revealed almost full densification for both prepared MAX phases. The measurements of mechanical properties showed a low increase in bending strength and Vickers hardness of V₄AlC₃ compared to V₂AlC MAX phase. Evaluation of corrosion behavior of developed MAX phases was carried out in 6.5 M HCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. Corrosion current density and corrosion potential of V₂AlC (5.3 ± 0.21 μA/cm² and -0.451 ± 0.01 V, respectively), and V₄AlC₃ (1.07 ± 0.22 μA/cm² and -0.091 ± 0.02 V, respectively) were measured and no passivation behavior was observed in their potentiodynamic polarization curves. However, EIS tests at open circuit potential confirmed more corrosion resistance of V₄AlC₃compared to V₂AlC. These tests also revealed the active dissolution of MAX phases in 6.5 M HCl solution at anodic potential of +0.1 V, while the impedance values of V₄AlC₃ were larger than those of V₂AlC. Microstructural investigation revealed the preferential dissolution of V₂AlC phase in grain boundaries after corrosion test. Moreover, the layered structure of V₂C MXenes was observed in some regions. After corrosion test, V₄C₃ MXene layers had larger thickness compared to V₂AlC. It was found that V₄AlC₃ with higher amount of Al₂O₃ and thicker layers has more corrosion resistance than V₂AlC MAX phase.     

Effect of solid solution formation on densification of spark plasma sintered ZrC ceramics with TiC as sintering aid

The densification of ZrC ceramics doped with different contents of TiC prepared by spark plasma sintering at the temperatures between 1750 and 1850°C has been investigated. The microstructure and mechanical properties of the ceramics have been characterised. It was shown that TiC additions effectively promoted the densification process by forming (Zr,Ti)C solid solution. The relative densities and mechanical properties of ZrC samples increased with the increasing of TiC content or the sintering temperature. Ceramic with the content of TiC up to 10 vol.-% sintering at 1850°C showed an excellent combination of properties including a relative density of 98.7%, hardness of 20.8?GPa and flexural strength of 605?MPa.     

Influence of TiN dopant on microstructure of TiB2 ceramic sintered by spark plasma

In this study, the impact of TiN as a sintering aid on the relative density and microstructure of TiB₂ ceramic was investigated. Monolithic TiB₂ and TiB₂ doped with 5?wt% TiN were sintered at 1900?°C for 7?min dwell time under the pressure of 40?MPa by spark plasma. The addition of TiN affected the microstructure of TiB₂-based sample considerably depicting the finer grains in the as-sintered ceramic. X-ray diffraction evaluation indicated that no interaction occurred between the initial materials. However, detail investigation by the map analysis and energy dispersive spectroscopy results revealed the formation of in-situ nano-sized hBN secondary phase in the TiN-doped TiB₂. In addition, TiN played a remarkable role on increasing the relative density of TiN-doped TiB₂ ceramic producing a nearly fully dense ceramic with relative density of 99.9% in comparison with the monolithic ceramic having 96.7% relative density.     

Microstructure and mechanical properties of B4C based ceramics with Fe3Al as sintering aid by spark plasma sintering

B₄C based ceramics were fabricated with different Fe₃Al contents as sintering aids by spark plasma sintering at relatively low temperature (1700 °C) in vacuum by applying 50 MPa pressure and held at 1700 °C for 5 min. The effect of Fe₃Al additions (from 0 to 9 wt%) on the microstructure and mechanical properties of B₄C has been studied. The composition and microstructure of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalyzer (EPMA) equipped with WDS (wavelength dispersive spectrometry) and EDS. The mixtures of B₄C and Fe₃Al underwent a major reaction in which the metal borides and B₄C were encountered as major crystallographic phases. The sample with 7 wt% of Fe₃Al as a sintering aid was found to have 32.46 GPa Vickers hardness, 483.40 MPa flexural strength, and 4.1 MPa m^1/2 fracture toughness which is higher than that of pure B₄C.     

Electrical properties of reduced 3YTZP ceramics consolidated by spark plasma sintering

3 mol% Yttria doped zirconia ceramics were consolidated by spark plasma sintering (SPS) at two sintering temperatures with the aim of achieving two different reduction levels. Microstructural characterization of the ceramics was performed by scanning electron microscopy (SEM). Electrical properties were investigated by means of impedance spectroscopy from room temperature up to 500 °C. The two ceramics presented a remarkably different electrical behavior. The effect of the extra electrons introduced by reduction during SPS on both the bulk and the grain boundary conductivity was analyzed and discussed.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

Silicon carbide ceramics: Mechanical activation,combustion and spark plasma sintering

Single-stage fabrication of SiC ceramics by a combination of self-propagating high temperature synthesis (SHS) and spark plasma sintering (SPS) is reported. SHS+SPS is demonstrated to be an efficient method for production of SiC ceramics with density 3.1 g/cm³, hardness of 24 GPa and toughness of 5 MPa m^1/2. The starting material for the process is fine (50–300 nm in size) highly reactive powder, which involves composite particles of elemental carbon and silicon. This powder was prepared using a high-energy ball milling (HEBM). To optimize precursor preparation conditions, the structure transformation in nano-composite Si/C particles at different HEBM stages is also investigated.     

Spark plasma sintering of TiB2-based ceramics with Ti3AlC2

A TiB₂–Ti₃AlC₂ ceramic was manufactured by spark plasma sintering at 1900 °C temperature for 7 min soaking time under 30 MPa biaxial pressure. The role of Ti₃AlC₂ additive on the microstructure development, densification behavior, phase evolution, and hardness of the ceramic composite were studied. The phase characterization and microstructural investigations unveiled that the Ti₃AlC₂ MAX phase decomposes at the initial stages of the sintering. The in-situ formed phases, induced by the decomposition of Ti₃AlC₂ additive, were identified and scrutinized by XRD and FESEM/EDS techniques as well as thermodynamics principles. The sintered TiB₂–Ti₃AlC₂ ceramic approached a near full density of ~99% and a hardness of ~28 GPa. The densification mechanism and sintering phenomena were discussed and graphically illustrated.