Pulsed Current Sintering Process Research Articles

Effect of Cyclic Uniaxial Pressure in Pulse-Current Sintering on Microstructure of Bi2Te3-Based Thermoelectric Materials

Various cyclic uniaxial pressure patterns have been applied to a powder of Bi0.4Sb1.6Te3 thermoelectric material during the pulse-current sintering process. The cyclic uniaxial pressure pattern (0 MPa to 100 MPa) was varied by changing the number of pressure cycles and the ratio of the pressing time to the time during which no pressure was applied (i.e., pressing time/pressureless time). Sintering was performed at 400°C for 10 min in vacuum. Electron backscattered diffraction analysis revealed that both the number of pressure cycles and the pressing time ratio strongly affected the microstructure of the sintered materials. Increasing the number of pressure cycles led to a monotonic increase in the intensity of the c-plane orientation. Increasing the pressing time ratio also enhanced the intensity of the c-plane orientation, although the intensity of the c-plane orientation decreased at larger pressing time ratios. These results indicate that constant pressure and pressureless sintering are not effective for attaining c-plane orientation and that the cyclic uniaxial pressure pattern is important for controlling the microstructure of the sintered material. Consequently, the thermoelectric properties of sintered materials could be enhanced by varying the cyclic uniaxial pressure pattern.

Preparation of Bi0.5Sb1.5Te3 Thermoelectric Materials by Pulse-Current Sintering Under Cyclic Uniaxial Pressure

The effects of applying cyclic uniaxial pressure during the pulse-current sintering process on the crystal alignment and thermoelectric properties of p-type Bi0.5Sb1.5Te3 were investigated. Sintering was performed at 673 K using pulse-current heating under 70 MPa or 100 MPa of cyclic uniaxial pressure. x-Ray diffraction patterns and electron backscattered diffraction analyses showed that application of the cyclic uniaxial pressure enhanced crystal grain orientation. The texture consisted of flattened crystal grains stacked in the thickness direction of the sintered materials. The hexagonal c-plane strongly tended to align in the direction perpendicular to the uniaxial pressure. Owing to the crystal alignment, the Hall mobility in the direction perpendicular to the uniaxial pressure became larger than that of equivalent samples prepared with a constant uniaxial pressure. As a result of the increase in Hall mobility, the resistivity of the material was decreased while the equivalent Seebeck coefficient was maintained and the power factor was improved.

Fabrication of a large scale transparent conducting film using transformed few-layered graphene nanoribbons obtained from unzipping of single wall carbon nanotubes

A large scale transparent conducting film has been fabricated using transformed few-layered graphene nanoribbons (FGNRs) obtained from unzipping of single wall carbon nanotubes and poly(3,4-ethylenedioxythiophene). FGNRs are fabricated from an unzipping process of single-walled carbon nanotubes with a high direct current pulse through a pulsed current sintering process. From control of the film thickness and conductivity, the optical transmittance values of films at a wavelength of 550 nm were 84% and 92%, and have sheet resistances of 150 and 2000 Ω sq−1. High resolution transmission electron microscopy, high resolution Raman spectroscopy, X-ray photoelectron spectroscopy, transmittance and electrical conductivity were used for the investigation of the FGNRs and FGNR transparent electrodes to characterize the films.

Fabrication of graphene layers from multiwalled carbon nanotubes using high dc pulse

Graphene layers are fabricated from multiwalled carbon nanotubes (MWCNTs) with a high direct current pulse through a pulsed current sintering process. We confirm the transformation of the structure from MWCNTs to graphene layers. Graphene layers are analyzed by field emission scanning electron microscopy, high resolution transmission electron microscopy, high resolution Raman, and x-ray diffraction.

Development of a Thermoelectric Module Using the Heusler Alloy Fe2VAl

A thermoelectric module was constructed using a Heusler Fe2VAl sintered alloy prepared by pulse-current sintering (PCS) using mechanically alloyed powder. A large-amplitude vibration mill was developed for powder preparation. The processing rate was increased 50-fold over that of laboratory planetary ball milling. In all, 25 pieces of millimeter-sized sintered alloy, which is useful for thermoelectric devices without cutting, were fabricated simultaneously using a single PCS process. The thermoelectric module consisting of the high-strength sintered alloy and electrode joint showed high durability. This module stably generated electric power on the exhaust pipe of a running motorcycle.

Fabrication of WC-TiC Based Hard Material by MA-PCS Process

WC-23%TiC-6%Ni-8%Co powder was prepared by a mechanical alloying for a short time. When the MA powder was consolidated by a pulsed current sintering process, the hard composite material with the hardness of 92.1 HRA and the transverse rapture strength of 1455 MPa was obtained. When Ti powder and graphite powder were used for mechanical alloying as starting materials instead of TiC powder, TiC particles were synthesized during the pulsed current sintering. Although the obtained sintered body had fine structure, the transverse rapture strength of it was lower than that of the case with TiC powder used. It would be attributable to the remaining pores and the unstable metallic binder phases in the sintered WC-23%TiC-6%Ni-8%Co body. The new process of the mechanical alloying for a short time combined with the pulsed current sintering was effective to obtain a hard composite material with higher strength than that of ordinary vacuum sintering process because of its fine microstructure.

Synthesis of diamond-reinforced Zr 65Al 10Ni 10Cu 15 metallic glass composites by pulsed current sintering

Zr 65Al 10Ni 10Cu 15 metallic glass matrix composites (MGMC) reinforced with diamond particles were fabricated using a pulsed current sintering process. The thermal stability, microstructure and mechanical properties of the sintered bulk MGMC were investigated. The pulsed current sintered composites not only showed nearly full relative density, but also thermal stability similar to that of the initial metallic glass powders. The addition of diamond particles increased the strength which was further improved as the volume fraction increased up to ∼10%. This corresponds to the change in fracture pattern of metallic glass after reinforcing with diamond particles.

短時間のメカニカルアロイング法によるＦｅ２ＶＡｌ合金粉末の合成

The Fe2VAl alloy was produced through the mechanical alloying and the pulsed current sintering process to clarify the optimum milling condition for obtaining homogeneous Fe-Al based intermetallic compounds. The homogeneity of the milled powder was evaluated from the absolute value of Seebeck coefficient, |S|, of the thermoelectric Fe2VAl alloy. It used to be that the synthesis of the homogeneous Fe2VAl powder yields after the milling for 720 ks in the case of using conventional planetary ball milling process. On the contrary, under the modified milling conditions optimized in this study, which were the orbital speed of 900 rpm and the inversed rotating speed of 300 rpm, enabled to cut down the milling time can be shortened to 10.8 ks to provide the satisfactory Fe2VAl powder. The |S| values of Fe2VAl alloy sintered from the powder produced through the optimum milling were comparable to those of alloys fabricated by a conventional melting technique.

Preparation of Fe<sub>2</sub>VAl Thermoelectric Module by RF Sputtering

An attempt to prepare Fe2VAl deposited film and the thermoelectric module using RF sputtering has been made. Sputtering target has been prepared using mechanical alloying of metallic powders and the subsequent pulse current sintering process. The obtained deposited film has had a lack of aluminum content compared to the composition of the starting material. Controlling of aluminum content for the preparation of Fe2VAl sputtering target has made it possible to obtain the desired material composition. The film has had the experimental thermoelectric force being similar to the one estimated from the measured thermoelectric data of the materials. Fe2VAl thermoelectric module of eight pairs with a film thickness of 4 μm has had an electric force of 31mV and 5.6μW.

In Situ Observations of Sintering Process during Pulsed Current Sintering of Al<sub>2</sub>O<sub>3</sub>, ZnO and WC Alloy

Pulsed Current Sintering (PCS) process possesses some problems that need to be resolved. We, therefore aims at understanding phenomena of PCS process by presenting some basic data on in situ sintering behavior of PCS. In order to observe in situ sintering behavior of PCS, a special graphite mold equipped with thermo couple and electrodes were designed to measure the temperature, electric current and voltage inside the powder during PCS process. We apply three types of raw materials, especially for ZnO (thermoelectric material) as semiconductor, Al2O3 as non-conductor and WC (Tungsten Carbide) as good conductor. The observation succeeded and some valuable data were obtained. The results showed that the temperature in the Al2O3 powder is 100 K higher than the graphite mold at the temperature of 1473K and ZnO powder is 150 K higher than the graphite mold at the temperature of 1373K. The electric current and voltage were measured for each powder during PCS process. In addition, their electric resistance properties were calculated. The electric resistance showed different behavior.

加圧力を制御したパルス通電焼結による機能性材料の作製

A pulsed current sintering is a new sintering process that powder in the conductive mold is heated by applying current under a pressure. This new sintering process is able to apply high-speed heating and densification at lower sintering temperature. Therefore, this process is effective for consolidation of nanocrystalline materials.In this study, we focus attention on the pressure during the pulsed current sintering of varied functional materials. In the ordinary pulsed current sintering process, the powder in the graphite die was consolidated under a pressure of about 30 MPa. The mechanically alloyed Fe-48at%Cr powder was consolidated in this process without brittle phase. In addition, the Fe-48at%Cr compact with a complex shape was able to be formed.The pulsed current sintering under a pressure of less than 10 MPa was effective to prepare the homogeneous porous materials. The porous material prepared by this process from copper fiber can be applied to heat sink for cooling CPU. The pulsed current sintering with a pressure of more than 500 MPa was effective to consolidate amorphous powder and to obtain a bulk nanocrystalline compact at s sintering temperature more than crystallized temperature of the amorphous powder. The nanocrystalline Ti-2at%Fe-10at%Si compact had high strength and was applied to the mold for consolidation of NaCl powder.

Fabrication of TiB<SUB>2</SUB>–Fe–Al Cermet Alloys Synthesized by Pulsed Current Process

Mechanical alloying and pulsed-current sintering were used to synthesize TiB 2 -Fe-Al cermet alloys consisting of titanium boride and iron aluminide intermetallic compound. A mixture was obtained to which Fe was minutely distributed by mechanically alloying TiB 2 powder and Fe powder for 180ks. Mixture powders of TiB 2 -20mass%Fe3Al and of TiB 2 -24 mass%FeAl were prepared by mixing an Al powder with this mechanically alloyed powder using a mortar. The added Al reacted with Fe after it had melted when these mixtures were heated. These powders were sintered for a short time and at low temperature using pulsed-current sintering process, but these sintered compacts had pores. Several iron aluminide intermetallic compounds were generated in the TiB 2 -20mass%Fe 3 Al. Therefore, the TiB 2 -20mass%Fe 3 Al compact was hard and brittle. However, the TiB 2 -24 mass%FeAl sintered body was composed only of TiB 2 and FeAl, and had better ductility. A slight weight increase resulted from oxidation even though it was heated to 800 K or more in the air, but both sintered compacts retained their pores.

メカニカルアロイングによるＦｅ‐４０ａｔ％Ａｌ＋ＷＣ粉の合成とその成形体の機械的特性

Elemental powders of iron, aluminum and tungsten carbide powders has been used as starting materials for mechanical alloying and mixed to give the desired composition of Fe-40at%Al+50, 65vol%WC. The powder mixtures were milled for 360 ks using a planetary ball mill. Milled powders have been consolidated using pulse current sintering process at 1373 K in vacuum. The compacts thus obtained consisted of FeAl and WC. The FeAl-WC has had almost the same vickers hardness as the specimen prepared using milled Fe-40at%Al powder and WC powder in the previous study. However, the transverse rupture strength of FeAl-WC prepared in this study was inferior to the latter process. This is due to the different microstructure of FeAl-WC prepared by the different process.

Fabrication and Output Evaluation of Si-Ge Multilayered Thermoelectric Element

A Si0.8Ge0.2 multilayered thermoelectric element with ten pairs of p-n layers was fabricated by simultaneous sintering and joining of Si-Ge powders with a sandwiched alumina cloth. Every p-n joint of the element was firmly connected without any cracks and the alumina layer inserted here remained unchanged even after a pulse-current sintering process. The maximum output, Pmax, of this element was determined in the temperature range of 473-1273 K, and then increased in proportion to the second order of the temperature difference between the hot and cold ends of the element. As a result, Pmax was 120 mW when the temperature difference was 500 K.

Preparation and Characterization of Nano-Crystalline Ti-2 at%Fe-10 at%Si Alloy by Mechanical Alloying and Pulsed Current Sintering Process

Ti-2 at%Fe-10 at%Si amorphous powder was synthesized by mechanical alloying (MA) of elementary Ti, Fe and Si powders using planetary ball milling for 1440 ks. Crystallization temperature of the obtained amorphous powder was ca. 830 K. Ti-2 at%Fe-10 at%Si amorphous powder was consolidated using a pulsed current sintering (PCS) process at a die temperature of 773 K under a pressure of 372 MPa. Then the compact was densified at a die temperature between 743 K and 763 K, which is around the crystallization temperature of the amorphous phase. The obtained Ti-2 at%Fe-10 at%Si alloy consists mainly of nanocrystalline α-Ti phase. The compressive strength of the nanocrystalline compact at room temperature was more than 1.7 GPa, much higher than that of a commercial Ti-6 mass%Al-4 mass%V alloy.

XPS Analysis of Dopant Penetration in Joined p-n Silicon-Germanium Semiconductor

The penetration behavior of dopants, boron and phosphorous, across a p-n junction in a joined silicon-germanium semiconductor, which had been applied to a thermoelectric element, was elucidated by X-ray photoelectron spectroscopic (XPS) analysis. The p-n joined disk sample was prepared from Si, Ge and B (79.7:20.1:0.21 in atomic ratio for p-type semiconductor), and Si, Ge and P (79.8:20.0:0.18 for n-type) powders one on the top of the other at 1523 K for 180 s under 49 MPa by a pulse current sintering process. Then, the XPS measurement and surface grinding of the joined sample were carried out alternately, which gave a series of photoelectron spectra as a function of distance from the p-n junction. The binding energy differences for Si2s and Ge3d peaks between the p-type and the n-type sides were 0.5 and 0.6 eV, respectively. An interlayer with a thickness of approximately 2 mm was found just in the p-type side judging from the above energy differences. The binding energies inside the interlayer along the thickness varied gradually between the original value of p-type Si-Ge and that of the n-type. It was considered that both the dopants coexisted in the interlayer, caused by the migration of phosphorous of the n-type side into the p-type side. The long-distance migration of phosphorous more likely occurred according to the vaporization-condensation mechanism. XPS analysis was proved to be most effective for the characterization of the p-n junction of semiconductors.

ＭＡ‐ＰＣＳプロセスによるターゲットを用いた（Ｂｉ２Ｔｅ３）０．２５（Ｓｂ２Ｔｅ３）０．７５熱電薄膜の作製

A p-type thermoelectric film of (Bi2Te3)0.25(Sb2Te3)0.75 was fabricated by radio frequency sputtering. The target for the sputtering was prepared by mechanical alloying (MA) and pulsed current sintering (PCS) process. The single phase of Bi0.5Sb1.5Te3 was obtained by planetary ball milling Bi powder, Te powder and Sb powder for 36ks. The obtained powder was consolidated by PCS into the cylindrical target of 50 mm in diameter and 4mm in thickness at 643 K under a pressure of 13 MPa. The sintered body consisted of Bi0.5Sb1.5Te3 without cracks. The sintered Bi0.5Sb1.5Te3 was joined to Cu backing plate with In for an insertion by PCS process. The thin film of (Bi2Te3)0.25(Sb2Te3)0.75 was fabricated on a polyimide substrate by RF sputtering. The thickness of this thin film was about 15-20μm. This film was amorphous state and crystallized at about 520 K. The performance of this p-type thermoelectric film after the crystallization were 2.0mV in voltage and 2.9μA in current at 20 K of the difference between heating side temperature 353 K and cooling side temperature 333 K.

ＦｅＡｌ‐ＷＣの組織と機械的特性に与えるＢ添加の効果

Effect of boron addition on structure and mechanical properties of FeAl-WC have been investigated. Elemental powders of iron, aluminum and boron powders have been used as starting materials for MA and mixed to give the desired composition of Fe-40at%Al+0-5 at%B. Boron addition leads to fine milled powders and improves mechanical properties of Fe-40at%Al compacts. Milled Fe-40 at%Al powders with 3at% B have been blended with WC powders and consolidated using pulse current sintering process at 1373-1473 K. 3at%B-added FeAI-50vol%WC has had high vickers hardness 1230 Hv and transverse rupture strength 1.77 GPa, while no B-added FeAl-50vol% WC has had 1080 Hv and 1.10 GPa respectively. This is caused by improvement of dispersion of WC paricles in FeAl matrix.

成形・加工のプロセスとその周辺技術 Ｔｉ‐Ｃｒ合金のメカニカルアロイングによる合成とその固化成形

Mg-10at%B, Mg-20at%B, Mg-60at%B, Mg-75at%B and Mg-20at%B-5at%C were synthesized by mechanical alloying (MA) of Mg powder, boron powder and graphite powder using a vibrational ball milling in argon gas atmosphere for 720ks. Mg-10at%B, like Mg, was agglomerated into a mass by continuous rolling during MA process. Boron addition was effect to produce powder, but the contamination of Fe from the vessel and balls increased with increasing boron content in the MA powder. Since Mg-20at%B synthesized by MA was very fine powder, it was able to consolidate at 573K under a pressure of 195MPa by pulsed current sintering process. The sintered body of Mg-20at%B has a homogeneous microstructure, a density of 1.82g/cm3 and a hardness of 90Hv. In case of Mg-20at%B-5at%C, hard particles were obtained during the MA process and changed into other phase during the sintering process.

放電焼結によるＴｉＮｉ形状記憶合金の作製

Ti-Ni shape memory alloys were fabricated by a pulse current sintering process from two types of starting powders prepared through different processes: mechanical alloying of TiH and Ni powders mixture and dehydrogenation(MA powder), and pre-mixed of Ti and Ni powders (mixed powder).The MA powder was composed of mainly TiNi phase and a small amout of Ti2Ni and TiNi3 phases. The composition of this powder remained unchanged after the pulse current sintering at 973-1173K for 300s, while the relative density of 100% was attained for the sintering body. On the contrary, the pulse current sintering of the mixed powder at 1273K for 60-300s, led to a distinguished compositional change and the relative density of 100%. The TiNi3 phase in the sintered body could be removed on the condition of longer sintering period or by heat treatment at 1273K for 18ks; however, the Ti2Ni phase could not be removed. The composition and the thermal characteristics (Ms=318K) of these sintered bodies were found to be almost the same as those of commercially available Ti-Ni shape memory alloys.