Nanocrystalline Formation and Magnetic Properties in Fe2O3-C System by Mechanical Alloying.

The effect of mechanical alloying (MA) on the solid state reaction of hematite and graphite system with a positive reaction heat was investigated using a mixture of elemental Fe2O3-C powders. The solid state reduction of hematite to Fe3O4 has been obviously observed after 3 hours of MA by a vibrating ball mill. A two-phase mixture of Fe3O4 and remaining Fe2O3 is obtained after 5 hours of MA. Saturation magnetization gradually increases with MA time due to the formation of Fe3O4 and then reaches 23 emu/g after 5 hours of MA. In addition, a Fe3O4 single phase is obtained by MA after 3 hours and subsequently heat treated up to 700°C. X-ray diffraction result shows that the average grain size of Fe3O4 prepared by MA for 5 hours and heat treatment to be in the range of 92 nm. The saturation magnetization of Fe3O4 prepared by MA and heat treatment reaches a maximum value of 56 emu/g for 5 hours MA sample. It is also observed that the coercivity of 5 hours MA sample annealed at 700 °C is still high value of 113 Oe, suggesting that the grain growth of magnetite phase during annealing process tends to be suppressed.

Preparation and Structural Observation of Co2MnSi Heusler Alloys by Mechanical Alloying.

A Co2MnSi Heusler alloy has been prepared by mechanical alloying (MA) method successfully using a mixture of elemental Co50Mn25Si25 powders. A two-phase mixture of amorphous phase and remaining Mn were obtained after 5 hours of MA without any evidence for the formation of Co2MnSi alloys. The saturation magnetization of MA powders decreased with MA time due to the magnetic dilution by alloying with nonmagnetic Mn and Si elements to 48 emu/g after 5 hours of MA. On the other hand, a Co2MnSi single phase was obtained by MA after 3 hours and subsequently heat treated up to 650 °C. X-ray diffraction result showed that the average grain size of Co2MnSi Heusler alloys prepared by MA for 5 hours and heat treatment to be in the range of 85 nm. The saturation magnetization of Co2MnSi Heusler alloys prepared by MA and heat treatment reached a maximum value of 112 emu/g for 5 hours MA sample. It was also observed that the coercivity of 5 hours MA sample annealed at 650 °C was fairly low value of 27 Oe.

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.

Synthesis and Spark Plasma Sintering of Soft Magnetic Composite in a Fe2O3­Al System by Mechanical Alloying.

We have applied mechanical alloying (MA) to produce soft magnetic composite material using a mixture of elemental Fe2O3­Al powders. An optimal milling and sintering conditions to obtain soft magnetic α-Fe/Al2O3 composite with fine microstructure were investigated by X-ray diffraction, differential scanning calorimeter and vibrating sample magnetometer measurements. It is found that the average grain sizes of α-Fe in α-Fe/Al2O3 composite ball-milled for 5 hours is estimated to be in the range of 50 nm. The saturation magnetization of ball-milled powders showed a maximum value of 88 emu/g after 30 min. of MA and reaches to 77 emu/g after 5 h of MA. The magnetic hardening due to the reduction of the α-Fe grain size with ball milling was also observed. Densification of the ball-milled powders was performed in the spark plasma sintering (SPS) machine at 1000 °C and 1100 °C. FE-SEM observation shows that the average grain size of α-Fe in α-Fe/Al2O3 composite sintered at 1000 °C is in the range of 100 nm, which is nearly same value estimated from the so-called Hall plot. It can be also seen that the coercivity of SPS sample sintered at 1000 °C is still high value of 92 Oe, suggesting that the grain growth of magnetic α-Fe phase during SPS process tends to be suppressed.

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.

Fabrication and Characterization of Thermoelectric CrSi2 Compound by Mechanical Alloying and Spark Plasma Sintering.

A mixture of elemental Cr-Si powders has been subjected to mechanical alloying (MA) at room temperature to prepare CrSi2 thermoelectric compound.The MA powders were sintered at 800-1000 °C Cunder 60 MPa using spark plasma sintering (SPS) technique. Due to the observed larger loss of Si relative Cr during ball milling, the starting composition was modified to Cr30Si70, Cr31.5Si68.5 and Cr33Si67 to get a single phase of CrSi2 compound. The single phase CrSi2 has been obtained by MA of Cr31.5Si68.5 mixture powders for 70 h and subsequently sintered at 1000 °C. X-ray diffraction data shows that the SPS compact sintered at 1000 °C consists of only nanocrystalline CrSi2 compound with a grain size of 250 nm. The value of Seebeck coefficient of CrSi2 compound increases with temperature and reaches maximum value of 245 µV/K at 300 °C.

Fabrication and Characterization of Thermoelectric Fe2VAl Alloy Powders by Mechanical Alloying.

A mixture of elemental Fe50V25Al25 powders has been subjected to mechanical alloying (MA) at room temperature to prepare the Heusler Fe2VAl thermoelectric alloy. Fe2VAI alloy with a grain size of 90 nm can be obtained by MA of Fe50V25Al25 powders for 60 h and subsequently annealed at 700 degrees C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies up to 900-1000 degrees C under 60 MPa. The shrinkage of consolidated samples during SPS was significant at about 400 degrees C. X-ray diffraction data shows that the SPS compact from 60 h MA powders consolidated up to 900 degrees C consists of only nanocrystalline Fe2VAl alloy with a grain size of 200 nm.

Nanocomposite formation in haematite-Mg, Ti system induced by high-energy ball milling.

Reactant materials of haematite and Mg, Ti powders have been milled, where pure metals are used as reducing agent. It is found that nanocomposite powders in which MgO and TiO2 are dispersed in alpha-Fe matrix with nano-sized grains are obtained by MA of Fe2O3 with Mg and Ti for 30 min and 5 hours, respectively. It is suggested that the shorter MA time for the nanocomposite formation in Fe2O3-Mg is due to a larger negative heat associated with the chemical reduction of reactant materials. X-ray diffraction results show that the average grain size of alpha-Fe in alpha-Fe/TiO2 nanocomposite powders is in the range of 40 nm. From magnetic measurement, we can also obtain indirect information about the details of the solid-state reduction process during MA.

Fabrication of MnSi1.73 thermoelectric material by mechanical alloying.

The effect of mechanical alloying (MA) on the formation of MnSi1.73 thermoelectric compound was investigated. Due to the observed larger loss of Si relative to Mn during MA, the starting composition of Mn-Si was modified to MnSi1.83 and MnSi1.88. Sintering was performed in a spark plasma sintering (SPS) machine up to 600-800 degrees C under 50 MPa. The single phase MnSi1.73 has been obtained by MA of MnSi1.88 mixture powders for 200 h. It is also found that the grain size of MnSi1.73 compound analyzed by Hall plot method is reduced to 40 nm after 200 h of milling. Additionally, X-ray diffraction data shows that the SPS compact from 200 h MA powders consolidated at 600 degrees C consists of only nanocrystalline MnSi1.73 compound with a grain size of 90 nm.

Annealing characteristics of nanostructured Cu-Fe-P alloy processed by accumulative roll-bonding.

Annealing characteristics of a nanostructured copper alloy processed by accumulative roll-bonding (ARB) were studied. A nano-grained Cu-Fe-P alloy processed by 8 cycles of the ARB was annealed at various temperatures ranging from 100 to 400 degrees C for 0.6 ks. The sample still showed an ultrafine grained (UFG) structure up to 250 degrees C, however above 300 degrees C it began to replace by equiaxed and coarse grains due to an occurrence of the conventional static recrystallization. The hardness of the annealed copper decreased largely above 300 degrees C. These annealing characteristics of the UFG copper alloy were compared to those of a high purity copper.

Nanocrystalline formation in immiscible Cu-Mo system subjected to mechanical alloying.

The mechanical alloying process has been studied on the Cu-Mo system, the atomic pair of which is characterized by a positive heat of mixing of +19 kJ/mol. The EXAFS and X-ray diffraction measurements have been employed to analyze the structural changes taking place during milling. Two phases mixture of nanocrystalline fcc-Cu and bcc-Mo with a grain size of 10 nm has been formed by MA of Cu30Mo70 powders for 200 hours. The structural analysis based on the EXAFS spectra revealed that bcc and fcc crystal structure clearly do not change around Mo and Cu atoms up to 200 h of milling, respectively. Studies of the thermodynamical considerations by DSC analyses confirmed that the alloying does not occur even after 200 hours of MA in Cu-Mo system.

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.

Annealing characteristics of nano-grained oxygen free copper processed by accumulative roll-bonding process.

Annealing characteristics of nano-grained oxygen free copper processed by accumulative roll-bonding (ARB) were studied. A nano-grained oxygen free copper fabricated by 8 cycles of the ARB was annealed at various temperatures ranging from 100 to 300 degrees C for 0.6 ks. TEM observation revealed that the ultrafine grains still sustained up to 150 degrees C, however above 200 degrees C they were replaced by equiaxed and coarse grains due to an occurrence of the static recrystallization. The tensile strength of the copper decreased largely above 200 degrees C. These annealing characteristics of the copper were compared with those of a commercially pure aluminum.