Effect of temperature and holding time on the densification of alumina obtained by two-step sintering

The two-step sintering technique is a process of controlling the sintering curve, which provides materials with higher density and smaller grain size when compared to conventional sintering. This technique was evaluated by optical dilatometry with three commercial alumina powders of different purity (92, 96 and 99 wt% of Al₂O₃) and particle size (between 0.73 and 2.16 µm). Different sintering conditions in the first (temperature, T1) and second (temperature, T2, and holding time, t2) steps were studied in order to evaluate the effect of these variables on densification and grain growth. Considering T1 as the temperature at which a relative density (D_rel) of 83% was achieved, and for the range of conditions tested, it was found that higher D_rel values and lower grain size of alumina were obtained with higher T2 and lower t2. Alumina with 99 wt% purity sintered at T1 of 1550 °C for 5 min and T2 of 1500 °C for 4 h showed the best relationship between higher densification (~96% relative density) and reduced grain size (0.94±0.15 µm). Thus, this work demonstrated that suppression of grain growth can also be obtained for commercial alumina.     

A preliminary study on the preparation of nanostructured Ti-doped Li4SiO4 pebbles by two-step sintering process

Li₄SiO₄ has been widely studied as attractive tritium breeding materials due to its innate merits. Considering the potential advantages of nanostructure in tritium breeding materials, a distinctive process was developed to obtain nanostructured Li₄SiO₄ pebbles. In brief, ultrafine precursor powders were synthesized by solvothermal method without using surfactants, and then indirect wet method was adopted to generate the green spheres with homogeneous microstructure. After that, the suitable sintering conditions were defined by studying the effects of sintering parameters on the grain size evolution, and nanostructured Ti-doped Li₄SiO₄ pebbles were first obtained by two-step sintering method. This study will be expected to provide references for fabricating other Li-based tritium breeding materials.     

Pure-phase BiFeO3 ceramics with enhanced electrical properties prepared by two-step sintering

A two-step sintering method was employed to fabricate BiFeO₃ ceramics and the influences of sintering conditions on the structural, dielectric, ferroelectric, and piezoelectric properties were investigated. It was found that high-density and pure-phase BiFeO₃ ceramics could be successfully achieved through the two-step sintering method by optimizing the sintering conditions. Furthermore, ferroelectricity and piezoelectricity were also improved by the two-step sintering method. The pure-phase BiFeO₃ ceramics obtained at T₁=850 °C and T₂=750 °C exhibited high resistivity (1.57×10⁶ Ω.m), large remanent polarization (P_r=0.80μC/cm²) and strong piezoelectric activity (d₃₃=42pC/N). All these results indicated that the two-step sintering method was an effective way to improve the properties of the BiFeO₃ ceramics.     

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

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.     

Densification and grain growth of nanocrystalline 3Y-TZP during two-step sintering

Two-step sintering (TSS) was applied on nanocrystalline yttria tetragonal stabilized zirconia (3Y-TZP) to control the grain growth during the final stage of sintering. The process involves firing at a high temperature (T1) followed by rapid cooling to a lower temperature (T2) and soaking for a prolonged time (t). It is shown that for nanocrystalline 3Y-TZP (27 nm) the optimum processing condition is T1 = 1300 °C, T2 = 1150 °C and t = 30 h. Firing at T1 for 1 min yields 0.83 fractional density and renders pores unstable, leading to further densification at the lower temperature (T2) without remarkable grain growth. Consequently, full density zirconia ceramic with an average grain size of 110 nm is obtained. XRD analysis indicated that the ceramic is fully stabilized. Single-step sintering of the ceramic compact yields grain size of 275 nm with approximately 3 wt.% monoclinic phase. This observation indicates that at a critical grain size lower than 275 nm, phase stabilization is induced by the ultrafine grain structure.     

Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si₃N₄ ceramics were prepared using novel two-step sintering method by mixing α-Si₃N₄ as raw material with nanoscale Y₂O₃–MgO via Y(NO₃)₃ and Mg(NO₃)₂ solutions. Si₃N₄ composite powders with in situ uniformly distributed Y₂O₃–MgO were obtained through solid–liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO₃ and densification at high temperature. Variations in O, Y, and Mg contents in Si₃N₄–Y₂O₃–MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO₂ on Si₃N₄ surface reacted with MgO to form low-melting-point MgSiO₃ compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y–Si–O–N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y₂Si₃O₃N₄ secondary phase occurred simultaneously. Two-step sintered Si₃N₄ ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si₃N₄ ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m·K), and fracture toughness (7.6 MPa?m^1/2) of Si₃N₄ ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si₃N₄ ceramics.     

Effects of two-step sintering on thermal and mechanical properties of aluminum nitride ceramics by impedance spectroscopy analysis

The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb₂O₃ and YbF₃ additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb₂O₃–YbF₃. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb₂O₃ and YbF₃ additives sintered using two-step regimes.     

Strategies and practices for suppressing abnormal grain growth during liquid phase sintering

Abnormal grain growth (AGG), where a small number of grains grow to sizes much larger than the neighboring matrix grains, is a frequent occurrence in liquid phase sintering of ceramics and cermets. As AGG can be detrimental to the material properties, a considerable amount of research on the nature, causes and suppression of AGG has been carried out. In this review, we outline the mixed control theory of grain growth and the principle of microstructural evolution that have been developed by Kang and coworkers over the last two decades. The theory and the principle, which are based on theories of crystal growth from a liquid, state that grain growth behavior is controlled by the nature of the solid-liquid interfaces, either atomically rough (macroscopically rounded) or smooth (macroscopically faceted). For grains with atomically rough solid-liquid interfaces, growth is controlled by diffusion of solute through the liquid phase and normal grain growth always occurs. For grains with faceted solid-liquid interfaces (or a mixture of rough and faceted interfaces), growth is interface reaction-controlled and diffusion-controlled below and above a critical driving force for growth, respectively. Depending on the relative values of the critical driving force for growth Δg_c and the maximum driving force for the largest grain in the system Δg_max, pseudo-normal, abnormal, and stagnant grain growth can take place. Based on this theory and principle, we present strategies for suppressing AGG by adjusting Δg_c and Δg_max to avoid AGG and examples of the successful use of these strategies.     

Enhanced resistivity and piezoelectric response in BiFeO3-BaTiO3 via two-step sintering

Due to the defects from Bi volatilization and Fe ion valence change during normal sintering (NS), the BiFeO₃-BaTiO₃ (BF-BT) ceramic is always accompanied by serious leakage current and deteriorated electrical performance, which impeded its further application. In this work, the two-step sintering (TSS) method was applied to the BF-BT ceramics preparation, and numerous microstructures of samples were observed and verified that it can decrease the sintering temperature from 1000 ℃ to 900 ℃ and enhance the relative density to above 96%. This reduced the defects and solved the issue of large leakage current, and the resistivity of TSS samples is an order of magnitude higher than NS samples and reaches the maximum among the already reported. Moreover, the experimental and theoretical results conclude that the TSS samples obtained a stronger intrinsic piezoelectric effect, which increased the strain from 0.13% to 0.19%. The TSS method displayed enormous potential for BF-BT preparation.     

Electrically conductive SiC ceramics processed by pressureless sintering

Polycrystalline SiC ceramics with 10 vol% Y₂O₃-AlN additives were sintered without any applied pressure at temperatures of 1900-2050°C in nitrogen. The electrical resistivity of the resulting SiC ceramics decreased from 6.5 × 10¹ to 1.9 × 10⁻² Ω·cm as the sintering temperature increased from 1900 to 2050°C. The average grain size increased from 0.68 to 2.34 μm with increase in sintering temperature. A decrease in the electrical resistivity with increasing sintering temperature was attributed to the grain-growth-induced N-doping in the SiC grains, which is supported by the enhanced carrier density. The electrical conductivity of the SiC ceramic sintered at 2050°C was ~53 Ω⁻¹·cm⁻¹ at room temperature. This ceramic achieved the highest electrical conductivity among pressureless liquid-phase sintered SiC ceramics.     

Microstructures,piezoelectric properties and energy harvesting performance of undoped (K0.5Na0.5)NbO3 lead-free ceramics fabricated via two-step sintering

Piezoelectric energy harvesters have become increasingly popular in the field of green energy because of the ability to convert low-frequency environmental vibrations into usable electricity. To fabricate high-performance energy harvesters, the key requirements are piezoelectric ceramics with a small grain size, of near-full density, the intended stoichiometric ratio and a high transduction coefficient. In this work, the effects of two-step sintering on the sinterability, microstructure, piezoelectric properties and energy harvesting performance of (K_0.5Na_0.5)NbO₃ were systematically investigated. Compared with conventional single-step sintering, two-step sintering samples were of higher density, increasing from 91 % to 95 % of theoretical, reduced mean grain size, down from 17 μm to 7.5 μm, and decreased evaporation of the alkali metals. This translated into an improved piezoelectric performance (d₃₃ ∼122 pC/N, k_p ∼36 % and Q_m ∼76), a higher transduction coefficient and energy conversion efficiency as well as a higher open-circuit voltage and power density. This demonstrates the potential of two-step sintering as a high through-put sintering technique for moderate-performance, pure KNN ceramics.     

Two-step sintering technique for enhancing mechanical and oxygen permeation properties of dual-phase oxygen transport membranes

The present work involved the application of a two-step sintering (TSS) technique for typical Ce_0.9Gd_0.1O_2?δ–La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ (GDC–LSCF) composites and its effect on the microstructure and resultantly mechanical and O₂-permeating properties. The samples obtained by conventional sintering (CS) performed at 1100, 1250, and 1400 °C for 3 h exhibited maximum flexural strength and hardness values of 142 MPa and 8.71 GPa, respectively. However, the application of a TSS procedure produced fine-grained microstructures with improved mechanical properties. In particular, with the use of a typical 1250/1200 TSS protocol, increases of approximately 31 % and 7% in the flexural strength and hardness values, respectively, were observed compared to those obtained using the CS method. Moreover, thick-film (～ 60 μm) GDC–LSCF membrane prepared by tape casting and the TSS technique showed remarkably 1.5–2 times higher oxygen permeation flux than membrane sintered by CS method. The results are discussed and explained in detail.     

Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 °C at a heating rate of 10 °C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 °C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 µm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 °C, attributed mainly to the enhance densification of the sintered bodies.     

Full densification of zirconium carbide ceramics sintered under high pressure at low temperature

Zirconium carbide (ZrC) ceramic is one of the most important and promising materials with a high melting point. However, its low diffusion coefficient affects its densification behavior, which dramatically limits its engineering application. For polycrystal ceramic materials, sintering by thermal diffusion is the most widely used method for consolidation. But in view of the difficult densification behavior of ZrC, it is necessary to develop new sintering methods together with another driving force. In the study, we reported an ultra-high-pressure sintering strategy to fully densify ZrC ceramic under 1.7 GPa at 1600℃. The sintered sample exhibited high density, fine grain size, excellent mechanical properties, and a large number of crystal defects, including dislocation networks and walls, which were similar to those in deformed metals. Its hardness increased to 2057.44 HV0.1 because of its unique microstructure.     

Fabrication and mechanical properties of homogeneous and gradient nanocomposites by two-step sintering

Homogeneous and functional gradient nanocomposites were prepared using a two-step sintering process at a pressure of 30 MPa. The effects of macro –micro fracture morphologies and element distributions on the mechanical properties of the composites were studied. The results showed that the fracture toughness of the inner layers of the gradient material was almost the same as that of the corresponding homogeneous material. The composition content difference between the gradient layers had a significant influence on the bending strength and hardness. There was little Ni and Mo diffusion in the gradient nanocomposites because of the relatively low contents and suitable processing parameters. A gradient distribution of the metals was ensured. The surface layer thickness had little influence on the flexural strength of the gradient nanocomposite, but it had an obvious influence on the hardness and fracture toughness of the surface. The five-layered gradient composite with a surface layer thickness of 85 μm exhibited the best mechanical properties, including a flexural strength of 1021 ± 35 MPa, fracture toughness of 7.60 ± 0.21 MPa m^1/2 and surface hardness of 19.26 ± 0.62 GPa.     

Pressure-enhanced densification of TaC ceramics during flash spark plasma sintering

Flash spark plasma sintering (FSPS) offers extremely high heating rates to consolidate ceramics at a short time. However, significant grain growth sometimes occurs accompanied by rapid densification. In this work, a FSPS apparatus available for applying pressure was used to sinter TaC ceramics from powder compacts without preheating. It is found that the use of a higher pressure can efficiently promote densification and retard significant grain growth. Dense bulk TaC ceramics (95.18%) with average grain size of 4.09 μm were obtained in 90 seconds under 80 MPa. Such a process should facilitate the fast preparation of refractory ceramics with fine-grained microstructure.     

High-pressure sintering of ultrafine-grained high-entropy diboride ceramics

Herein the ultrafine-grained (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)B₂ high-entropy diboride ceramics were successfully fabricated by high-pressure sintering technology for the first time. The results showed that the grain size, relative density, and Vickers hardness of the as-fabricated samples all increased gradually with increasing sintering temperatures from 1373 K to 1973 K. The relative density and mean grain size of the as-sintered samples at 1973 K were 97.2% and 684 nm, respectively, and simultaneously they exhibited excellent comprehensive mechanical properties, combining a Vickers hardness of 26.2 GPa and a fracture toughness of 5.3 MPa·m^1/2, which were primary attributed to the fine grain strengthening mechanism and microcrack deflection toughening mechanism.     

Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics

With the cold sintering process (CSP), it was found that adding acetic acid to an aqueous solution dramatically changed both the densities and the grain microstructures of the ZnO ceramics. Bulk densities >90% theoretical were realized below 100°C, and the average conductivity of CSP samples at around 300°C was similar to samples conventionally sintered at 1400°C. Frequently, ZnO is also used as a model ceramic system for fundamental studies for sintering. By the same procedure as the grain growth of the conventional sintering, the kinetic grain growth exponent of the CSP samples was determined as N=3, and the calculated activated energy of grain growth was 43 kJ/mol, which is much lower than that reported using conventional sintering. The evidence for grain growth under the CSP is important as it indicates that there is a genuine sintering process being activated at these low temperatures and it is beyond a pressurized densification process.     

Slip casting of highly concentrated ZnO suspensions: Rheological studies,two-step sintering and resistivity measurements

The paper presents research on elaboration of well dispersed and stable aqueous suspensions of ZnO fine powder. Within the work the influence of the type and concentration (0.2 wt% - 1.2 wt%) of selected dispersing agents (i.a. poly(acrylic acid)-based polyelectrolyte and tetramethylammonium hydroxide), solid loading (30 - 50 vol%) and milling time (1–3 h) on the rheological properties of the slurries was investigated. Two-step sintering (970/920 °C, 2 h) was applied to sinter the green bodies obtained by slip casting.The lowest viscosity of ZnO suspensions was obtained for the addition of 0.4 wt% of poly(acrylic acid)-based polyelectrolyte (PAA) and TMAH. ZnO suspension containing PAA had negative zeta potential in the whole pH range. The highest solid loading obtained in the study was 50 vol%. The applied two-step sintering allowed to obtain samples of high density (above 96% of TD) and homogeneous microstructure of average grain size of 640 nm. ZnO sintered bodies were characterized by different electric properties at the core part and outer part of the sample which was caused by the differences in concentration of oxygen vacancies.