Low–temperature spark plasma sintering of transparent ceramics by using SiC molding set

Hydroxyapatite (HAP) and alumina ceramics were sintered at low temperatures by using the SiC molding set during spark plasma sintering (SPS). Transparent ceramics were obtained at 800 ˚C and 1000 ˚C for HAP and alumina, respectively. The SiC is electrically conductive (3x104 S/m), so that the molding set (mold, punch and spacer) was directly heated during SPS without external heating. Compared to the conventional graphite molding set, the voltage level during heating was higher due to lower conductivity. When the graphite molding set was used, the temperature required for transparent HAP and alumina was about 950 ˚C and 1150 ˚C, respectively. The SiC molding set lowered the temperatures for transparent ceramics by 150 ˚C (Fig. 1). For transparent HAP, the transmission is almost similar between the samples sintered at 800 ˚C with the SiC set and at 950 ˚C with the graphite set. For transparent alumina, the transmission is considerably affected by the heating rate. At \u3c20 \u3e˚C/min, the transmission of the sintered alumina increases with an increase in the heating rate and shows a maximum at 20 ˚C/min. This is opposite to the behavior for the graphite molding set, where the transmission decreases with increasing heating rate. The dependence on the heating rate is explained by considering the effect of applied voltage during SPS. The high voltage level seems to enhance the diffusion during sintering. At a heating rate of 50 ˚C/min, the transmission of the alumina decreases due to the accelerated grain growth, and the delay behavior of dynamic grain growth was also observed. In this study, the effects of the SiC molding set were examined on the densification temperature, transmission, voltage level and grain growth

Grain growth behavior during spark plasma sintering of ceramics

During sintering, most of densification process proceeds in the intermediate stage where channel-like open pores and large isolated pores shrink by the movement of particles or grains towards the pores. The grain rearrangement without significant shape change, one of the characteristics of sintering, is a result of the grain-boundary sliding which is the most important mechanism for high-temperature deformation, such as superplastic deformation. The grain-boundary sliding is an essential process during densification. During spark plasma sintering of alumina, the effects of heating rate, pressure and loading schedule on the grain size were examined. Usually, high heating rates results in small grain sizes because of short heating time. However, when alumina was densified at low temperatures, high heating rates accelerated grain growth, though the total heating time was reduced. The grain growth rate after full densification was also accelerated for high heating rates. The accelerated grain growth might result from the generation of defects during densification. The densification in the intermediate stage of sintering includes the deformation of powder particles, and the deformation occurs mainly by grain-boundary sliding or grain re-arrangement. The defects generated during grain-boundary sliding may enhance the grain-boundary mobility and accelerate the grain growth rate, that is the dynamic grain growth. It is considered, therefore, that the high deformation rate at high heating rates accelerated grain growth during sintering. The accelerated grain growth also appeared for high-pressure sintering. The grain size after sintering increased with the applied pressure. High pressures lowered the deformation temperature and increased the deformation rate. As a result, the high deformation rate during heating may generate defects and enhance the grain-boundary mobility. Lastly, the loading schedule during heating also affected the deformation and the grain growth. Applying pressure at low temperatures or at high rates may generate more defects and resultantly accelerate the grain growth. These unusual grain growth behaviors during spark plasma sintering are explained by using a concept of dynamic grain growth [1]. Hence, one of our conclusions is that the deformation of grain-boundary sliding plays an important role in both densification and grain growth during sintering. [1] BN Kim et al., Scripta Mater., 80 (2014) 29

High temperature tensile behavior of zirconia ceramics under DC current

These Flash sintering phenomena, which occurs by applying DC current directly to ceramic powder compacts, has been the subject of many paper of ceramic sintering. This is because the flash event can succeed to lower the sintering temperature/time of several ceramic powders. On the other hand, Conrad and his colleagues examined the effect of electric fields on the high temperature tensile properties of 3Y-TZP and confirmed that the fields can lower the tensile flow stresses of 3Y-TZP enough to attain superplasticity. The enhanced deformation was explained by suppressed grain growth due to the electric bias effect. However, the mechanism/phenomena of the flash event are still unclear. In order to clarify the effect of electric current on high temperature deformation, therefore, the present study was carried out to examine the tensile behavior of polycrystalline zirconia ceramics under the several temperature and electric field/current conditions. By applying the DC electric power higher than a critical value Ec, the flash event similar to that of powder sintering occurs even in dense zirconia ceramics. At around 1000 °C, for example, the Ec value is about 100 - 200 mW/mm3, which is slightly larger than those reported in the powder compacts. For lower than Ec, the applied electric current increases sample temperature depending on the applied value, but does not enhance the rate of deformation. For higher than Ec, on the other hand, the electric current enhances the rate of the deformation to about several times as compared with that of without current conditions. The enhanced deformation cannot be interpreted only by the increment of sample temperatures and is likely to occur by the flash event. After the deformation under the electric current conditions, the tested sample shows slight gray color even under air condition. This suggests that the enhanced deformation would be related to oxygen vacancy formation. In the presentation, we will discuss the detailed current effect obtained at wide range testing conditions

Effective colloidal processing for densification before SPS

In conventional dry processing, fine particles tend to agglomerate spontaneously due to Van der Waals attractive forces. Since the agglomeration of particles forms large residual pores in green bodies, elevated temperatures are necessary for densification. Colloidal processing is a very effective technique for controlling the pore size distribution in green compacts before sintering. The green compacts having small residual pores with a narrow size distribution is expected to enhance the densification at low sintering temperature during SPS. We already reported that colloidal processing for controlling the packing structure in green compact is effective for densification in SPS in the case of SiC. A commercially available SiC (6H) powder with the average particle size of 0.55 mm was used as the starting materials. When using the dry processing for consolidation, the density of the sample sintered by SPS in a vacuum atmosphere at 1950°C was 92% of the theoretical value. Aqueous suspensions with dispersed particles were prepared by adjusting pH and consolidated by slip casting to prepare the dense green compacts. The relative density of SiC prepared by SPS was increased with increasing temperature and reached more than 97% at 1950°C as shown in Fig. 1. In this presentation, this processing was applied to fabrication of transparent alumina. Commercially available Al2O3 powder with the average particle size of 0.4 mm was used as the starting materials. Suspensions with 30 vol% solid were consolidated by slip casting. The green compacts before sintering were further densified by cold isostatic pressing at 392 MPa for 10 min and calcined at 500°C for 1 h in air in order to burn off the dispersant. Final sintering was carried out at 1150°C under a uniaxial pressure of 100 MPa using an SPS. After rapid heating to 600°C, the temperature was raised from 600°C to 1150°C using a heating rate of 5°C/min. After holding samples at the sintering temperature for 10 min and then subsequently annealing them at 1000°C for 10 min, we obtained a sintered disk with a diameter of 25 mm and a thickness of 2 mm. Fig. 2(a) shows the photograph of the sample from the green compact prepared by slip casting, Fig2(b) is the sample densified by SPS from the as-received powder directly. The transparency of the sample prepared by slip casting is clearer than that of the sample by SPS from the as-received powder directly. Please click Additional Files below to see the full abstract

Predictors of spontaneous viral clearance and outcomes of acute hepatitis C infection

Background/AimsThis study evaluated the predictors of spontaneous viral clearance (SVC), as defined by two consecutive undetectable hepatitis C virus (HCV) RNA tests performed ≥12 weeks apart, and the outcomes of acute hepatitis C (AHC) demonstrating SVC or treatment-induced viral clearance.MethodsThirty-two patients with AHC were followed for 12-16 weeks without administering antiviral therapy.ResultsHCV RNA was undetectable at least once in 14 of the 32 patients. SVC occurred in 12 patients (37.5%), among whom relapse occurred in 4. SVC was exhibited in 8 of the 11 patients exhibiting undetectable HCV RNA within 12 weeks. HCV RNA reappeared in three patients (including two patients with SVC) exhibiting undetectable HCV RNA after 12 weeks. SVC was more frequent in patients with low viremia than in those with high viremia (55.6% vs. 14.3%; P=0.02), and in patients with HCV genotype non-1b than in those with HCV genotype 1b (57.1% vs. 22.2%; P=0.04). SVC was more common in patients with a ≥2 log reduction of HCV RNA at 4 weeks than in those with a smaller reduction (90% vs. 9.1%, P<0.001). A sustained viral response was achieved in all patients (n=18) receiving antiviral therapy.ConclusionsBaseline levels of HCV RNA and genotype non-1b were independent predictors for SVC. A ≥2 log reduction of HCV RNA at 4 weeks was a follow-up predictor for SVC. Undetectable HCV RNA occurring after 12 weeks was not sustained. All patients receiving antiviral therapy achieved a sustained viral response. Antiviral therapy should be initiated in patients with detectable HCV RNA at 12 weeks after the diagnosis

Improved reversibility in lithium-oxygen battery: Understanding elementary reactions and surface charge engineering of metal alloy catalyst

Most Li-O-2 batteries suffer from sluggish kinetics during oxygen evolution reactions (OERs). To overcome this drawback, we take the lesson from other catalysis researches that showed improved catalytic activities by employing metal alloy catalysts. Such research effort has led us to find Pt3Co nanoparticles as an effective OER catalyst in Li-O-2 batteries. The superior catalytic activity was reflected in the substantially decreased overpotentials and improved cycling/rate performance compared to those of other catalysts. Density functional theory calculations suggested that the low OER overpotentials are associated with the reduced adsorption strength of LiO2 on the outermost Pt catalytic sites. Also, the alloy catalyst generates amorphous Li2O2 conformally coated around the catalyst and thus facilitates easier decomposition and higher reversibility. This investigation conveys an important message that understanding elementary reactions and surface charge engineering of air-catalysts are one of the most effective approaches in resolving the chronic sluggish charging kinetics in Li-O-2 batteries.

Prognostic Value of Postoperative CEA Clearance in Rectal Cancer Patients with High Preoperative CEA Levels

PURPOSE: We determined the prognostic value of carcinoembryonic antigen (CEA) clearance after tumor resection with serial evaluation of postoperative CEA levels in rectal cancer. METHODS: Between 1994 and 2004, we retrospectively reviewed 122 patients with rectal cancer whose serum CEA levels were measured on the preoperative day and postoperative days 7 and 30. Patients with preoperative CEA levels <5.0 ng/ml were excluded. An exponential trend line was drawn using the three CEA values. Patients were categorized into three groups based on R(2) values calculated through trend line, which indicates the correlation coefficient between exponential graph and measured CEA values: exponential decrease group (group 1: 0.9 < R(2) < or = 1.0), nearly exponential decrease group (group 2: 0.5 < R(2) < or = 0.9), and randomized clearance group (group 3: 0.5 < or = R(2)). We then analyzed the CEA clearance pattern as a prognostic indicator. RESULTS: With a median follow-up of 57 months, the 5-year overall survival was 62.3% vs. 48.1% vs. 25% and the 5-year disease-free survival was 58.6% vs. 52.7% vs. 25% among groups 1, 2, and 3 (P = 0.014, P = 0.027, respectively) in patients with stage III rectal cancer. For those with stage II rectal cancer, the 5-year overall survival rate of group 1 was significantly better than groups 2 and 3 (88.8% vs. 74.1%, respectively, P = 0.021). CONCLUSIONS: the postoperative pattern of CEA clearance is a useful prognostic determinant in patients with rectal cancer. Patients with a randomized pattern of CEA clearance after tumor resection should be regarded as having the possibility of a persistent CEA source and may require consideration of intensive follow-up or adjuvant therapy.ope