Earticle

Purpose: The aims of this study are compare with microstructure and mechanical properties of Co-Cr-Mo alloys fabricated by powder metallurgy(P/M) process and casting process respectively. Method: Microstructure and micro-hardness were tested by SEM and Vickers Hardness Tester. The sintered specimen was produced by furnace-coolling after sintering, however the casting specimen were produced thru air-cooling and water-cooling after the casting. For observation of phase transformation during sintering, DSC analyzing was carried out. Results: Mean pore size of sintered Co-Cr-Mo alloy was 4.32㎛ and that of casting alloy was 1.63㎛. Hardness of sintered alloy was lower than water-quenched casting alloy. Conclusion: Proper sintering temperature of Co-Cr-Mo alloy was above 1,200℃ and pore size of casting specimen were finer than sintered specimen, but hardness were similar.

Purpose: The aim of this study was to investigate the influence of short and long duration sintering on microstructure and flexural strength of zirconia. Methods: To conduct three-point bending test, Zirconia specimens are milled according to ISO 6872 guidelines(N=18, n=9 per group). Two specimens group(n=8) is sintered for 10 hours(Standard schedule) and 3 hours(Speed schedule) at the peak temperature of 1550°C with silicon carbide sintering furnace. Flexural strength of specimens are measured by instron. After coating each specimen(n=1), microstructure of specimens is observed using Scanning Electron Microscope(SEM). T-test was utilized to statistically assess the data. Results: The mean and standard deviation value of the flexural strength for standard schedule group are 578.15±57.48Mpa, that of speed schedule are 465.9±62.34Mpa. T-test showed significant differences in flexural strength between two zirconia specimen group which applied standard schedule and speed schedule respectively(p<0.05). Conclusion: The result of this study showed that the increase in sintering time led to increased grain size, and also to a positive effect on the flexural strength.

Purpose: Generally dental technicians clinically decide the sintering temperature of zirconia artificial teeth to match the color of the teeth. However, the sintering temperature influence the microstructure and mechanical strength of ceramic body. In this study, to evaluate the free choice of sintering temperature which leads to color the problems in zirconia false teeth, the variation of microstructure, mechanical strength, and colortone of zirconia ceramics according to the change of sintering temperature was investigated. Methods: Bar type specimens were prepared from commercial zirconia blocks by cutting and polishing into 0.8 cm(L) x 1.0 cm(W) x 4.8 cm(H). Specimens were fired from 1,400 to 1,700℃ at 50℃ intervals and held for 1hour at highest temperature. Apparent porosity, water absorption, firing shrinkage, bulk density, bend strength, whiteness were tested. Microstructures were observed by SEM. Results: When fired above 1450℃, all specimens showed 0% apparent porosity and water absorption, 20% firing shrinkage, and 6.1g/cm2 bulk density regardless of firing temperatures. SEM photomicrographs showed grain growth of zirconia occurred above 1,600℃. Whiteness was also largely changed above this temperature. Maximum bend strength of 1,050 MPa was obtained at 1,550℃. Bend strength lowered slightly above this temperature and showed 950ßÁ at 1,700℃. Conclusion: In order to fit the colortone of zirconia artificial teeth, arbitrary choice of firing temperature higher than 1,500℃, up to 1,700℃ did not influence the mechanical strength.

Tialite (Al2TiO5) is a ceramic material of very low thermal expansion coefficient, excellent thermal shock resistance, high refractoriness and good corrosion resistance. However, industrial applications of this material are hindered by two major limitations: decomposition to its parent oxides, alumina and rutile between 800 and 1280 oC and its low thermal mechanical properties. In the present study addition of talc with a good stabilizing effect has been studied on aluminum-titanate (tialite)- based ceramics. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the effect of talc on phase evolutions, sintering, and the microstructure of aluminum-titanate, and also density measurements were performed after sintering. The results showed that the presence of talc reduces the unreacted alumina and rutile significantly. The formation of Mg0.3Al1.4Ti1.3O5 and other solid solutions of MgO and aluminum-titanate were observed.

Thermoelectric conversion efficiency of thermoelectric elements can be increased by using a structure combining n-type and p-type semiconductors. From the above point of view, attention was directed at ZnO as a candidate n-type semiconductor material and investigations were made. As the result, a dimensionless figure of merit ZT close to 0.28 (1073K) was obtained for specimens produced by the PCS (Pulse Current Sintering) method with addition of specified quantities of $TiO_2$, CoO, and $Al_2O_3$ to ZnO. It was found that the interstitial $TiO_2$ in the ZnO restrains the grain growth and CoO acts onto the bond between grains. The influence of the inclusion of $TiO_2$ and CoO onto the sintering behavior also was investigated.

Alumina/mica and spinel/mica composites were fabricated by sintering of compacts containing 20 mass% fluoromica ($KMg_3AlSi_3O-{10}F_2$) glass and alumina or spinel. In both composites, mica precipitated as plate-like crystals at temperatures lower than $1300^{\circ}C$ and melted at $1300^{\circ}C$ to $1400^{\circ}C$. In alumina/mica composites, alumina and glass reacted to produce spinel, and the densification progressed by the solution-precipitation of alumina. Consequently, the glass composition changed and the mica did not precipitate at temperatures higher than $1400^{\circ}C$. However, mica precipitated after a reheating process. In spinel/mica composites, the glass composition did not change. After the mica phase melted, it recrystallized during slow cooling. The relative density reached the maximum at $1500^{\circ}C$ for alumina/mica and at $1300^{\circ}C$ spinel/mica composites, and decreased at further high temperatures.

This study systematically explores, for the first time, the relationships among sintering behavior, physico-mechanical properties,microstructure, and radiation shielding of reactive alumina ceramics. High-purity reactive α-Al2O3 powder with bimodalparticle size distribution was pressurelessly sintered at 1550?1600 °C for 1?3 h. Nearly full densification (≥ 99% theoreticaldensity) was achieved at 1550 °C?3 h and above. XRD and SEM?EDX confirmed single-phase corundum (α-Al2O3) formation. Increasing sintering temperature and dwell time enhanced densification and grain coarsening. The highest hardness (18.37 ±0.45 GPa) occurred at 1550 °C?1 h, while the best fracture toughness (4.19 ± 0.62 MPa·m1/2) and flexural strength (387 ± 37MPa) were obtained at 1600 °C?3 h. Crack deflection, bridging, branching, and nanopore-related mechanisms contributedto toughness improvement. Color values (L ? 90?95, a ? 0?1.7, b ? 9?12, ΔE ? 0?3.8) showed high stability. The best γ-rayshielding was achieved at 1600 °C?3 h, with linear attenuation coefficients of 29.959 cm?¹ (0.662 MeV), 22.789 cm?¹ (1.173MeV), and 21.351 cm?¹ (1.332 MeV). These results indicate that reactive α-Al2O3 ceramics can be readily densified, offeringexcellent mechanical performance and radiation attenuation for advanced shielding panels and blocks.

0.2ZnAl2O4/0.8Ba0.5Sr0.5TiO3 ceramics with good dielectric temperature stability were synthesized by controlling sinteringbehavior. The relationship between sintering conditions, microstructure, and dielectric properties of ceramics was studied. Cubic structures were confirmed in all ceramics. Double sintering (DS) behavior can effectively improve the density, and thegrain size has no obvious change compared with non-repeated sintering. The maximum dielectric permittivity (220 at 10 kHz)and minimum leakage current (3.98×10-6 A/cm2 at 0.25 kV/cm) are obtained in the DS ceramics, which is superior to thoseof the single sintering ceramics. This can be ascribed to the higher relative density of the DS ceramics in contrast to the singlesintering ceramics. More importantly, the temperature stability of the DS sample in all samples is optimal due to the highsurface energy at grain boundaries and improved density. This work demonstrates a route to produce ceramics with weaktemperature sensitivity for microwave applications.

It is feasible to add homogeneously minute amounts (0.25 wt%) of Al₂O₃ into 3 mol% yttria stabilized tetragonal zirconia polycrystals using a co-precipitation procedure to produce precursory gel nano-powders. X-ray diffraction, scanning electron microscopy, high-angle annular dark-field-field emission gun transmission electron microscopy, relative density determination, shrinkage, and resistivity measurements using impedance spectroscopy were combined to elucidate the microstructure and sintering behavior of zirconia ceramics produced using 0.25 wt% added Al₂O₃, possibly the limit of solubility of Al₂O₃ in the ZrO₂ structure. Two modes of sintering behavior may be described as: (I) The total solid solution of Al₂O₃ in the solid solution of ZrO₂ structure via a precursory co-precipitation procedure; calcining at 800 oC and sintering at 1275 oC can be shown to indicate the viable solubility of Al₂O₃ in the ZrO₂ structure, designated as stage I of the thermal treatment; and (II) At the higher sintering temperature, above 1275 oC, the segregation of Al₂O₃ at triple junction occurs. Higher bulk and grain boundary resistivity values are found for the sample sintered at 1275 oC, which probably resulted from the dissolution of Al₂O₃ in the ZrO₂ structure.

Ultrafine-grained WC-Co-VC-Cr3C2 cemented carbide was consolidated from homogeneously synthesized powder using sparkplasma sintering at 1300 °C under 60 MPa. The densification behavior of WC-Co-VC-Cr3C2 was determined based on theshrinkage strain with sintering kinetics. As the Cr3C2 addition increased, the secondary (Cr, W)Cx phase was attributed toabnormal grain growth in the basal facet of WC, whereas grain boundary strengthening was induced by the separation of theWC/Co interface with the addition of VC. The mechanical properties of WC-Co-VC-Cr3C2 cemented carbide depend on thegrain size and lattice strain of the hard phase, in contrast to the toughening mechanism, and were correlated with the crackpropagation and fracture resistance. The WC-Co-Cr3C2 cemented carbide with the best combination of mechanical propertieswas found, including a hardness of 1958.9 kg/mm2 and fracture toughness of 7.3 MPa?m1/2.

Molybdenum-tantalum (Mo-Ta) alloy sputtering targets are widely used in electronic applications owing to their excellent corrosion resistance, high thermal and electrical conductivity, and low electrical impedance. In this study, the sintering behavior and microstructural evolution of Mo-Ta alloys fabricated by spark plasma sintering (SPS) were investigated as a function of sintering temperature in the range of 1650-1800 °C. X-ray diffraction and microstructural analyses indicate that densification and alloying of the mixed Mo and Ta powders occur simultaneously during the SPS process. Increasing the sintering temperatures significantly enhances densification, and the compact sintered at 1750 °C achieves a relative density exceeding 99%, which is essential for high-quality sputtering target applications. The sintered alloys exhibit a clear temperature-dependent grain growth behavior together with a homogeneous microstructure and randomly oriented grains. These results demonstrate that appropriate control of sintering temperature enables the fabrication of dense and microstructurally uniform Mo-Ta alloys, providing valuable guidelines for optimizing sputtering target performance.

Sintering behavior of the Fe-0.8Mn-0.5C powder system was studied on the specimens with a density of ${\sim}7.0g/cc$ sintered at $1120^{\circ}C$ for 30 min in a gas mixture of $7%H_2/93%N_2$ with the inlet dew point of $-60^{\circ}C$. During the atmosphere monitoring ($CO/CO_2$-content and dew point) was showed, that carbothermical reduction occurs in two different temperature ranges; three peaks of dew point profile also can be distinguished during sintering cycle as well. Following sintering the Mn-content distribution and microstructures around the Mn-source were micro-analytical evaluated; the results showed that manganese travels through porous iron matrix up to ${\sim}60{\mu}m$.

Composite ceramics between Al2O3 (6 mol% MgO); AM and ZrO2 (4 mol% Y2O3); ZY with 40:60, 50:50, and 60:40 ratiowere prepared. The samples were sintered under different sintering method between normal sintering and two-step sinteringmethod. The effects of the different contents of AM and ZY and sintering condition on the structural analysis, densification,microstructure and mechanical behavior were studied. It was found that composites with higher proportions of ZrO2 showedhigher weight percentages of t-ZrO2 and c-ZrO2 phases which is obtained high density and good mechanics properties. TheFracture toughness mechanisms of composites are also discussed in detail around the sintering parameters. The AM:ZYcomposite ceramics that provides good mechanical properties were revealed in both sintering method in optimum conditions.

The effect of sintering temperature on the microstructure, electrical and dielectric properties of (V, Mn, Co, Dy, Bi)-codoped zinc oxide ceramics was investigated in this study. An increase in the sintering temperature increased the average grain size from 4.7 to $10.4{\mu}m$ and decreased the sintered density from 5.47 to $5.37g/cm^3$. As the sintering temperature increased, the breakdown field decreased greatly from 6027 to 1659 V/cm. The ceramics sintered at $900^{\circ}C$ were characterized by the highest nonlinear coefficient (36.2) and the lowest low leakage current density ($36.4{\mu}A/cm^2$). When the sintering temperature increased, the donor concentration of the semiconducting grain increased from $2.49{\times}10^{17}$ to $6.16{\times}10^{17}/cm^3$, and the density of interface state increased from $1.34{\times}10^{12}$ to $1.99{\times}10^{12}/cm^2$. The dielectric constant increased greatly from 412.3 to 1234.8 with increasing sintering temperature.

Because ZrB2 has some excellent physical performance and chemical stability, it has been widely applied in a lot of fields, but sintering of ZrB2 is too difficult to be densified and easy oxidation at high temperature. To keep advantages and improve disadvantages of ZrB2, the shell-core structure A12O3-Y2O3/ZrB2 composite powders are prepared by the co-precipitation methods, the high density YAG/ZrB2 ceramics is prepared by the spark plasma sintering (SPS). The increasing ratio of mehanical properties is faster at sintering temperature from 1300o C to 1700o C, the increasing ratio of mehanical properties is faster under sintering pressure from 5 MPa to 20 MPa, the increasing ratio of mehanical properties is faster for holding time from 1 minute to 4 minutes. When the sintering conditons is 1700o C, 20 MPa and 4 minutes, the Young’s modulus and the fracture toughness are 430 GPa and 3.76 MPam1/2, respectively. The YAG is coated on the surface of ZrB2 crystal, that is to say, the YAG is show on the crystal boundary, which is help for the densification and the oxidation resistance at hightemperature condition.

Three kinds of $Si_3N_4$ powders (M-11, SN-ESP, and SN-E10) were gas-pressure sintered at $1700-1900^{\circ}C$ for 2 h under 18 atm $N_2$. Their densification behavior was investigated and compared as well as the mechanical properties and microstructure of the resulting ceramics. SN-ESP and SN-E10 started to reach nearly full densification at $1750^{\circ}C$ and showed almost no decomposition up to $1900^{\circ}C$. In contrast, M-11 was not fully densified until $1800^{\circ}C$ and showed about $3\%$ weigh loss at $1900^{\circ}C$ indicating poor thermal stability. SN-ESP and SN-E10 showed much higher strength both at room temperature and $1200^{\circ}C$ than M-11 when fully densified. Compared with SN-ESP, SN-E10 was not only a little better in strength (both at room temperature and $1200^{\circ}C$) and fracture toughness but also much higher in the Weibull modulus due to more interlocked microstructure by well elongated grains.