Sintering Activation Energy Research Articles

A Study of the Impact of Graphite on the Kinetics of SPS in Nano- and Submicron WC-10%Co Powder Compositions

This study investigates the impact of carbon on the kinetics of the spark plasma sintering (SPS) of nano- and submicron powders WC-10 wt.%Co. Carbon, in the form of graphite, was introduced into powders by mixing. The activation energy of solid-phase sintering was determined for the conditions of isothermal and continuous heating. It has been demonstrated that increasing the carbon content leads to a decrease in the fraction of η-phase particles and a shift of the shrinkage curve towards lower heating temperatures. It has been established that increasing the graphite content in nano- and submicron powders has no significant effect on the SPS activation energy for “mid-range” heating temperatures, QS(I). The value of QS(I) is close to the activation energy of grain-boundary diffusion in cobalt. It has been demonstrated that increasing the content of graphite leads to a significant decrease in the SPS activation energy, QS(II), for “higher-range” heating temperatures due to lower concentration of tungsten atoms in cobalt-based γ-phase. It has been established that the sintering kinetics of fine-grained WC-Co hard alloys is limited by the intensity of diffusion creep of cobalt (Coble creep).

Sintering mechanism and activation energy of MgAl2O4 synthesized by preceramic organomagnesiumoxanealumoxan

A detailed study of the new method’s ceramic spinel powder was carried out in the paper. A thorough analysis of the oligomer pyrolysis temperature effect on the kinetics and sintering mechanism was carried out in this work. The dependence of the change in the shrinkage and the shrinkage rate of spinel samples during non-isothermal sintering at three heating rates-5, 10, 20 °C/min were obtained. Analyzing the obtained dependencies can be concluded that an increase in the pyrolysis temperature leads to a shift in the spinel shrinkage curves towards higher values of the sintering start temperature. The data obtained were used to determine the sintering parameter of spinel powder, which corresponds to the change in the dominant sintering mechanism from viscous flow to bulk diffusion, increasing the pyrolysis temperature.

Sintering mechanisms of partially stabilized zirconia (TZ3Y) with ZnO and Co3O4 as sintering additives

In this work, the sintering mechanisms of 3 mol% yttria-stabilized zirconia (TZ3Y) and TZ3Y containing 0.3 wt% of ZnO or Co3O4 were analyzed by determining the apparent activation energy of sintering from dilatometry data and observing the evolution of microstructure. Dilatometry test was performed at a heating rate ranging from 5 to 20 °C/min from room temperature up to 1500 °C. The main mechanism of each sintering additive for the studied samples was revealed from the variation of the sintering activation energy as a function of the relative density and scanning electron microscopy images. The mechanism for the ZnO additive was the formation of a liquid phase at the beginning of the sintering process, while the Co3O4 additive acted via solid-state sintering by increasing cation diffusivity.

Sintering and grain growth behaviour of magnesium aluminate spinel: Effect of lithium hydroxide addition

Lithium hydroxide, LiOH, in the amounts ranging from 0.1 to 1.2 wt% has been used as a sintering aid to improve the densification of MgAl2O4. The addition of 0.3 wt% LiOH promotes densification and limits grain growth. The activation energy of sintering, calculated using master sintering curve approach, decreases from 790 ± 20 kJ.mol−1 to 510 ± 20 kJ.mol−1 with the addition of 0.3 wt% of LiOH. In addition, MgAl2O4 was also mixed with 10 wt% of LiOH to amplify the formation of reaction products. High-temperature XRD results showed that secondary phases (MgO and LiAlO2) are produced above 1040 °C. The secondary phases start to disappear at T > 1200 °C, and MgAl2O4 is produced. While adding small amounts of LiOH, up to ca. 0.3 wt%, is beneficial for densification and suppressing grain growth, there exists a critical concentration of Li+ that is accounted for by the preferential incorporation of lithium ions into MgAl2O4 crystal lattice.

Effect of TiN content on properties of NiFe2O4‐based ceramic inert anode for aluminum electrolysis

AbstractNiFe2O4‐based ceramic inert anodes for aluminum electrolysis doped with various TiN nanoparticles were prepared by a two‐step cold‐pressing sintering process to investigate how TiN affected the sintering behavior and properties of the composites. The differential scanning calorimetry‐thermogravimetry (DSC‐TG), X‐ray diffraction (XRD), and microstructure analysis results indicated that the Ti and N were evenly distributed in the NiFe2O4 matrix to form a solid solution. The maximum linear shrinkage and linear shrinkage rate were enhanced with the increase of TiN nanoparticles contents, and the sintering activation energy of initial stage was lowered from 382.63 to 279.58 kJ mol−1 with the TiN nanoparticles additive range from 0 to 9 wt%. When the content of TiN nanoparticles was 7 wt%, the relative density, bending strength, and elastic modulus reached their maximum values of 97.24%, 73.88 MPa, and 3.77 GPa, respectively, whereas the minimum static corrosion rate of NiFe2O4‐based ceramic of 0.00114 g cm−2 h−1 was obtained, mainly attributed to the relatively dense and stable microstructure. The electrical conductivity of NiFe2O4‐based ceramics presented a clear ascending trend with increasing TiN nanoparticles content and elevated temperature, attributed to the increased concentration and migration rate of carrier.

Revealing dehydrogenation effect and resultant densification mechanism during pressureless sintering of TiH2 powder

The use of TiH2 powder as a sintering precursor can produce nearly full-density titanium and titanium alloys with good mechanical properties. Unfortunately, there is a lack of research on the effect of lattice defects generated during the dehydrogenation of TiH2 powder, and the underlying sintering diffusion mechanism and activation energy have yet to be determined. In this work, we report a two-step sintering strategy to reveal the dehydrogenation effect and resultant densification mechanism during the pressureless sintering of a TiH2 powder precursor. The results show that, compared with hydrogenated-dehydrogenated (HDH) Ti powder, TiH2 powder, an intermediate of HDH-Ti powder, exhibited a higher instantaneous densification rate, greater onset relative density, rapid grain growth, and thus a smaller grain size. It also showed a grain boundary diffusion mechanism below 91% relative density and half the sintering activation energy in the intermediate sintering stage. Fundamentally, this was attributed to lattice defects generated during the dehydrogenation of TiH2 powder, which was confirmed by the greater relative density of a sintered TiH2 compact due to the two-step sintering strategy designed herein. Interestingly, the sintered sample obtained from the TiH2 powder precursor has a satisfying combination of strength and ductility that is far superior to other bulk Ti materials, especially sintered bulk Ti obtained from HDH-Ti powder. The results obtained in this paper provide theoretical guidance for using pressureless sintering to produce nearly full-density Ti and Ti alloys with good mechanical properties for structural applications.

Effect of SiO2/BaO ratio on sintering behavior, crystallization behavior, and properties of SrO–BaO–B2O3–SiO2 glass-ceramics

SrO–BaO–B2O3–SiO2 glass-ceramics with a high dielectric constant and low dielectric loss were prepared. The influence of the SiO2/BaO ratio on the sintering behavior, crystallization behavior, and properties were systematically investigated. The results showed that the increasing amount of liquid phase from increasing the SiO2/BaO ratio promoted sintering, resulting in an initial decrease in the sintering activation energy. However, the viscosity of the liquid phase continued to increase, leading to an increase in the sintering temperature and sintering activation energy when the SiO2/BaO ratio reached 0.90. As the SiO2/BaO ratio increased, the fraction of the primary phase BaSi2Al2O8 increased, and its grains grew; the minor phase SrTiO3 was suppressed, resulting in changes in the properties of the glass-ceramics. The sample with a SiO2/BaO ratio of 0.69, which was sintered at 850 °C for 10 min, showed excellent properties: a coefficient of thermal expansion (CTE) of 10.63 × 10−6 K−1 (25–500 °C), a dielectric constant (ϵr) of 16.24 (1 MHz), a dielectric loss (tanδ) of 3.39 × 10−3 (1 MHz), a flexural strength of 138.80 MPa, and a quality loss of acid corrosion of 1.06 × 10−4 g mm−2. The results indicated that the SrO–BaO–B2O3–SiO2 glass-ceramics used in this study have great potential for low-temperature co-fired ceramics applications.

Low-temperature sintering kinetics and dielectric properties of Ba5Nb4O15 with B2O3–SiO2 glass

For the first time, B2O3–SiO2 (BS) glass-doped Ba5Nb4O15 (BN) ceramics for low-temperature co-fired ceramic (LTCC) applications were facilely synthesized by the conventional solid-phase synthesis strategy. The influences of glass additive content on phase evolution, microstructure, and sintering activation energy were systematically deciphered associated with the microwave dielectric properties of the ceramics. While remaining original phase of Ba5Nb4O15, trace BS additive doping will effectively lessen the sintering temperature from 1350 to 925 °C excluding sacrificing microwave dielectric properties significantly. Optimized dielectric property is achieved via further time modulation: with 0.6 wt.% doping BS glass, Ba5Nb4O15 ceramics sintered at 925 °C for 4 h exhibits best characteristics of er = 41.2, Q × f = 22,238 GHz, τf = 53.1 ppm/°C. Therefore, Ba5Nb4O15 ceramics doped with BS glass are potential candidates for LTCC.

Microstructural evolution and magnetic properties of pressureless-sintered nanosized iron prepared by a facile combustion-based route

High-activity iron nanopowders were prepared through a facile combustion-based route, combining solution combustion synthesis and hydrogen reduction. The as-synthesized iron nanopowders were densified by pressureless sintering. The densification behavior, microstructure, mechanical properties and magnetic properties of the iron bulk sintered at different temperatures were studied in detail. At a low sintering temperature of 700 °C, the relative density of the sintered iron reaches 97.3%, because the iron nanopowders exhibit a low sintering activation energy of 123 KJ/mol. The iron bulk sintered at 700 °C exhibits relatively regular equiaxed grains with an average grain size of 0.45 μm, and has a maximum tensile strength of 510.1 MPa, a high microhardness of 201.9 HV, and a saturation induction of 1.75 T. As the sintering temperature increases, the grain size and magnetic properties of the sintered iron enhance, while the mechanical strength decreases. For the sintered iron prepared at 1300 °C with the average grain size ~ 80.9 μm, the saturation induction value reaches 1.78 T and the tensile strength is 334.9 MPa.

Exploring the potential of the mechanical/thermal properties and co-shielding ability of Bi2O3-doped aluminum borate ceramics against neutron/gamma radiation

The efficient optimisation of radiation shielding materials (RSMs), which protect people from potential radiant threats, is highly desirable; however, it remains challenging. This study addresses the low-cost fabrication of the ceramic-based RSMs, aluminium borate-based ceramics using Bi2O3 as a novel simultaneous shielding agent and sintering promoter. The phase compositions, microstructures, sintering kinetics, and performances of the as-prepared Bi2O3 doped aluminium borate ceramics (BDABCs) are systematically researched. Finally, co-shielding tests for neutron and gamma radiation are performed. The results demonstrate that Bi2O3 can positively influence the sintering densification process of BDABCs via the evident reduction in the sintering activation energy. The migration of the Bi2O3–B2O3 liquid phase affects the pore structure, crystal morphology, and thermal conductivity of the samples. The obtained BDABCs exhibited highly reliable mechanical properties with a maximum elastic modulus and modulus of rupture of 124.3 GPa and 54.9 MPa, respectively; controllable thermal conductivity from 1.32 to 6.16 W m−1 K−1; and 12 wt% Bi2O3-doped sample (1400 °C × 3 h, 1.5 cm) shows the best radiation shielding performance, including 58.6% neutron and 26.6% γ rays. The obtained results manifest the enormous potential of BDABCs as structural materials and functional RSMs.

Influence of Y2O3 on microstructure, crystallization, and properties of MgO-Al2O3-SiO2 glass-ceramics

The effect of Y2O3 on the microstructure, crystallization, and properties of MgO-Al2O3-SiO2 (MAS) glass-ceramics was investigated. Y2O3 addition increased the crystallinity and promote the formation of Indialite, and thereby adjusted the coefficient of thermal expansion. Sintering kinetics and crystallization kinetics indicated that doping Y2O3 reduced the sintering activation energy and the crystallization activation energy. A proper amount of Y2O3 improved the densification of sintering process and the precipitation of crystal phase, and thus significantly enhanced mechanical properties. MAS glass-ceramics with 1 wt% Y2O3 achieved the best performances: a high Young's modulus of 89 GPa, a high flexural strength of 211 MPa, an appropriate CTE of 4.68 × 10−6 /°C, a low dielectric loss of 1.99 × 10−3, and a low dielectric constant of 5.99.

Self-reinforced and high-thermal conductivity silicon nitride by tailoring α-β phase ratio with pressureless multi-step sintering

A self-reinforced Si3N4 ceramic with high thermal conductivity was prepared through multi-step pressureless sintering with Y2O3 and MgO additives. The steps consisted of phase control and densification stages to provide additional crystallization sites using a partial α-β phase transformation, which also established a bimodal microstructure. The optimal temperature for the intermediate stage that exhibited the maximum flexural strength was determined by comparing the phase content and linear shrinkage. Thus, 1400 °C, which resulted in 14.4 wt% β-Si3N4, was found to be optimal. After final sintering, a flexural strength of 932 MPa and a thermal conductivity of 74 W/mK were achieved, which are 11.5% and 8.0% higher, respectively, than those obtained using conventional pressureless sintering. The rate constant and activation energy of the pressureless sintering with the Y2O3–MgO additive system were also calculated to evaluate the potential of the α-β transformation to obtain the proper quantity of β-Si3N4 seeds.

Controlled current-rate AC flash sintering of uranium dioxide

Uranium dioxide (UO2) pellets with controlled microstructures were densified up to 93.4% of their theoretical density in less than 25 minutes at a furnace temperature of 873 K, utilizing controlled current-rate alternating current (AC) flash sintering (FS). Using this AC-FS method it was possible to control the sintering rate and thermal gradients, resulting in dense pellets with no appreciable hourglassing and good mechanical integrity. Moreover, the apparent sintering activation energy for FS and for conventionally sintered samples was estimated to be 108 kJ mol−1 and 380 kJ mol−1, respectively using the master sintering curve method. The apparent activation energy for FS was remarkably close to those reported in the literature for spark plasma sintering of UO2. Both these field assisted sintering methods utilize fast heating rates and electrical effects that are likely enhancing the grain boundary diffusion mechanism. Finally controlled current-rate AC-FS has been demonstrated as a technological advancement, capable of producing ceramic nuclear fuels in a fraction of the conventional processing time.

Comparison of Conventional and Flash Spark Plasma Sintering of Cu–Cr Pseudo-Alloys: Kinetics, Structure, Properties

Spark plasma sintering (SPS) is widely used for the consolidation of different materials. Copper-based pseudo alloys have found a variety of applications including as electrodes in vacuum interrupters of high-voltage electric circuits. How does the kinetics of SPS consolidation for such alloys depend on the heating rate? Do SPS kinetics depend on the microstructure of the media to be sintered? These questions were addressed by the investigation of SPS kinetics in the heating rate range of 0.1 to 50 K/s. The latter conditions were achieved through flash spark plasma sintering (FSPS). We also compared the sintering kinetics for the conventional copper–chromium mixture and for the mechanically induced copper/chromium nanostructured particles. It was shown that, under FSPS conditions, the observed maximum consolidation rates were 20–30 times higher than that for conventional SPS with a heating rate of 100 K/min. Under the investigated conditions, the sintering rate for mechanically induced composite Cu/Cr particles was 2–4 times higher compared to the conventional Cu + Cr mixtures. The apparent sintering activation energy for the Cu/Cr powder was twice less than that for Cu–Cr mixture. It was concluded that the FSPS of nanostructured powders is an efficient approach for the fabrication of pseudo-alloys.

Isothermal and non-isothermal sintering characteristics of mechanically alloyed nonequiatomic Fe2CoCrMnNi high-entropy alloy powder

ABSTRACT Mechanically alloyed high-entropy alloy (HEA) has taken considerable attention due to its ease of fabrication. Recent trend progresses towards the development of non-equiatomic HEAs to enhance the material properties compared to equiatomic alloys. Sintering behaviour of widely investigated equiatomic FeCoCrMnNi HEA has been reported. However, no report exists on the sintering kinetics of non-equiatomic HEA, when the concentration of a particular element is increased in the alloy powder. The present work attempts to study the isothermal and non-isothermal sintering behaviour of mechanically alloyed non-equiatomic Fe2CoCrMnNi HEA over a temperature range of 950–1100°C in an argon atmosphere. Activation energies for sintering and diffusivity parameters of Fe2CoCrMnNi HEA were calculated from dilatometric measurements through the sintering models. The result indicates the grain boundary diffusion as a dominating sintering mechanism for this alloy powder.

Experimental study of the influence of different carbon content on the shrinkage kinetics and structure evolution of ultralow-cobalt hard alloys during spark plasma sintering

The features of the compaction of plasma-chemical WC - (0.3; 0.6; 1) %Co nanopowders with the addition of 0.3 and 0.5% graphite during high-speed spark plasma sintering have been investigated. It was shown that an increased concentration of oxygen adsorbed on the surface of plasma-chemical nanoparticles during mixing with graphite, as well as the effect of graphite itself, which leads to a decrease in the activation energy of sintering due to a decrease in the intensity of the formation of particles of the η-phase in the “oxidized” WC-Co nanopowders, are the main structure features of ultralow-cobalt hard alloys. Samples of finegrained ultralow-cobalt hard alloys with increased hardness and fracture toughness were obtained (for a WC-0.6% Co-0.3% C hard alloy with an average grain size of ∼ 1-1.5 pm, the hardness is Hv = 20.2-20.5 GPa with a minimum crack resistance coefficient K ic = 9.2-10.4 MPa-m1/2).

Sintering Behavior and Fracture Morphology of NiFe2O4/Nano-TiN Ceramics Synthesized under Argon Atmosphere

NiFe2O4/nano-TiN ceramics were synthesized by a two-step cold-pressing sintering process. Sintering behavior, fracture morphology, mechanical performance and high-temperature conductivity were investigated. The results indicated that besides NiO and NiFe2O4, new Ni3TiO5 and metallic phases (iron and nickel) formed in the sintered samples. TiN accelerated the grain growth along with considerable quantity of micro-pores in the sintered samples. Compared to the un-doped samples, the temperature for the onset of sintering of the NiO-Fe2O3-2.0 wt.% TiN system decreased from 1093 to 985 °C. TiN addition changed the sintering mechanism of the NiO-Fe2O3 from grain boundary diffusion to volume diffusion. TiN additions reduce the apparent sintering activation energy and porosities and improve the density, bending strength, and pyroconductivity. Three-dimensional fracture morphology of the composite ceramics synthesized under argon atmosphere, exhibits self-similarity characteristics and the fractal dimension for NiFe2O4-1.0 wt.% TiN ceramics is 2.0813.

Pressureless Sintering Kinetics of NiFe2O4 Ceramic Fabricated by Slip Casting

In this work, a systematic research is carried out to investigate the sintering kinetics of NiFe2O4 ceramic obtained by slip casting and pressureless sintering. The sintering shrinkage behaviors showed the linear shrinkage and linear shrinkage rate of the green body in the axial and radial directions, both increased with increasing sintering temperature, though the maximum linear shrinkage rate in the radial direction was acquired at a lower temperature (1280.7°C) than that in the axial direction (1305.4°C) for a denser compact. The temperature related to the maximum densification rate was about 1316.5°C while the relative density was around 72%. The characteristic sintering kinetics window exhibited that the sintering process could be typically divided into three stages. The sintering activation energy of the initial stage was 268.34 kJ mol−1, and the initial stage of the sintering process was controlled by both grain boundary diffusion and volume diffusion mechanisms. The grain growth kinetic analysis illustrated the grain growth exponent (n) reduced from 2.959 to 2.169 when the sintering temperature increased from 1300 to 1375°C, while the activation energy for grain growth decreased with both the increasing of sintering temperature and the shortening of holding time. It implied that the atomic diffusion process controlled the grain growth. In addition, it was observed that increases in the bending strength and elastic modulus reached its maximum value of 70.36 ± 1.03 MPa and 3.44 ± 0.53 GPa, respectively, mainly associated with the relatively dense microstructure.

Outstanding sintering resistance in pyrochlore-type La2(Zr0.7Ce0.3)2O7 for thermal barrier coatings material

Sintering of advanced thermal barrier coatings (TBCs) at high temperatures is key challenge as it can adversely affect service performance and thermal fatigue resistance of TBCs. In this study, sintering behavior of pyrochlore-type La2(Zr0.7Ce0.3)2O7 (LZ7C3) was investigated using experiments and molecular dynamics. Meanwhile, the corresponding dynamic process and behind mechanism were uncovered. Results showed that novel LZ7C3 exhibited significantly higher sintering resistance than host La2Zr2O7 (LZ) and typical 8 wt% yttria-stabilized zirconia (8YSZ) at temperature up to 1773 K, which indicated that pyrochlore-type LZ7C3 is a promising TBC candidate to replace conventional 8YSZ at high temperatures. Further study also revealed that initial stage played crucial role in sintering process, and the sintering mainly occurred at grain boundary (GB) region. Intrinsic sintering activation energy of LZ7C3 GB (695.248 J mol−1) is larger than that of LZ GB (384.171 J mol−1) and 8YSZ GB (173.303 J mol−1), which resulted in outstanding sintering resistance for LZ7C3. Furthermore, no obvious enrichment of second phase was observed at the GB of LZ7C3. This study thus concluded that hindering the atomic diffusion of GB, as well as introducing foreign atom with larger mass and bond energy may act as effective strategy to enhance the sintering resistance of TBCs materials.

Master sintering curve and activation energy of sintering of ZrO2-doped Al2O3

Master sintering curve (MSC) was used to make an estimation of the apparent activation energies of undoped Al2O3 and ZrO2-doped Al2O3 samples in the intermediate density range (60–83%). ZrO2 at low concentrations of 2000 ppm and 5000 ppm were used for doping Al2O3. Dilatometric studies were performed using three different heating rates (5, 7.5, and 10 °C/min) on undoped and ZrO2-doped Al2O3 samples to construct the MSCs. The validity of the MSCs was verified by experimental runs on the undoped and doped Al2O3 compacts. The apparent activation energies of Al2O3 was found to be 539 kJ/mol, and the same for the ZrO2-doped Al2O3 samples were 670 kJ/mol (for 2000 ppm doping) and 790 kJ/mol (for 5000 ppm doping), respectively. The effects of the low concentrations of ZrO2 on the densification behaviour and activation energy of sintering of Al2O3 were underpinned.