Activation Energy For Grain Growth Research Articles

Grain growth kinetics, phase evolution during annealing and strain rate sensitivity of TRIP-assisted, dual-phase, non-equiatomic high entropy alloy

Transformation-induced plasticity-assisted dual-phase high-entropy alloys (TRIP-DP-HEAs) belong to a recently developed class of HEAs. TRIP-DP-HEAs have been reported to exhibit superior strength-ductility combination at room temperature as well as at high temperatures. Small amounts of carbon additions to TRIP-DP-HEAs are known to further improve the strength as a result of interstitial solid solution and nanoprecipitate strengthening effects. Current investigation compares the grain growth kinetics in a metastable, dual-phase (FCC and HCP), Fe49.5Mn30Co10Cr10C0.5, carbon-containing high entropy alloy (C-DP-HEA) with that in the corresponding carbon-free variant, Fe50Mn30Co10Cr10 HEA (DP-HEA). The alloys after being subjected to cold rolling, were annealed at 900 °C, 1000 °C and 1100 °C for 3, 30, 60 and 120 min, followed by water quenching. Slower grain growth kinetics was observed in C-DP-HEA due to Zener pinning by carbide particles. The activation energy for grain growth in C-DP-HEA was determined to be 339.71 kJ/mol, which is larger than that of DP-HEA (282.68 kJ/mol). The microstructures of the C-DP-HEA samples, after annealing at 900 °C for different durations of times and quenching, were investigated. The HCP phase fraction in the microstructure initially decreased with increase in annealing time/grain size and later increased with further increase in annealing time/grain size. This is explained based on the counteracting effect of grain boundary density and back stresses acting in samples having increased grain size. Strain rate sensitivity (SRS) parameter, ‘m’, was evaluated for both C-DP-HEA and DP-HEA. The strain rate jump tests were conducted at two quasi-static strain rates, 1 × 10−3 s−1 and 5 × 10−3 s−1, and at room temperature as well as at higher temperatures, 900 °C, 1000 °C and 1100 °C. The ‘m’ value was found to depend on the grain size, test temperature, and the presence of carbon in the alloy.

Fabrication, sintering behavior, and strength of tritium breeding ceramic pebbles with Pb addition

Lithium ceramics are regarded as potential candidates for tritium breeding in fusion reactors. Lead (Pb) was added to the ceramic to improve the strength and tritium release properties. The Li2TiO3-0.5Li4SiO4–Pb ceramic powders with the Pb contents of 5 wt% and 20 wt% were prepared by microwave-induced solution combustion synthesis. It was confirmed that the Pb was in the form of the Li2PbO3 phase in the powders. The Li2TiO3, Li4SiO4, and Li2PbO3 crystallites were composited at nanoscale level, which was confirmed by the transmission electron microscopy (TEM). During high-temperature sintering, the Li2PbO3 was transformed into PbO, which facilitated the densification process at low temperatures. The chemical state in the sintered body was characterized by X-ray photoelectron spectroscopy (XPS) with 3 keV Ar+ sputtering. The binding energies for Pb were 137.2 and 142.2 eV. There was almost no change in peak position during Ar+ sputtering, showing that Pb was firmly adherent with oxygen atoms. Besides, the as-synthesized powders were used for the pebble fabrication. The average crush load of Li2TiO3-0.5Li4SiO4–20%Pb pebbles sintered at 1223 K was 124 N, which was much higher than that without Pb addition. It was proved that the crush load increased almost linearly with the increase in the size of the pebbles. Thus, the bending strength of the sintered ceramic body was used to evaluate the mechanical properties. Further, the grain growth kinetics was studied. It was confirmed that the Pb addition reduced the activation energy of grain growth, which realized the densification of the ceramic pebbles at low temperatures.

Role of mixed entropy in sintering of manganite-based perovskite oxides: Faster densification with delayed grain growth

High entropy perovskite oxides (HEPOs) have garnered significant attention due to their superior performance compared to conventional perovskite oxides. However, there is a limited understanding of the sintering behavior of HEPOs. This study aims to obtain a mechanistic understanding of how the sintering of the manganite-based perovskite oxides varies with the mixed entropy. Based on systematic sintering experiments, detailed microstructural characterization, and molecular dynamics simulations, we found that the densification process of manganite-based perovskite oxides with increasing mixed entropies follows a similar pattern to conventional perovskite oxides. The mixed entropy significantly influences the final and middle stages of densification but has negligible effects in the initial stage. More importantly, the densification becomes easier and faster with the increased mixed entropy, while the grain growth is delayed to the higher relative density and the corresponding activation energy for grain growth is also significantly enhanced. Consequently, this creates a more ideal situation for achieving denser materials with smaller grain sizes during sintering, shedding light on rational optimization of the sintering and resulting properties of high entropy perovskite oxide.

How does cold sintering affect the microstructural evolution of high-entropy pyrochlore during pressureless sintering?

The challenge of producing dense fine-grained high-entropy ceramics is hereby addressed by a combined cold and conventional pressureless sintering method for the first time. It provides nearly fully dense high-entropy pyrochlores with better microstructural homogeneity, and only half the grain size of conventional pressurelessly sintered samples with identical density. Both the finer and more homogeneous microstructures benefit better mechanical properties in hardness and reduced Young’s modulus. The kinetics and activation energies of both densification and grain growth were quantified and discussed to understand how cold sintering affects the microstructural evolution during the following pressureless sintering. More importantly, cathodoluminescence analysis suggested that cold sintering could cause more defects on the surface of the original particles, which could promote the grain boundary diffusion and have drag effects on grain boundary motion. The lessons learned here offer scientific understanding and technological guidance towards pressureless sintering of dense bulk high-entropy ceramics.

The phase-stabilized behavior of Sc2O3–Y2O3 co-doped ZrO2 nanopowders by co-precipitation synthesis

Single stabilizer-doped ZrO2 has drawbacks such as inferior mechanical properties and thermal stability, making it difficult to obtain ZrO2 ceramics with good density and high crystallinity. Thus, it is expected that ZrO2 is doped and modified by two or more stabilizers. In this paper, the Sc2O3–Y2O3–ZrO2 nanocomposite ceramic powders were prepared using ZrOCl2⋅8H2O, ScCl3⋅6H2O, and YCl3⋅6H2O as raw materials and polyvinylpyrrolidone as dispersant. They calcined at diverse temperaturesby the pressureless sintering-assisted co-precipitation method after being pressed into disc shape. Sc2O3 and Y2O3 were added as ZrO2 stabilizers to ameliorate the lack of insufficient mechanical properties and high-temperature stability of ZrO2 ceramics caused by single doping. To study the stability of the phases at high temperatures, the specimens were characterized by TG-DTG, XRD, Raman, SEM, FT-IR, etc. The effects of different pH valuesand different calcination temperatures on the Sc2O3–Y2O3–ZrO2 nanocomposite ceramics were investigated. It was demonstrated that ternary doping has very excellent phase stability. After calcination no monoclinic phase appeared, and the precursor samples were transformed from amorphous to a large amount of tetragonal phases and a few cubic phases. The samples at pH = 10 and calcination temperature of 1000 °C showed the best stabilization, the most uniform grain development, the best crystallinity, the most regular morphology, the highest tetragonal phase content of 89.2 %, the stabilization rate of 94.58 %, and the highest relative density of 98.75 %. The results show that this nanocrystalline powder can be used to prepare high-density nanoceramics. Furthermore, the activation energy for grain growth of Sc2O3–Y2O3–ZrO2 nanocomposite ceramics was calculated as 13.87 kJ/mol. This paper provides a theoretical and experimental research basis for the controlled synthesis of Sc2O3–Y2O3–ZrO2 nanocomposite ceramics using a pressureless sintering-assisted co-precipitation method.

Thermal stability of electrodeposited nanostructured high-entropy alloys

Over the past decade, the study of nanostructured high-entropy alloys (HEAs) has attracted great attention due to their high strength (grain boundary hardening) and improved thermal stability over conventional nanostructured metals and alloys. However, a lingering issue yet to be resolved by these studies is that of commercial viability; the synthesis processes used to date are costly, not easily scalable, and energy intensive. This work examines the thermal stability and mechanical properties of electrodeposited nanostructured HEAs, towards establishing a cost-effective and versatile synthesis route to commercialize this underutilized material class. Nanocrystalline, amorphous, and nanoglass alloys of NiFeCoW, NiFeCoMo, and NiFeCoMoW were systematically characterized to assess the stability of their structure and mechanical properties, compared against conventional nanocrystalline materials and HEA counterparts. The structural stability of these electrodeposited HEAs was shown to improve with increasing number of elements, from a peak temperature for grain growth of ∼270 °C in pure nanocrystalline Ni, to ∼500 °C in electrodeposited HEAs, corresponding to a near doubling of the activation energy for grain growth from 1.3 eV (Ni) to a maximum of 3.1 eV (NiFeCoW). This improved stability was matched with a 60 % increase in hardness after annealing to the point of nanograin nucleation (∼500 °C), to a maximum hardness of ∼8.4 GPa. Such hardening trends followed an extrinsic inverse-to-regular Hall-Petch relationship, where the inverse regime was largely dominated by grain boundary relaxation and nanoglass structural breakdown. With these results, we have established electrodeposition as a promising candidate route for fabricating nanostructured HEAs for applications at elevated temperatures.

Synthesis of nano- and ultra-fine refractory carbide powders by low-temperature calcium-carbothermic process and their grain growth during sintering

Metal carbides are widely used in various applications as catalysts, components of composite materials, cutting tools, and abrasive materials where the particle/grain size has a great impact on target properties. Here, we report a facile way to synthesize nano- and ultra-fine carbides of IVb-Vb groups, including high-entropy carbides, which are of interest nowadays due to their superior properties, by the combination of calcium-hydride reduction of metal oxides and the direct reaction of the reduced metals with carbon in one production step. By tailoring the synthesis parameters, it is possible to obtain the carbide powders of a size in the range of 50–1500 nm with high crystallinity. The average particle size is found to be dependent on the homologous synthesis temperature and follows a power law. We have also shown the possibility of obtaining ultrafine-grained bulk carbides (d = 0.24–0.9 μm) via spark-plasma sintering at a temperature of 1500–1650 °C with a relative density >95 % without using additives. The HEC (Ti,Zr,Nb,Hf,Ta)C has a higher activation energy of grain growth (collective recrystallization) compared to binary TaCx (x = 0.8 and 1), which confirms the effect of lattice distortion on diffusion coefficients. The suggested fabrication route can be widely used for nano and ultrafine carbide powder preparation on the industrial scale.

Grain growth of ZnO nanocrystals synthesized in polyol medium with polyvinylpyrrolidone additives

In this work, we have investigated the influence of annealing temperature on the grain growth of zinc oxide nanocrystals synthesized in ethylene glycol and isothermally calcined at Ta: 300 °C, 500 °C, 700 °C for 60 min at the ambient atmosphere. The effect of polyvinylpyrrolidone (PVP) addition and annealing temperature on ZnO nanocrystal's phase composition, structural (scattering domain size of nanocrystallites L, unit lattice parameters a, c, and unit volume Vunit, theoretical density ρx) parameters, morphological (size, shape, specific surface area SSA) features, chemical composition, and grain growth kinetics were determined. It was found that Ta caused the increase in L of single-phase ZnO nanocrystals from 18 nm to 40 nm (with adding PVP) and to 45 nm (without adding PVP). Despite the surfactant composition, submicron grain sizes (D = 120–150 nm) were observed at Ta = 700 °C. The activation energy of grain growth was Q = 121.4 kJ/mol (without adding PVP) and Q = 126.4 kJ/mol (with adding PVP), while grain growth exponent was n = 1.9 (without adding PVP) and n = 2.0 (with adding PVP). ZnO nanocrystals annealed at 700 °C are forthcoming in the design of the active layers of solar cells, and at (300–500) °C are suitable for thermoelectric converters.

Thermal Stability of Electrodeposited Fe-55wt%Ni Alloy and Effect of Low-Temperature Heat Treatment on Magnetic Properties

Electrodeposited nanocrystalline soft magnetic materials have emerged as one of the hottest research topics in the field of magnetic materials due to they are easy to implement in miniaturization, lightweight, and energy-saving of electronic devices. The thermal stability and grain growth process of electrodeposited Fe-55wt%Ni alloy were investigated. Results indicated that the grain growth was rapid at a temperature of about 678 K, while the exothermic peak appeared in DSC with an exothermic heat of about 12 ± 1 J g−1. The activation energy for grain growth was obtained through the optimized Kissinger equation and isothermal kinetics calculations, and the growth mechanism was evaluated based on the calculation results. Below 678 K, the activation energy required for grain growth was low, which implied the growth mechanism was the rearrangement of atoms at the grain boundary; Above 678 K, the growth mechanism was grain boundary diffusion. After the low-temperature heat treatment, the coercivity (H c) decreased and the saturation magnetization (M s) increased slightly, which was attributed to the reduction of internal stress and the ultra-fine nanocrystalline structure. The optimal heat treatment process was 573 K heat treatment for 5 h, where M s and H c of 160 emu g−1 and below 1 Oe, respectively.

Sintering kinetics of cerium-stabilised tetragonal zirconia

Zirconia ceramics stabilised by CeO2 (Ce-ZrO2) exhibit low strength owing to their coarse grain sizes, which are typically larger than 1–2 μm. A promising approach to enhance the mechanical properties of Ce-ZrO2 is refining of the grain size. This study focuses on achieving fully dense Ce-ZrO2 ceramics with sub-micron grain sizes by identifying a sintering kinetic window that facilitates active sintering while suppressing grain growth. Both non-isothermal and isothermal sintering techniques were systematically employed to investigate the sintering and grain growth behaviour of the Ce-ZrO2 ceramics. The activation energies for sintering and grain growth were also determined. The existence of a sintering kinetic window for the Ce-ZrO2 ceramics was confirmed by a comparative analysis of the calculated activation energies. Accordingly, we successfully prepared fully dense sub-micron Ce-ZrO2 ceramics (with relative density >98%) by sintering within this kinetic window, leading to an average grain size of 0.55 μm. This is a significant step towards tailoring the microstructure of Ce-ZrO2 ceramics to improve their mechanical performance.

Grain growth mechanism and thermal stability of Li2TiO3 pebbles fabricated by wet method

Li2TiO3 ceramic pebbles with uniform structure, high density and good mechanical property were successfully fabricated by wet process. The Li2TiO3 pebbles sintered at 1000 °C for 4 h had satisfactory density (91% T.D.) and crush load (a.v. 27.1 N). The apparent activation energy of Li2TiO3 grain growth was 201 ± 7 kJ/mol. Accelerated densification was observed in a reducing atmosphere, this might be attributed to the oxygen vacancies generated by the reduction of Ti4+ to Ti3+ in 0.1%H2+He atmosphere, which facilitate the diffusion of ions through the lattice. The grain growth was controlled by lattice diffusion under reducing atmosphere, and the apparent activation energy was 168 ± 6 kJ/mol. The mechanical results show that the crush load of Li2TiO3 pebbles decreased with the increase of temperature, and the average crush load at 500 °C was 13.9 N. In addition, the thermal cycling and long-time annealing experiments suggested that Li2TiO3 pebbles have good structural and performance stability. The Li2TiO3 ceramics prepared by wet process were satisfactory tritium breeder materials with ideal application potential.

Efficient synthesis of forsterite via high-temperature thermochemical reactions from boron mud waste without briquetting

Forsterite is a highly demanded material in high-temperature industries due to its superior performance in elevated temperatures. This study proposes synthesizing high-quality forsterite via high-temperature thermochemical reactions using abundant boron mud waste. The new synthesis method capitalizes on the high reactivity of fine raw powder materials, enabling the reaction to reach completion at low temperatures rapidly. The chemical characteristics, microscopic morphology, and structure of synthesized samples are systematically studied using XRD and SEM. The results confirm that boron mud can be efficiently transformed into forsterite after 10 min of reactions at 1500 °C. The synthesized products attain over 98.7% densification and contain more than 85.0% forsterite with well-developed grains. The activation energy of forsterite grain growth in the 1100–1500 °C temperature range is 165.5 kJ/mol. This study provides a low-cost method for producing forsterite and an efficient and environmentally friendly solution for treating solid waste.

Thermal stability of microstructure and their influences on mechanical properties of precipitation-hardened medium-entropy alloy Ni43.4Co25.3Cr25.3Al3Ti3

The thermal stability of the coherent, nanoscale, L12 precipitates and fine grains in a strong, ductile medium-entropy alloy (MEA) Ni43.4Co25.3Cr25.3Al3Ti3 were analyzed and related to the room-temperature mechanical properties. Increasing the aging temperature and prolonging the soaking time produced more L12 precipitates and reduced the matrix lattice constant. The fine grains exhibit excellent thermal stability with a very low coarsening rate (1.73 × 10−24 m3/s at 700 °C, 4.78 × 10−23 m3/s at 800 °C, and 4.32 × 10−22 m3/s at 900 °C), which mainly results from the strong pinning from the high density of precipitates and high activation energy for grain growth (∼272 kJ/mol). The L12 precipitates remain spherical and have a coherent relationship with the matrix during ripening from tens to hundreds of nanometers. They exhibit better thermal stability (6.45 × 10−30 m3/s at 700 °C, 2.03 × 10−28 m3/s at 800 °C, and 7.16 × 10−27 m3/s at 900 °C) than the L12 precipitates in nickel-based superalloys by 1–2 orders of magnitude with an activation energy of 341 kJ/mol. The critical size for the transition from dislocation shear of the L12 precipitates to dislocation looping is 23.5–30.4 nm. No significant coarsening of either the grain size or L12 precipitates caused a small decrease in the YS and UTS of MEA aged at 700 °C. In comparison, both the YS and UTS of MEA aged at 800–900 °C dramatically decreased, which is caused by the coarsening of the L12 nanoparticles and the average grain size. This decrease in strength was accompanied by a slight increase in ductility at all three aging temperatures.

Investigating the Trade‐off between Optical and Mechanical Properties during Constrained Sintering of Mesomorphic Ceramic Films

AbstractRobust, transparent, and birefringent inorganic films are demanded for polarization control of high‐power lasers. While single crystals or films obtained via glancing angle deposition exhibit desirable optical properties and laser damage resistance, these methods are limited by cost and scalability. Mesomorphic ceramics as inorganic solids with liquid crystalline superstructure offer appealing transparency and birefringence but lack mechanical robustness due to their high porosity. Here, the effect of sintering on optical and mechanical properties of mesomorphic ceramics is evaluated. Films prepared by blade coating are sintered under varying conditions. Constrained sintering accomplished crystallite growth, densification, and morphological changes including necking as well as cracking while preserving the crystallographic orientation. The extent of sintering as a function of thermal treatment is quantified by morphology, surface area loss, and crystallite growth. Moreover, activation energies for surface diffusion and grain growth are estimated by surface area analysis and X‐ray diffraction peak narrowing, respectively. After sintering, birefringence decreases while Young's modulus and hardness improve as the film densifies. Upon partial sintering, mesomorphic ceramics retain transparency, high birefringence, and enhanced modulus. Laser‐induced damage threshold is measured as well. The reported results represent an important step toward the assembly and sintering of robust waveplates with high laser damage resistance.

Arrhenius constitutive model and dynamic recrystallization behavior of 18CrNiMo7-6 steel

In order to analyze the thermal deformation and dynamic recrystallization behavior of 18CrNiMo7-6 steel, the isothermal compression tests are carried out on Gleeble-3500 thermal simulator machine at deformation temperatures 973.15 K–1273.15 K with strain rates 0.001 s−1–1 s−1. Based on the true stress σ0.3, the Arrhenius constitutive models are constructed by using characterization parameters and physical base parameters, and the determination coefficients between the predicted values of the two models and the measured values are 0.98378 and 0.97613, respectively. The constitutive models of characterization parameters and physical base parameters considering strain effect are also constructed, and a comparative analysis is made. The results show that the predicted values of the characterization parameters constitutive model is closer to the measured values in the low strain region. After reaching the peak strain, the predicted values of the two constitutive models are close to the measured values, and both have high prediction accuracy. The dynamic recrystallization models are established to characterize the recrystallization behavior. The recrystallization activation energy of 18CrNiMo7-6 steel is 169.6793 kJ mol−1, the ratio of critical strain to peak strain is about 0.47074, and the ratio of critical stress to peak stress is about 0.94059. The activation energy of 50% dynamic recrystallization is 29.9844 kJ mol−1. The dynamic recrystallized grain size increases with increasing deformation temperature or decreasing strain rate, and the activation energy of grain growth is −49.9237 kJ mol−1.

Thermal stability behaviors of ultrafine-grained Cu-Cr-Zr alloy processed by friction stir processing and rolling methods

In order to investigate the effect of different microstructures on thermal stability, the thermal stability behaviors of ultrafine-grained Cu-Cr-Zr alloy processed by friction stir processing (FSP) and cold rolling (CR) methods was contrastively investigated by comparing with pure copper in this study. In addition to the precipitate strengthening, the FSP and CR samples were mainly strengthened by high angle grain boundaries (HAGBs) and dislocations, respectively. When there were no precipitates in Cu matrix, the onset temperature of softening of CR sample was 50 °C higher than that of the FSP sample at the same strength increment. But the precipitates in the FSP and CR Cu-Cr-Zr samples stabilized the deformed microstructures and precipitate coarsening occurred before the recrystallization of deformed microstructures. Therefore, the thermal stability differences disappeared in the FSP and CR Cu-Cr-Zr samples with the same onset temperature of softening (400 °C). In Cu-Cr-Zr alloys, although the stored energy of the FSP sample (39.1 J/mol) was 17.1% higher than that of the CR sample, the activation energy (∼175 kJ/mol) for grain growth of FSP sample was the same as that for recrystallization of CR sample. The different decrease degrees of thermal stability caused by different stored energy are completely masked by the kinetic constraint of precipitates.

Thermal stability, microstructure evolution and grain growth kinetics of ultrafine grained Al 7075 alloy processed by cryogenic temperature extrusion machining

In this study, an investigation was conducted over the effect of annealing on microstructure and precipitation evolution, microhardness, thermal stability, as well as grain growth kinetics of Al 7075 alloy processed by cryogenic temperature extrusion machining (CT-EM). A severely deformed microstructure with refined grains and high dislocation density was obtained in the initial CT-EM sample. There was an improvement in the hardness of the chip sample due to the precipitation strengthening in the case of sample annealed at 150 °C, when the dislocation density decreased. With the increase of the annealing temperature, more dislocation-free equiaxed grains were present in the microstructure. The recrystallization process started at 150 °C and got completed at 300 °C. The ultrafine grained (UFG) Al 7075 alloy fabricated by CT-EM was thermally stable up to 350 °C, which could be attributed to the pinning effect of precipitates. The differential scanning calorimeter (DSC) results indicated the efficiency of CT-EM treatment in facilitating precipitate kinetics. The activation energy of grain growth in the Al 7075 alloy subjected to CT-EM was higher than that of materials processed by other severe plastic deformation (SPD) processes.

Synthesis, sintering, and grain growth kinetics of Hf6Ta2O17

A systematic study of the solid-state synthesis, pressureless sintering, and grain growth kinetics of Hf6Ta2O17 is presented. The ideal conditions for solids-state synthesis of Hf6Ta2O17 powder with minimal particle necking was 1250 °C for 2 h in air. The resultant powder has an average particle size of 210 ± 70 nm. The combined synthesis and ball-milling procedure produces highly sinterable Hf6Ta2O17 powder, achieving > 97 % of theoretical density after pressureless sintering at 1600 °C for 2 h in air. The grain growth mechanism was sensitive to processing conditions, appearing to be primarily driven by surface diffusion below 1600 °C and grain boundary diffusion above 1650 °C. The respective activation energies for grain growth were found to be QS = 659 ± 79 kJ mol−1 and QGB = 478 ± 63 kJ mol−1.

Improving the Thermal Stability of the Fine-Grained Structure in the Cu-15Ni-8Sn Alloy during Solution Treatment by the Additions of Si and Ti.

Grain refinement has been found to be an effective method for simultaneously enhancing strength and toughness. To avoid the sharp coarsening of grains in Cu-Ni-Sn alloys during solution treatment and thereby overcoming the tradeoff between strength and ductility, this work attempted to modify the composition and improve the thermal stability of the fine-grained structure in Cu-Ni-Sn alloys. The grain growth behavior during a solution treatment of the Cu-15Ni-8Sn alloys with/without Si and Ti additions was systematically investigated. The result reveals that compared to the grain size of 146 μm in the based alloy (without trace additions) after solution processing at 1073 K for 2 h, the fine-grained structure with a size below 20 μm is maintained owing to the benefit from Si and Ti addition. It was observed that the addition of Si and Ti offer the inhibition effect on the dissolution of the γ phase and Ni16Si7Ti6 particles after solution treatment. The grain boundary movement is severely hindered by these two aspects: the pinning effect from these particles, and the drag effect induced by additional solute atoms. Based on the analysis of grain growth kinetics, the activation energy of grain growth is increased from 156 kJ/mol to 353 kJ/mol with the addition of Si and Ti.

Grain size stabilization in pure, Co, Mg and Al-doped nickel oxide

Grain size stabilization is essential for employing nickel oxide (NiO) for energy-related applications. Hence, it is investigated in NiO by adding Co, Mg and Al as solutes. Pure and doped NiO with 2–5 mol% Co, Mg, and Al are synthesized at 720, 920 °C. The composition distribution maps from energy dispersive spectroscopy reveal that Al-concentration is the highest at NiO grain boundaries. This is supported by its highest calculated segregation enthalpy (ΔHseg). This leads to ∼82% reduction in grain size in Al-doped NiO (∼24 nm, 5%Al, 720 °C) vis-à-vis ∼33% and ∼0.6% in Mg-doped and Co-doped NiO, respectively. Isothermal annealing of pure and 5% doped NiO at 620–920 °C for up to 300 min is conducted to study the grain size stability. Al-doped NiO yields ∼14 nm followed by Mg, Co-doped and pure NiO at ∼41, ∼53 and ∼55 nm, respectively at 620 °C. The estimated activation energy (Q) for grain growth is ∼504 kJ mol−1 in Al-doped NiO. Those in pure, CO, Mg-doped NiO are within ∼121–138 kJ mol−1. The highest Q in Al-doped NiO results in its lowest stabilized grain size during isothermal annealing. This work is useful in selecting a suitable solute and its concentration for grain size stabilization.