Role of Particle Substructure in the Sintering of Monosized Titania

Monosized titania particles (∼0.35-μ diameter) prepared by controlled hydrolysis of titanium tetraethoxide in ethanol were found to be porous agglomerates of ∼6-nm primary particles. The sintering behavior of compacts constituted of monodispersed agglomerates was evaluated, and changes in macroscopic dimensions were correlated with changes in particle microstructure and chemistry. The total volume shrinkage during sintering was ≥87%. Five contributions to the total shrinkage and the temperature ranges for the associated processes were identified: removal of chemisorbed water (from ambient to 250°C), crystallization to anatase (between 250° to 425°C), intra-agglomerate densification (425° to 800°C), conversion of anatase to rutile (600° to 800°C), and inter-agglomerate densification (>800°C). Approximately one-half the compact shrinkage was the result of agglomerate substructure changes. Studies of the agglomerate structural evolution indicated the intra-agglomerate densification and crystallite growth rates are the secondary factors, after compact packing, that influenced microstructure development.     

Fabrication of transparent Y2O3 ceramics by two-step spark plasma sintering

Transparent Y₂O₃ ceramics were successfully fabricated by spark plasma sintering applying a two-step pressure and heating profile. Through the shrinkage curve of the single-step SPS profile, it was confirmed that shrinkage occurred at 800°C–1250°C, and it was selected as the two-step pressure profile. After the first-step SPS stage at 1250°C, the second-step SPS stage, which had the highest real in-line transmittance, was completed at 1500°C. The two-step SPS profile improved the shrinkage behavior and was able to achieve sufficient densification without excessive coarsening. As a result, the normalized real in-line transmittance to 1 mm was 80.6% at 1100 nm, which is close to the theoretical transmittance of 81.6%. The two-step pressure and heating profile in the SPS process was a significant advantage in manufacturing ceramics that were transparent and had sufficient densification.     

Pressure Effect on the Homogeneity of Spark Plasma-Sintered Tungsten Carbide Powder

A combined experimental/numerical methodology was developed to aid full densification of pure ultrafine tungsten carbide powder by means of Spark Plasma Sintering (SPS) operating in Current Control mode. Applied pressure ranged from 5 to 80 MPa while the current intensity was set and held constant at 1400 A. The developed SPS model used a moving-mesh technique to account for the electrothermal contact resistance change during both shrinkage and punch sliding follow-up. The pressure dependence on the electrothermal contact resistance was also taken into account by the model. The experimental and numerical results showed the effects of pressure on grain growth, residual porosity, and hardness observed along the sample radius. Upon increasing sintering pressure, complete densification was obtained by reducing the peak temperature measured at the die surface. By combining experimental and modeling results, a direct correlation between compact microstructure homogeneity and sintering parameters (i.e., temperature and applied pressure) was established.     

Investigation of the sintering mechanisms of GDC pellets obtained by the compaction of nanostructured oxide microspheres

The sintering behavior of green pellets obtained from nanostructured Ce_0.8Gd_0.2O_1.9 submillimetric microspheres is studied in the present paper. Corresponding shrinkage rate curve shows a two‐step densification in dynamic conditions, with the presence of two successive extrema, at 1200 K and 1500 K. To fully understand this non‐common densification behavior, an iterative study was performed. Multiple characterizations point out multiscale organization of the matter with temperature giving rise to differential sintering stages of two different particle size classes. Concerning 1200 K‐first shrinkage rate maximum, it corresponds to the densification of nanometric aggregates of crystallites into submicrometric pre‐sintered aggregates, resulting in a specific porous microstructure with residual open porosity. As‐generated porosity combined with submicron size of pre‐sintered aggregates thus prevent from a homogeneous sintering illustrated by a single maximum shrinkage rate. Finally, the second maximum shrinkage rate at 1500 K can then be associated to optimal temperature for submicrometric particles sintering.     

Effect of sintering temperature on functional properties of alumina membranes

Supported membranes were prepared from different submicron alumina powders. The evolution of pore size, hardness and permeability were monitored after sintering the films at temperatures ranging from 1000 to 1400 °C. These functional properties and the microstructure of the films were compared with the free-standing membranes. Sintering at temperature range from 1000 to 1200 °C maintained the narrow, monomodal pore size distribution of the supported membranes. The effect of sintering temperature on the hardness of the membranes was weak. The permeability was also independent on the sintering temperature. When sintering temperature was raised up to 1300 and 1400 °C, the pore size increased significantly and distribution was changed to bimodal containing fraction of large pores. The hardness of the membranes increased while significant densification was not observed. Permeability increased due to the large pore size and the high porosity. In sintering of the free-standing membranes pore size remained almost unchanged, density increased when sintering temperature was raised, hardness was dependent on the density and permeability decreased continuously. The substrate did not have effect on the grain growth, which was dependent on the sintering temperature. Evolution of the properties of the free-standing membranes suggests local densification. The rigid substrate restricts the sintering shrinkage leading to densification of small areas. This local densification opens large flow channels between agglomerates. This increases the pore size, broadens the pore size distribution and increases the permeability. The macroscopic densification of the film is small.     

Densification and Shrinkage During Liquid-Phase Sintering

The process of densification and shrinkage during the final stage of liquid-phase sintering is described. The densification occurs by the liquid filling of pores during grain growth. The pore filling results in an instantaneous drop of liquid pressure in the compact and causes gradual accommodation of grain shape. The grain shape accommodation by the growth causes the specimen shrinkage. At the same time, the grains tend to restore their spherical shape, resulting in microstructure homogenization around filled pores. The process of densification and shrinkage appears to be determined by the growth of grains during sintering.     

Fabrication of transparent Ce3+-doped (Gd,Lu)3Al5O12 ceramics by two-step spark plasma sintering

Transparent Ce³⁺:(Gd,Lu)₃Al₅O₁₂ with microstructure control was fabricated by two-step spark plasma sintering. In the two-step profile, the heating rate was changed from 50 to 5°C/min at the first step temperatures. During the initial stage of shrinkage, the holding time of the first step sintering could induce densification by suppressing the microstructure coarsening. As compared to the single-step profile, the two-step profile showed a smaller grain size, which decreased with a decrease in the first step temperature. The porosity of the two-step profile was lower than that of the single-step profile, and the lowest porosity was obtained at the first step temperature of 1000°C, which was the starting point of shrinkage. The TS-1000 specimen showed the highest transmittance among all specimens because of the microstructure control offered by the two-step profile. Thus, by employing the two-step profile, the transmittance could be increased from 50.1% (SS-1250) to 56.5% (TS-1000).     

Addition of h-BN for enhanced machinability and high mechanical strength of AlN/Mo composites

Ceramic/metal composites, as high-temperature structural materials, have limited applications because of their poor machinability. AlN/Mo/Mo₂B machinable composites were fabricated by adding h-BN to AlN/Mo composites by plasma-activated sintering at various temperatures and at an axial pressure of 30 MPa under argon atmosphere. The sample sintered at 1500 °C exhibited excellent machinability for cemented carbide tools, with a bending strength of approximately 500 MPa and a Vickers hardness value of 9.34 GPa. The effects of sintering temperature on the microstructure, mechanical properties, and machinability were investigated, and the reactions introduced by adding h-BN were studied. These reactions reinforced the composite and newly introduced a Mo₂B phase, which showed better ability to deflect cracks and prevent them from moving deep inside the composite, thus improving its machinability. This research provides a possible solution for improving the machinability of ceramic/metal composites without sacrificing the strength.     

Preceramic Polymer‐Derived SiAlON as Sintering Aid for Silicon Nitride

Two kinds of sintering additives based on the polysiloxanes or polysilazanes filled with nano‐sized powders as SiAlON precursors were tested for the densification of Si₃N₄‐based ceramics. The results showed that both systems can be successfully used as additives for the preparation of Si₃N₄ ceramics with favorable mechanical characteristics. The ceramics were sintered with 18 wt% of preceramic polymer‐based mixture, and good fracture resistance and high hardness values were obtained after sintering in optimized conditions (temperature, dwell time, nitrogen pressure). Higher densification temperatures and longer holding times were required for sintering of samples with polysilazane‐based precursors. The best toughness values were approximately 5 MPa·m^0.5, while the highest hardness was about 19 GPa. The differences in mechanical properties of the prepared composites can be related to the phase composition, microstructure and different chemical bonds present in the ceramic residue generated upon pyrolysis and final densification.     

Evolution of microstructure,mechanical, and optical properties of Y2O3-MgO nanocomposites fabricated by high pressure spark plasma sintering

A high-pressure spark plasma sintering (SPS) process was applied for consolidating Y₂O₃–MgO nanocomposites. This approach enabled to fabricate a fully dense infrared (IR) transparent nanocomposites, which possess an average grain size of ∼70 nm and high hardness, at a relatively low sintering temperature of 1130 °C under a high pressure of 300 MPa. The light transmittance was improved with increasing pressure and reached to the maximum transmittance of 64.5% at a wavelength of 0.2–1.6 μm owing to the fine-grained microstructure. The Vickers hardness exhibited 16.6 ± 0.7 GPa for the grain size of 74 nm, which is significantly higher than that of the sub-micro grains obtained at a conventional sintering pressure of 70 MPa (11.9 ± 0.8 GPa). The hardness rigorously followed the Hall–Petch relationship, that is, it is enhanced with a reduction of the grain size. Successful fabrication of the high-performance Y₂O₃–MgO nanocomposites indicates that the nanopowder processing followed by the high-pressure sintering process can be applied for fabricating fully dense fine-grained nanocomposites with excellent optical and mechanical properties.     

Sintering kinetics and properties of NiFe2O4-based ceramics inert anodes doped with TiN nanoparticles

Sintering kinetics of NiFe₂O₄-based ceramics inert anodes for aluminum electrolysis doped 7 wt% TiN nanoparticles were conducted to investigate densification and grain growth behaviors. The linear shrinkage increased gradually with the increasing sintering temperature between 1000 and 1450°C, whereas the linear shrinkage rate exhibited a broad peak. The maximum linear shrinkage rate was obtained at 1189.4°C, and the highest densification rate was achieved at the relative density of 75.20%. Based on the pressureless sintering kinetics window, the sintering process was divided into the initial stage, the intermediate stage, and the final stage. The grain growth exponent reduced with increased sintering temperature, whereas the grain growth activation energy decreased by increasing sintering temperature and shortening dwelling time. The grain growth was mainly controlled by atomic diffusion. NiFe₂O₄-based ceramics possessed high-temperature semiconductor essential characteristics. The electrical conductivity of NiFe₂O₄-based ceramics first increased and then decreased with increasing sintering temperature, reached their maximum value (960°C) of 33.45 S/cm under 1300°C, mainly attributed to the relatively dense and uniform microstructure. The thermal shock resistance of NiFe₂O₄-based ceramic was improved by a stronger grain boundary bonding strength and lower coefficient of linear thermal expansion.     

Strengthening of Porous Mullite and Zirconia CMC Matrices by Evaporation/Condensation

A processing method using evaporation/condensation sintering in an HCl atmosphere was developed for strengthening porous materials without shrinkage. Strengthening without shrinkage is useful in preventing voids and cracks that might be formed during constrained densification, e.g., a porous matrix in a continuous fiber reinforced ceramic composite. Mixtures of mullite and zirconia (monoclinic, tetragonal (3 mol% Y₂O₃), and cubic (8 mol% Y₂O₃)) were studied and exposed to HCl vapor at temperatures up to 1300°C. It was observed that the evaporation–condensation mass transport process produced a porous material with minimal shrinkage. As the crystal structure of the starting tetragonal and cubic zirconia powders did not change after extensive coarsening, it appeared that zirconium and yttrium were transported in the same proportion via evaporation/condensation. The process produced significant coarsening of the zirconia grains, which made the material resistant to densification when heated to 1200°C in air. Because the sintering produced coarsening without shrinkage, the pores also coarsened and a porous microstructure was retained. Mixtures of mullite and zirconia were used because mullite does not densify under the processing conditions used here, namely, heat treatments up to 1300°C. The mullite particles acted as a non-densifying second phase to further inhibit shrinkage when the mullite/zirconia composite was heated up to 1200°C in air. The coarsened cubic zirconia plus mullite mixture had the least densification after heat treatments in air of 100 h at 1200°C.     

Pressureless sintering of transparent AlON ceramic with assimilable γ-Al2O3 as sintering promoting additives

Transparent aluminum oxynitride (AlON) ceramic was successfully fabricated without doping sintering additive by the pressureless sintering method. Γ-Al₂O₃ nano-powder that can be assimilated by the AlON matrix was used for promoting the densification of AlON during the sintering. The sintering behavior of AlON and the effects of γ-Al₂O₃ nano-powder on the phase, hardness, and transmittance of AlON have been investigated in detail. The mechanisms of γ-Al₂O₃ nano-powder on the AlON green body modifying and the sintering promoting are revealed. The transmittance of the AlON ceramic is dramatically enhanced by doping γ-Al₂O₃ nano-powder and the 2 mm thick sample doped with 2.5 wt% γ-Al₂O₃ nano-powder shows an inline transmittance above 81% at 1500 nm.     

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO3

Microstructural evolution and densification behavior of porous kaolin-based mullite ceramic added with MoO₃ were investigated. The results indicated that MoO₃ addition not only lowered the secondary mullitization temperature to below 950?°C, but also facilitated effectively the anisotropic growth of mullite grains. Fine mullite whiskers grew and interlocked with one another in the pre-existing pore regions, in-situ forming a stiff 3D skeleton structure of mullite whiskers, which arrested further densification of the sample. On the other hand, due to the great capillary attraction of small pores, the liquid phase tended to spread over small grains, which favored the growth from small mullite grains into whiskers at the expense of the liquid phase. Consequently, competitive mechanisms of sintering and crystal growth of mullite functioned, which further limited the sample densification. As a result, the total linear shrinkage of the sample added with MoO₃ after firing at 1400?°C was only ??2.75%, and its porosity was retained at as high as 67%.     

Sintering and densification mechanisms of tantalum carbide ceramics

Dense tantalum carbide (TaC) ceramics were prepared using TaC nanopowder via spark plasma sintering (SPS). The effects of the sintering temperature and applied pressure on the densification and grain growth behaviour of TaC ceramics were investigated. The results showed that high temperature and pressure promoted sintering densification, while their increase caused an increase in the grain size of TaC ceramics. A highly dense TaC ceramic (∼97.19%) with a fine grain size of 2.67 μm was obtained by sintering at 1800 °C for 10 min under 80 MPa. The Vickers hardness, Young's modulus and fracture toughness were 15.60 GPa, 512.66 GPa and 3.59 MPa·m^1/2, respectively. The densification kinetics were investigated using a creep deformation model. Diffusion and grain boundary sliding were proven to be the dominant densification mechanisms based on the stress and grain size exponents combined with the microstructural characteristics. The apparent activation energy of the mechanism controlling densification was 252.94 kJ/mol.     

The Effect of Applied Stress on the Densification of a Low-Temperature Cofired Ceramic-Filled Glass System Under Constrained Sintering

The effect of uniaxial stress on the mechanical response and densification behavior of a low-fire borosilicate glass (BSG)+alumina system during constrained sintering of a multilayer BSG+alumina/alumina laminate has been investigated. Compared with free sintering, the pressure-less constrained sintering of BSG+alumina exhibits poorer densification, and larger porous bulk viscosity at a given temperature. This is caused by the in-plane tensile stress and anisotropic development generated in the transverse directions of the laminate during constrained sintering. The applied uniaxial stress required in the thickness direction to densify BSG+alumina under constrained sintering varies in the range of 50–400 kPa at 700°–800°C. The above results are in agreement with those calculated using the viscous analogy for the constitutive relationships of a porous sintering compact.     

Sintering behavior of calcium lanthanum sulfide ceramics in field‐assisted consolidation

In this study, calcium lanthanum sulfide (CaLa₂S₄, CLS) ceramics with the cubic thorium phosphate structure were sintered at different temperatures by field‐assisted sintering technique (FAST). Densification behavior and grain growth kinetics were studied through densification curves and microstructural characterizations. It was determined that the densification in the 850°C‐950°C temperature range was controlled by a mixture of lattice or grain‐boundary diffusion, and grain‐boundary sliding. It was revealed that grain‐boundary diffusion was the main mechanism controlling the grain growth between 950°C and 1100°C. The infrared (IR) transmittance of the FAST‐sintered CLS ceramics was measured and observed to reach a maximum of 48.1% at 9.2 μm in ceramic sintered at 1000°C. In addition, it was observed that the hardness of the CLS ceramics first increased with increasing temperature due to densification, and then decreased due to a decrease in dislocations associated with grain growth.     

In situ synthesis,mechanical properties,and microstructure of reactively hot pressed WB4 ceramic with Ni as a sintering additive

In this study, tungsten tetraboride (WB₄) ceramics were synthesized in situ from powder mixtures of W and amorphous B with Ni as a sintering aid by reactive hot pressing method. The as-synthesized ceramics exhibited porosity as low as 0.375% and ultra-high Vickers hardness (H_v), as much as 49.808?±?1.683?GPa (for the low load of 0.49?N). It was seen that the addition of Ni greatly improved the sinterability of WB₄ ceramic. Besides, the flexural strength and fracture toughness of WB₄ ceramic were measured for the first time to be 332.857?±?36.763?MPa and 4.136?±?0.259?MPa?m^1/2, respectively, suggesting that the ceramic has good mechanical properties. The effects of sintering temperature and holding time on the densification, Vickers hardness, and mechanical properties of WB₄ ceramics were also investigated systematically as part of our study. The results indicated that increasing the sintering temperature can obviously improve the densification and mechanical properties of the ceramics. The bulk density and Vickers hardness of WB₄ ceramic sintered at 1650?°C for 60?min under 30?MPa revealed the highest values of 6.366?g?cm^?3 and 27.948?±?0.686?GPa (for the high load of 9.8?N), respectively. The flexural strength increased to the highest value of 332.857?±?36.763?MPa for sintering temperature up to 1550?°C, but decreased slightly as the sintering temperature further increased to 1650?°C. On the other hand, the fracture toughness increased gradually with increasing temperature. It was also found that Vickers hardness showed a similar trend as the densification of the samples with increasing temperature and holding time. Besides, no obvious improvements in the densification, mechanical properties, and Vickers hardness of the samples with sintering time were observed in this study. The microstructure and fracture behaviours of the as-synthesized WB₄ ceramic were also revealed, and the toughening mechanism has been discussed.     

Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam-gelcasting

Excessive sintering shrinkage leads to severe deformation and cracking, affecting the microstructure and properties of porous ceramics. Therefore, reducing sintering shrinkage and achieving near-net-size forming is one of the effective ways to prepare high-performance porous ceramics. Herein, low-shrinkage porous mullite ceramics were prepared by foam-gelcasting using kyanite as raw material and aluminum fluoride (AlF₃) as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF₃ content on the shrinkage, porosity, compressive strength, and thermal conductivity of mullite-based porous ceramics were investigated. The results showed that with the increase of content, the sintering shrinkage decreased, the porosity increased, and mullite whiskers were produced. Porous mullite ceramics with 30 wt% AlF₃ content exhibited a whisker structure with the lowest shrinkage of 3.5%, porosity of 85.2%, compressive strength of 3.06 ± 0.51 MPa, and thermal conductivity of 0.23 W/(m·K) at room temperature. The temperature difference between the front and back sides of the sample reached 710°C under high temperature fire resistance test. The low sintering shrinkage preparation process effectively reduces the subsequent processing cost, which is significant for the preparation of high-performance porous ceramics.