Chung-Hyo Lee

Fabrication of Mg2Si thermoelectric materials by mechanical alloying and spark-plasma sintering process.

A mixture of pure Mg and Si powders with an atomic ratio 2:1 has been subjected to mechanical alloying (MA) at room temperature to prepare the Mg2Si thermoelectric material. Mg2Si intermetallic compound with a grain size of 50 nm can be obtained by MA of Mg66.7Si33.3 powders for 60 hours and subsequently annealed at 620 degrees C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies up to 800-900 degrees C under 50 MPa. The shrinkage of consolidated samples during SPS was significant at about 250 degrees and 620 degrees C. X-ray diffraction data shows that the SPS compact from 60 h MA powders consolidated up to 800 degrees C consists of only nanocrystalline Mg2Si compound with a grain size of 100 nm.

Effect of mechanical alloying on the formation of Sm2Fe17Nx compound

Elemental powders of iron and samarium were mechanically alloyed in the concentration range of SmxFe100−x (x=11, 13, 15, 17). A two-phase mixture of amorphous Sm−Fe and α-Fe phases was formed in all compositions studied. The effect of the starting composition on the formation of Sm2Fe17 intermetallic compound was investigated by annealing mechanically alloyed powders. When the Sm content was 15 at.%, the annealed powders consisted of nearly a Sm2Fe17 single phase. For the preparation of a hard magnetic Sm2Fe17Nx compound, additional nitriding treatment was performed under an N2 gas flow at 450°C for various time intervals. It was found that nitrogenation for 3 hours was just enough to allow the formation of the Sm2Fe17Nx compound. The coercivity increased when the nitrogenation time increased up to 3 hours and then tended to decrease with further nitrogenation.

Fabrication and characterization of Mn-Si thermoelectric materials by mechanical alloying

The semiconducting MnSi1.73 compound has been recognized as a thermoelectric material with excellent oxidation resistance and stable characteristics at elevated temperature. In the present work, we applied mechanical alloying (MA) technique to produce MnSi1.73 compound using a mixture of elemental manganese and silicon powders. The mechanical alloying was carried out using a Fritsch P-5 planetary mill under Ar gas atmosphere. The MA powders were characterized by the X-ray diffraction with Cu-Kα radiation, thermal analysis and scanning electron microscopy. Due to the observed larger loss of Si relative to Mn during mechanical alloying of MnSi1.73, the starting composition of a mixture Mn-Si was modified to MnSi1.83 and then MnSi1.88. The single MnSi1.73 phase has been obtained by mechanical alloying of MnSi1.88 mixture powders for 200 hours. It is also found that the grain size of MnSi1.73 compound powders analyzed by Hall plot method is reduced to 40 nm after 200 hours of milling.

Mechanical Alloying Effect in Immiscible Cu-Based Alloy Systems.

The mechanical alloying effect has been studied on the three Cu-based alloy systems with a positive heat of mixing. The extended bcc solid solution has been formed in the Cu-V system and an amorphous phase in the Cu-Ta system. However, it is found that a mixture of nanocrystalline Cu and Mo is formed in the Cu-Mo system. The neutron diffraction has been employed as a main tool to characterize the detailed amorphization process. The formation of an amorphous phase in Cu-Ta system can be understood by assuming that the smaller Cu atoms preferentially enter into the bcc Ta lattice during ball milling.

1053 TREATMENT WITH CLEVUDINE FOR 24 WEEKS RESULTS IN SUSTAINED ANTIVIRAL RESPONSE ASSOCIATED WITH CONTINUED HBsAg DECREASE 24 WEEKS POST-TREATMENT IN HBeAg-POSITIVE PATIENTS

viral replication, with mean maximum reductions in HCVRNA ranging from −0.97 to −2.13 (log10 IU/mL). In this study, the safety and efficacy of FBV in combination with pegylated interferon alfa-2a (pegIFN) and ribavirin (RBV) was evaluated. Methods: Eligible patients (treatment naive, HCV genotype 1) were randomized 1:1:1:1 to receive FBV (200, 300 or 500mg BID) or placebo in combination with pegIFN (180 ug/wk) and RBV (1000/1200mg/day) for 4 weeks. Patients are continuing on open label pegIFN/RBV for an additional 44 weeks. Results from the Week 4 analysis are described here. HCVRNA was measured using the Roche COBAS Taqman (LLD= 25 IU/mL). Results: A total of 35 patients were enrolled and 34 completed Week 4. The majority were Caucasian (69%), genotype 1a (80%) and male (51%). The most frequently reported AEs were headache, fatigue, insomnia and nausea. There were no trends towards increased frequency or severity of AEs with increasing dose of FBV. The incidence and severity of laboratory test abnormalities was similar across treatment groups. There was one treatment-related SAE (elevated creatinine) in the FBV 300mg group, which resolved following IV hydration. No other renal AEs were reported. The mean reduction (log10 IU/mL) in HCVRNA at Day 4 for placebo (n = 8), 200 (n = 10), 300 (n = 9) and 500 (n = 8) mg BID was −0.58, −2.29, −2.72 and −2.83, respectively, and −2.10, −4.46, −4.67 and −3.62 at Day 28. The percent of patients achieving RVR (undetectable HCVRNA by Week 4) for placebo, 200, 300 and 500mg BID was 0%, 60%, 75% and 63%, respectively. Of those patients treated with 200, 300 and 500mg BID who achieved a RVR, 33%, 33% and 80% achieved undetectable HCVRNA within two weeks of treatment, respectively. Conclusions: The addition of FBV to pegIFN/RBV was well tolerated and markedly increased the percentage of patients achieving RVR. Assessment of response to pegIFN/RBV therapy subsequent to Week 4 is ongoing.

Neutron diffraction study in amorphous Ni30Ta70 alloy powders by mechanical alloying

A mixture of elemental Ni and Ta powders with an atomic ratio of 3∶7 was subjected to mechanical alloying (MA). An amorphous Ni30Ta70 alloy was formed after 80 hrs of milling, the amorphization by rapid quenching technique of which has not been reported. The atomic structural changes were observed by neutron diffraction in the amorphization process during MA. The radial distribution function RDF(r) shows that peaks of fcc-Ni and bcc-Ta crystal broaden first and gradually approach those characteristic of an amorphous phase with increasing MA time. A local atomic environment around Ni and Ta atoms was studied by analyzing the first peak in the total pair distribution function g(r) after the completion of amorphization. We reach our conclusion from this analysis that the amorphization in the Ni30Ta70 alloy takes place by the penetration of smaller Ni atoms into the bcc-Ta lattice.

Synthesis and Magnetic Properties of Fe2CrSi Heusler Alloys by Mechanical Alloying

We have applied mechanical alloying (MA) to prepare nanocrystalline Fe₂CrSi Heusler alloy using a mixture of elemental Fe50Cr25Si25 powders. Its structural characterizaion and magnetic properties have been studied by X-ray diffraction, differential scanning calorimeter and vibrating sample magnetometer measurements. α-(Fe,Cr,Si) BCC phases coupled with amorphous phase are obtained after 40 hours of MA. The saturation magnetization (Ms) of MA powders decreases with MA time due to the magnetic dilution by alloying with nonmagnetic Cr and Si elements. A Fe₂CrSi Heusler alloy can be synthesized by MA for 40 h coupled with subsequent heat treatment. X-ray diffraction result shows that the average grain size of Fe₂CrSi Heusler alloys prepared by MA for 40 h and heat treatment at 500 °C is in the range of 88 nm. Ms of MA powders after heat treatment at 500 °C and 650 °C are found to be 63 emu/g and 79 emu/g, respectively. Ms of Fe₂CrSi Heusler alloys by MA in this study is low value compared with other Fe-based Heusler alloys, probably being attributed to the formation of nonmagnetic Cr3Si phase with further annealing.

Fabrication and Magnetic Properties of Co2MnAl Heusler Alloys by Mechanical Alloying

We have applied mechanical alloying (MA) to produce nanocrystalline Co2MnAl Heusler alloys using a mixture of elemental Co50Mn25Al25 powders. An optimal milling and heat treatment conditions to obtain a Co2MnAl Heusler phase with fine microstructure were investigated by X-ray diffraction, differential scanning calorimeter and vibrating sample magnetometer measurements. α-(Co, Mn, Al) FCC phases coupled with amorphous phase are obtained after 3 hours of MA without any evidence for the formation of Co2MnAl alloys. On the other hand, a Co2MnAl Heusler alloys can be obtained by the heat treatment of all MA samples up to 650 °C. X-ray diffraction result shows that the average grain size of Co2MnAl Heusler alloys prepared by MA for 5 h and heat treatment is in the range of 95 nm. The saturation magnetization of MA powders decreases with MA time due to the magnetic dilution by alloying with nonmagnetic Mn and Al elements. The magnetic hardening due to the reduction of the grain size with ball milling is also observed. However, the saturation magnetization of MA powders after heat treatment increases with MA time and reaches to a maximum value of 105 emu/g after 5 h of MA. It can be also seen that the coercivity of 5 h MA sample annealed at 650 °C is fairly low value of 25 Oe.

Fabrication and Spark Plasma Sintering of Magnetic alpha-Fe/MgO Nanocomposite by Mechanical Alloying.

Solid-state reduction has occurred during mechanical alloying of a mixture of Fe2O3 and Mg powders at room temperature. It is found that magnetic nanocomposite in which MgO is dispersed in alpha-Fe matrix with nano-sized grains is obtained by mechanical alloying of Fe2O3 with Mg for 30 min. Consolidation of the ball-milled powders was performed in a spark plasma sintering (SPS) machine up to 800-1000 degrees C. X-ray diffraction result shows that the average grain size of alpha-Fe in a-Fe/MgO nanocomposite sintered at 800 degrees C is in the range of 110 nm. It can be also seen that the coercivity of SPS sample sintered at 800 degrees C is still high value of 88 Oe, suggesting that the grain growth of magnetic alpha-Fe phase during SPS process tends to be suppressed.

Characterization and consolidation of thermoelectric CrSi 2 compound prepared by mechanical alloying

Mechanical alloying was carried out to produce CrSi2 thermoelectric compound using a mixture of elemental Cr33Si67 powders. An optimal milling and heat treatment conditions to obtain the single phase of CrSi2 compound with fine microstructure were investigated by X-ray diffraction and differential scanning calorimetry measurement. CrSi2 intermetallic compound with a grain size of 70 nm could be obtained by MA of Cr33Si67 powders for 70 hours and subsequently annealed at 650 o C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies at 600~1000 o C under 60 MPa. The shrinkage of MA samples during SPS consolidation process increased gradually with increasing temperature up to 1000 o C and relatively significant at about 600 o C. We tend to believe that these behaviors are deeply related to form a CrSi2 compound during heating process, as can be realized from the DSC measurement. Electrical conductivity and Seebeck coefficient of sintered bodies were measured up to 900 o C. Seebeck coefficient and power factor of Cr33Si67 compact prepared by MA and SPS at 1000 o C showed the maximum value of 125 µV/K at 400 o C and 4.3 × 10 −4 W/mK 2 at 350 o C, respectively.