• Journal of the Korean Ceramic Society
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• v.40 no.11
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• pp.1106-1112
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• 2003

For the purpose of making clear the role of oxygen in the thermoelectric properties of FeSi$_2$, thermoelectric measurements and spectroscopic characterization were conducted for the oxidized specimens fabricated from ($\alpha$+$\varepsilon$)-phases and/or $\beta$-phase. Addition of oxygen to FeSi$_2$ prevented both densification during sintering and transformation from metallic phases to semiconducting phase during annealing treatment. In an specimens, electrical conductivity and thermal conductivity decreased with oxidation time. The Seebeck coefficient was positive and small for pure FeSi$_2$. And/or the oxidized specimens fabricated from ($\alpha$+$\varepsilon$)-phases. However, it was negative and showed a maximum peak at about 500 K for the oxidized FeSi$_2$ fabricated from $\beta$-phase. The value of maximum peak increased with oxidation time.

• Korean Journal of Materials Research
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• v.12 no.5
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• pp.375-381
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• 2002

N-type ${\beta}-FeSi_2$ with a nominal composition of $Fe_{0.98}Co_{0.02}Si_2$ powders has been produced by mechanical alloying process and consolidated by vacuum hot pressing. As-milled powders were of metastable state and fully transformed to ${\beta}-FeSi_2$ phase by subsequent isothermal annealing. However, as-consolidated $Fe_{0.98}Co_{0.02}Si_2$ consisted of untransformed mixture of ${\alpha}-Fe_2Si_ 5$ and $\varepsilon$-FeSi phases. Isothermal annealing has been carried out to induce the transformation to a thermoelectric semiconducting ${\beta}-FeSi_2$ phase. The transformation behavior of ${\beta}-FeSi_2$ was investigated by utilizing DTA, a modified TGA under magnetic field, SEM, and XRD analyses. Isothermal annealing at $830^{\circ}C$ in vacuum led to the thermoelectric semiconducting ${\beta}-FeSi_2$ phase transformation, but some residual metallic $\alpha$ and $\varepsilon$ phases were unavoidable even after prolonged annealing. Thermoelectric properties were remarkably improved by isothermal annealing due to the transformation from metallic $\alpha$ and $\varepsilon$ phases to semiconducting phases.

• Korean Journal of Materials Research
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• v.17 no.3
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• pp.132-136
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• 2007

Oxidation resistance of sintered ${\beta}-FeSi_{2}$ was investigated at intermediate temperature range in air atmosphere. Fully dense and porous bodies of ${\beta}-FeSi_{2}$ samples were fabricated by using the Spark Plasma Sintering (SPS). They were annealed at $900^{\circ}C$ for 5days to obtain ${\beta}-FeSi_{2}$ phase. The bulk samples were oxidized at $800,\;900\;and\;950^{\circ}C$ in air atmosphere. The high temperature oxidation tests reveal that amorphous $SiO_{2}$ layer, similar to Si was formed and grew parabolically on ${\beta}-FeSi_{2}$. Accelerated oxidation is not observed as well as cracks and grain boundary oxidation. Granular ${\varepsilon}-FeSi$ was developed below the oxide layer as a result of oxidation of ${\beta}-FeSi_{2}$. Oxidation resistance of sintered ${\beta}-FeSi_{2}$ was excellent for high-temperature thermoelectric application.

• Journal of Powder Materials
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• v.15 no.5
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• pp.345-351
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• 2008

The effects of the dopant (Mn) ratio on the microstructure and thermoelectric properties of $FeSi_2$ alloy were studied in this research. The alloy was fabricated by a combination process of ball milling and high pressure pressing. Structural behavior of the sintered bulks were systematically investigated by XRD, SEM, and optical microscopy. With increasing dopan (Mn) ratio, the density and ${\varepsilon}-FeSi$ phase of the sintered bulks increased and maximum density of 94% was obtained in the 0.07% Mn-doped alloy. The sintered bulks showed fine microstructure of ${\alpha}-Fe_{2}Si_{5}$, ${\varepsilon}-FeSi$ and ${\beta}-FeSi_2$ phase. The semiconducting phase of ${\beta}-FeSi_2$ was transformed from ${\alpha}-Fe_{2}Si_{5}+{\varepsilon}-FeSi$ phase by annealing.

• Korean Journal of Materials Research
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• v.12 no.3
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• pp.176-181
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• 2002

Fabrication of ${\beta}-FeSi_2$ was attempted by making use of the combined process of mechanical alloying (MA) and spark plasma sintering (SPS). MA was performed under the Ar gas atmosphere using mixed powders of pure iron and silicon having the mole fraction of 1:2. SPS process was performed at 800-85$0^{\circ}C$ with the applied pressure of 50MPa and the holding time was ranging from 0 to 30min. The mechanically alloyed powder by cyclic operation of rotor for 15hrs consisted of $\varepsilon$-FeSi and Si phases. When this mechanically alloyed powder was sintered by SPS process above 85$0^{\circ}C$, $\varepsilon$-FeSi and ${\alpha}-Fe_2Si_5$ phase were formed. Bulk product sintered at 82$0^{\circ}C$ for 30min consisted of ${beta}-FeSi_2$ phase with a small fraction of $\varepsilon$-FeSi and the density of sintered specimen was 75.3% theoretical density. It was considered that the MA/SPS combined process was effective for the preparation of ${\beta}-FeSi_2$ without heat treatment process after sintering.

• Korean Journal of Materials Research
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• v.11 no.2
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• pp.132-136
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• 2001

Iron- silicide has been produced by mechanical alloying process and consolidated by hot pressing. As-consolidated iron silicides were consisted of $\beta$-FeSi$_2$ phase, and untransformed mixture of $\alpha$-$Fe_2Si_5$ and $\varepsilon$-FeSi phases. Isothermal annealing has been carried out to induce the transformation to a thermoelectric semiconducting $\beta$-$FeSi_2$ phase. The condition for $\beta$-FeSi$_2$ transformation was investigated by utilizing DTA, SEM, TEM and XRD analysis. The phase transformation was shown to be taken place by a vacuum isothermal annealing at $830^{\circ}C$ for 24 hours. The mechanical and thermoelectric properties of $\beta$-FeSi$_2$ materials before and after isothermal annealing were characterized in this study.

• Korean Journal of Materials Research
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• v.8 no.7
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• pp.584-590
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• 1998

The microstructures of finely distributed Si-phases in $\beta$-$FeSi_2$ thermoelectric matrix, were produced by heat-treating the melt-cast ingots of single $\alpha$-$Fe_2Si_5$ phase at 730~85$0^{\circ}C$ for 4~20 hours, or by resistance-hot-pressing the mechanically alloyed powders ordinarily consisting of $\varepsilon$-FeSi and Si phases at 760~85$0^{\circ}C$ for 10 minutes of composition. $(Fe_{0.98}Mn_{0.02})_xSi_2(x{\leq$}1) The size and interspacing of dispersed Si-phases were able to control within a range of 0.05~0.27$\mu\textrm{m}$ and 0.2~0.6$\mu\textrm{m}$ by variations of heat treatment temperature and sintering temperature as well as the composition. respectively. The dispersion of Si- phases was expected to be effective for the reduction of thermal conductivity responsible for the increment of thermoelectric figure of merit.

• Korean Journal of Materials Research
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• v.14 no.3
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• pp.218-223
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• 2004

P-type $\beta$-FeSi$_2$ with a nominal composition of $Fe_{0.92}Mn_{0.08}Si_2$ powders has been produced by mechanical alloying process. As-milled powders were spray dried and consolidated by atmospheric plasma thermal spraying as a rapid sintering process. As-milled powders were of metastable state and fully transformed to $\beta$-$FeSi_2$ phase by subsequent isothermal annealing. However, as-thermal sprayed $Fe_{0.92}Mn_{0.08}Si_2$ consisted of untransformed mixture of $\alpha$-$Fe_2Si_{5}$ and $\varepsilon$-FeSi phases. Isothermal annealing has been carried out to induce transformation to the thermoelectric semiconducting $\beta$-$FeSi_2$ phase. Isothermal annealing at $845^{\circ}C$ in vacuum gradually led to the thermoelectric semiconducting $\beta$-$FeSi_2$ phase transformation, but some residual metallic $\alpha$ and $\varepsilon$ phases were unavoidable even after prolonged annealing. Thermoelectric properties of $\beta$-$FeSi_2$ materials before and after isothermal annealing were evaluated. Seebeck coefficient increased and electric conductivity decreased with increasing annealing time due to the phase transition from metallic phases to semiconducting phases. Thermoelectric properties showed gradual increment, but overall properties appeared to be inferior to those of vacuum hot pressed specimens.

• Proceedings of the Materials Research Society of Korea Conference
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• 2006.05a
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• pp.33.1-33.1
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• 2006

• Journal of Industrial Technology
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• v.19
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• pp.107-113
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• 1999

The microstructural evolution during mechanical alloying of elemental Fe and Si powders, average composition $Fe_{30}Si_{70}$ and $Fe_{50}Si_{50}$, has been investigated by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Differential scanning calorimetry (DSC). Mechanical alloying was performed by using a SPEX 8000 Mixer/Mill under argon atmosphere with/without hexane as a process control agent (PCA). In the presence of PCA, the milling process was dominated by fracture resulting in the decrease in particle size to about $1{\mu}m$. The structural development with milling time depended on the average composition of starting powders. The mixture of $Fe_{50}Si_{50}$ and $Fe_{30}Si_{70}$ resulted in the formation of FeSi(${\varepsilon}$ - phase) and $FeSi_2$(${\beta}$ - phase), respectively. In the case of $Fe_{33.3}Si_{66.7}$, a mixture and $FeSi_2({\beta})$ was formed. These results were discussed by considering the thermodynamics and kinetics concerning the milling process.