Highly-stable,nondegradable M2A-reinforced YSZ ceramic composites prepared by SPS

Magnesia-rich spinel (M₂A)-reinforced yttria-stabilized zirconia (YSZ) ceramic composites have been fabricated by SPS using m-ZrO₂, Y₂O₃, and waste-derived M₂A powders. The results showed that m-ZrO₂ in M₂A-free YSZ0 specimen is partially stabilized upon temperature rising into tetragonal phase whereas YSZ10-50 composites containing 10-50 wt% M₂A demonstrated variant behavior where their m-ZrO₂ is stabilized into a cubic form. YSZ10-50 composites SPSed at 1400°C for 30 minutes revealed greater than 99% relative density, 10-13 GPa Vickers hardness, 3.5-5 MPa m^0.5 fracture toughness and less than 0.5% apparent porosity. Aging hydration test for five hours soaking at 134°C and water vapor pressure of 2 bar, resulted in a conversion of ~2.8% of t-ZrO₂ to m-ZrO₂ for M₂A-free YSZ0 composite but it does not affect the YSZ10-50 composites. The outcomes indicate that M₂A has significantly improved both the densification and stabilization behavior of m-ZrO₂ through facilitating the diffusion of Y³⁺, Mg²⁺, and Al³⁺ ions inside zirconia lattice structure. In this regard, this study opens the door wide for producing fully stabilized c-ZrO₂ ceramics that could be potentially utilized as industrial ceramics for a broad scope of structural applications of high temperature, high-stress and corrosive environmental conditions.     

Spark Plasma Sintering (SPS) of NASICON Ceramics

Spark plasma sintering (SPS) method was used to obtain dense NASICON ceramics with a high-electrical conductivity, which was compared with conventional solid-state sintering. The fully dense NASICON was achieved at a relatively low-sintering temperature of 1100°C, whereas the apparent density of the specimen prepared by conventional sintering was 74% of the theoretical density. The highest conductivity of 1.8 × 10⁻³ Scm⁻¹ at 25°C, which is comparable to the best value reported, was achieved using the SPS process. Considering the phase, density, and microstructure, it appears that there is more room for improved conductivity by controlling the amount of monoclinic zirconia and the resistive grain-boundary glass phase.     

Impact of spark plasma sintering (SPS) on mullite formation in porcelains

The effect of the spark plasma sintering (SPS) process on mullite formation in porcelains was studied using X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy. SPS affected the kinetics and morphology of formed mullite. After sintering at 1100°C, unlike conventional sintering, SPS promoted the formation of mullite due to the combination of vacuum and applied pressure. Mullite crystal growth was altered by the atmosphere (vacuum), dwell time (0‐15 minutes), and temperature (1000‐1200°C). The applied pressure caused the mullite needles to orient perpendicular to the direction of the applied load. Depending on SPS dwell time, the mullite formed after sintering at 1100°C also had different crystal structure (tetragonal for short dwell time of 0‐5 minutes and orthorhombic for a long dwell time of 10‐15 minutes). Dissolution of mullite was observed at 1100°C by extending the dwell time by up to 15 minutes and the dissolved mullite reprecipitated on the small needles (~40 nm) and coarsened via Oswald ripening resulting in larger mullite needles (~60 nm).     

High thermal conductivity of spark plasma sintered silicon carbide ceramics with yttria and scandia

A fully dense SiC ceramic with a room‐temperature thermal conductivity of 262 W·(m·K)^?1 was obtained via spark plasma sintering β‐SiC powder containing 0.79 vol% Y₂O₃‐Sc₂O₃. High‐resolution transmission electron microscopy revealed two different SiC‐SiC boundaries, that is, amorphous and clean boundaries, in addition to a fully crystallized junction phase. A high thermal conductivity was attributed to a low lattice oxygen content and the presence of clean SiC‐SiC boundaries.     

放电等离子低温烧结h-BN-SiC复相陶瓷的结构与力学性能

以纳米h-BN和Si C粉为原料、B2O3为烧结助剂,利用放电等离子烧结(SPS)制备h-BN-Si C复相陶瓷,研究了烧结压力(20~50 MPa)对h-BN-Si C复相陶瓷结构与力学性能的影响。结果表明:在不同烧结压力下,h-BN-Si C复相陶瓷中h-BN晶粒的c轴倾向于平行压力方向,增大烧结压力能够提高复相陶瓷的致密化和力学性能,但较大的烧结压力(40 MPa)降低了c轴倾向于平行压力方向的取向度和断裂韧性。在40 MPa烧结压力时获得了较佳的综合性能,复相陶瓷的相对密度、抗弯强度和断裂韧性分别达到98%、289.2 MPa和3.45 MPa·m1/2,比同条件制备的纯h-BN陶瓷的抗弯强度和断裂韧性分别提高了约138.4%和64.3%。复相陶瓷断裂为典型的沿晶断裂模式,微裂纹及裂纹偏转提高了复相陶瓷的断裂韧性。     

Rapidly fabricating Y2O3 transparent ceramics at low temperature by SPS with mesoporous powder

Fine-grained and dense highly transparent Y₂O₃ ceramics have been successfully prepared using high sintering activity mesoporous Y₂O₃ powders without any additive by spark plasma sintering (SPS). The influences of the sintering temperature on microstructure, density, optical, and mechanical properties of SPS-sintered Y₂O₃ ceramics were studied in detail. As results, the optimal Y₂O₃ ceramics with high relative density of 99.90% and fine average grain size of 140 nm were obtained at a low sintering temperature of 1140°C and a moderate load pressure of 60 MPa for 5 min. Meanwhile, the dense Y₂O₃ ceramics with 1 mm thickness after annealing show a high linear transmittance of 78% (close to 94% of the theoretical value) at 2.4–3 µm wavelength. In additions, the Vickers hardness and fracture toughness of samples can reach 8.48 GPa and 1.45 MPa m^1/2, respectively. This result proves that the high activity of mesoporous Y₂O₃ is considered to be an important means for preparing high-performance fine Y₂O₃ ceramics at low sintering temperature.     

Effect of titanium diboride on the homogeneity of boron carbide ceramic by flash spark plasma sintering

A novel method, namely flash spark plasma sintering (FSPS), combining flash sintering and electric field assisted sintering, was utilized to densify boron carbide/titanium diboride (B₄C/TiB₂) composites. Further, sintering homogeneity of the composites with different contents of TiB₂ was systematically investigated and theoretical model was built. Results indicated that addition of 50?wt% TiB2 led to the densification of B₄C/TiB₂ composite by up to 97.7% with regional range 1.9% at 1872?°C under pressure of 4?MPa in 60?s. The preferential pathway of TiB₂ network proves to disperse the central current and distribute thermal flow throughout the specimen possibly via tunneling, electronic field emission effect at first stage and lower-resistance composite pathway latter, contributing to the increased homogeneity.     

Effects of sintering aids on microstructure,mechanical, and optical properties of AlON ceramics synthesized by SPS

Aluminum oxynitride (AlON) ceramics doped with different sintering aids were synthesized by spark plasma sintering process. The microstructures, mechanical, and optical properties of the ceramics were investigated. The results indicate that the optimal amount of sintering aids is 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO. The addition of La³⁺ and Mg²⁺ decreases the rate of grain boundary migration in ceramics, promotes pore elimination, and inhibits grain growth. The addition of Y³⁺ facilitates liquid-phase sintering of AlON ceramics. Moreover, the addition of Mg²⁺ effectively promotes twin formation in the ceramics, which hinders crack propagation and dislocation motion when the ceramics are loaded. Hence, the AlON ceramic doped with 0.06 wt% La₂O₃ + 0.16 wt% Y₂O₃ + 0.30 wt% MgO exhibits a relative density of 99.95%, an average grain size of 9.42 μm, and a twin boundary content of 10.3%, which contributes to its excellent mechanical and optical properties.     

Consolidation by spark plasma sintering (SPS) of polyetheretherketone

The present study deals with the consolidation of an ultra‐high performance polymer, the poly(ether ether ketone) (PEEK), for structural applications, using the powder metallurgy (PM) way, and more precisely the Spark Plasma Sintering (SPS) processing. The effects of SPS parameters such as temperature, pressure, and dwell time on density and mechanical properties of PEEK were investigated via a Design of Experiments (DoE). A temperature of 250 °C, a pressure of 40 MPa, and a dwell time of 20 min have been identified as the optimal SPS process parameters. In these conditions, a density of 1.31 g / cm³ was reached and homogeneous mechanical properties in the volume determined by means of compression tests were found with a compressive modulus of 2.75 GPa, a yield strength of 134 MPa, and a maximum compressive strain of 43%. These results are better than those of commercial products obtained by injection molding. The pressure appears to be a significant parameter on PEEK properties and plays positive or negative roles according to the responses of DoE studied. To our knowledge, it is one of the first studies based on the application of the PM techniques for PEEK consolidation showing the possibility to process below its melting point. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44911.     

Yb doped MgO transparent ceramics generated through the SPS method

Yb doped (0, 0.02, 0.1 and 0.5 at%) MgO transparent ceramics were synthesized through spark plasma sintering (SPS) at the relatively low temperature of 1100 °C for 5–60 min under a pressure of 105 MPa. The effects of dopant concentration and sintering holding time on the densification and microstructure evolution of MgO ceramics were investigated. All ceramics reached a relative density greater than 99.20%. The 0.02% Yb-doped MgO ceramic sintered at 1100 °C for 60 min showed the highest in-line transmittance, of 80% at 1030 nm, a value close to that of MgO single crystals. Yb dopant improved the transmittance, degree of densification and control of grain growth. Herein, the influence of Yb doping on the crystalline phase and microstructure was explored, and the photoluminescence properties of Yb in transparent MgO ceramics were investigated.     

烧结工艺对Ti/Al2O3复合材料性能的影响

利用放电等离子烧结技术探讨了烧结工艺对40%(体积分数)Ti/Al2O3复合材料性能的影响。实验结果表明,复合材料的性能受烧结温度的影响最为显著,过度延长保温时间会使晶粒发生异常长大,材料性能降低。烧结温度1300℃,保温时间8min制备的复合材料力学性能最佳,其弯曲强度、断裂韧性、显微硬度和相对密度分别为1002.22MPa,19.73MPa·m1/2,18.14GPa和99.74%。     

Study on the effect and mechanism of zirconia on the sinterability of yttria transparent ceramic

Transparent Y₂O₃ ceramics were fabricated by solid-state reaction using high purity Y₂O₃ and ZrO₂ powder as starting material. The results indicated that ZrO₂ additive can improve the transparency of Y₂O₃ ceramic greatly. The best transmittance appears with 3 at.% ZrO₂ doped Y₂O₃ transparent ceramic with transmittance at 1100 nm of 83.1%, which is up to 98.6% of the theoretical value. The microstructure is uniform and no secondary phase is observed in the ceramic with the average grain size of 15 μm. The mechanism of ZrO₂ improving the transparency of Y₂O₃ ceramic is analyzed in detail. On this basis, Yb³⁺ doped Y₂O₃ transparent ceramic was also fabricated and spectroscopic properties were investigated.     

Densification of 8Y‐Tetragonal‐Stabilized Zirconia Optoceramics with Improved Optical Properties by Y Segregation

8% Yttria‐stabilized zironcia (8YSZ) transparent ceramics have a wide technological applications. Segregation of the Y around the grain boundaries is favored by slow heating rate. The optimized sintering parameters helped in obtaining transparent ceramics of 8YSZ with a high percentage of cubic phase in addition to the presence of tetragonal phase. HRTEM was used to verify the grain growth suppression and to observe the presence of the cubic phase. The presence of cubic phase has suppressed the grain growth, which increased the transparency in the visible and infrared region without the addition of dopants or by utilizing high pressure.     

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.     

Properties of MgO transparent ceramics prepared at low temperature using high sintering activity MgO powders

Transparent MgO ceramics are successful fabricated via spark plasma sintering at lower temperature using the high sintering activity powders synthesized by precipitated method. The samples were detected by XRD, SEM, TEM, BET, UV-Vis-NIR, microhardness, and so on. The results show that all ceramics prepared at 700°C-900°C are visually transparent and the sample sintered at 860°C for 5 min exhibits the superior transmittance of 60% (800 nm). It is also found that the mechanical and thermal properties of MgO ceramics are all increasing firstly and then decreasing with the increase in the sintering temperature. And the maximum value of hardness, fracture toughness, MSP strength, and Young's modulus of MgO ceramics is 8.25 GPa, 2.01 MPa·m^1/2, 206 MPa, and 286 GPa, respectively. Moreover, the thermal conductivity of MgO ceramics sintered at 860°C can reach 48.4 W/mK at room temperature.     

Influence of process parameters in gel casting of a pure yttria nanopowder to fabricate transparent ceramics

The goal of this research is to fabricate pure transparent yttria ceramics through gel casting and vacuum sintering. A specific processing method has been used and optimized for this purpose. A pure yttria nanopowder was synthesized as the starting material to produce pure transparent ceramics through a low-temperature sintering process. It was attempted to minimize the undesirable nanopowder hydration by using the as-synthesized yttria nanopowder and a rapid deagglomeration and slurry preparation process. The synthesized nanopowders were deagglomerated to enhance the efficiency of both powder shaping and sintering stages. Carrageenan was used as the gelling agent because it is a low-cost and abundant material, and because the temperature is the only catalyst needed for its gelation; therefore, it is possible to control its gelation to obtain high-density and pure optical ceramics. The effect of the deagglomeration method and the processing parameters, including the amounts of dispersant, gelling agent, solid loading, pH, and deagglomeration time, on the rheology of slurry, density, and microstructure of the obtained green yttria ceramics was examined and optimized in order to obtain high solid loading nanoyttria suspensions of 38 vol%, which is more than those obtained in many of the previous investigations. The precise gelling temperature and time were measured, and green gel cast ceramics with a density of 63 % of the theoretical density were produced. A rapid deagglomeration and slurry preparation method was used instead of using a conventional planetary ball-milling approach to minimize the risk of the hydrolysis of yttria nanopowder. No sintering aid was necessary, and transparent yttria ceramics with 99 % of the theoretical density were produced after vacuum-furnace sintering at 10^-2 mbar and 1715 °C.     

Gel-casting of transparent magnesium aluminate spinel ceramics fabricated by spark plasma sintering (SPS)

In this paper, a transparent magnesium aluminate spinel ceramic was fabricated through the newest colloidal gel casting method, using a synthetic powder with the average particle size of 90 nm and Isobutylene-Maleic Anhydride (ISOBAM) additive. ISOBAM served as both a dispersant and a gelation agent to achieve a dense body. Also, the suspension rheological behavior was optimized by the solid loading of 85 wt%, the additive content of 0.7 wt%, and the gelation time of 350 s. This led to a green body with a density equal to 65% of theoretical density and the green strength of 14.48 MPa. The results revealed that the reduction of porosity and the uniform distribution of pores in the green body (smaller than half of the initial powder particle size, 35 nm), as accompanied by spark plasma sintering (SPS), resulted in the final body density of 99.97%, as well as the high in-line transmittance of 86.7% at the wavelength of 1100 nm.     

Optical and mechanical properties of transparent alumina fabricated by high-pressure spark plasma sintering

Transparent alumina was fabricated from untreated commercial powder by high-pressure spark plasma sintering (HPSPS) at temperatures of 1000, 1050 and 1100 °C under pressures of 250-800 MPa. It was established that transparency strongly depends on the HPSPS parameters. At all temperatures, there was a certain point when increasing the pressure led to decreasing transparency. At 1100 °C, relatively high pressure led to excessive grain growth, as well as the formation of creep-induced porosity at the center of the samples. Hardness values decreased with pressure due to grain growth, correlated with the Hall-Petch relationship. The optimal combination of optical and mechanical properties (68% in-line transmittance at a wavelength of 640 nm and a hardness value of about 2300 HV2) was achieved after sintering at 1050 °C under 600 MPa.     

Electron spin resonance of transparent alumina ceramics fabricated by spark plasma sintering

Transparent α‐alumina ceramics are fabricated using spark plasma sintering. Paramagnetic defects related to the optical properties of the ceramics have been investigated using electron spin resonance (ESR) analyses. An isotropic ESR signal at g = 2.003 (S = 1/2) with a linewidth of 0.5 mT is formed during sintering. The g = 2.003 signal intensity has a weak correlation with the absorbance in the visible region but does not correlate with the real in‐line transmission (RIT) at 650 nm. An ESR signal with a fine structure attributed to Fe³⁺ was detected in both the α‐Al₂O₃ starting powder and the sintered ceramic samples. The degree of c‐axis orientation of the grains has been determined using the Fe³⁺ signal intensity, which depends on the angle between the directions of the c‐axis and the applied magnetic field. The ESR analysis indicated that the c‐axis tends to be oriented in the direction of the sintering pressure. The degree of c‐axis orientation was found to correlate with the RIT in highly densified ceramics.     

Fabrication and characterization of SPS sintered SiC-based ceramic from Y3Si2C2-coated SiC powders

The Y₃Si₂C₂ coating was in-situ synthesized on the surface of SiC powders to form SiC-Y₃Si₂C₂ core-shell structure by using a molten salt technique. Phase diagram calculations on Si-Y-C ternary phase at different temperatures well illustrated that the Y₃Si₂C₂ phase can be stable with SiC but will be in liquid state at 1560?°C. The liquid Y₃Si₂C₂ explained the enhanced consolidation of SiC ceramics and its disappearance after spark plasma sintering. Such Y₃Si₂C₂ coating could not only effectively improve the sintering, but also their mechanical and thermal properties of resultant ceramics. Typically, at 1700?°C, the bulk SiC ceramic presented a mean grain size of 2.5?um and relative density of 99.5% when the molar ratio of Y to SiC is 1:4 in molten salts; the Young’s modulus, indentation hardness and fracture toughness measured by indentation test were 451.7?GPa, 26.3?GPa and 7.9?M?Pam^1/2, respectively; the thermal conductivity is about 145.9?W/(m?K). Excellent thermal and mechanical properties could be associated with the fine grain size, optimized phase composition and improved grain boundary structure.