Plasma Activated Sintering Method Research Articles

Effect of Al<sub>2</sub>O<sub>3</sub> on Microstructure and Ionic Conductivity of Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub><sub>12 </sub></sub> Solid Electrolytes Prepared by Plasma Activated Sintering

High performance Al-doped Li7La3Zr2O12 (LLZO) solid electrolytes were successfully prepared by plasma activated sintering method. The effect of Al2O3 concentration on the microstructure and ionic conductivity of Li7La3Zr2O12 was investigated. Without addition of Al2O3, only tetragonal phase was obtained at temperature range of 1000-1200 °C. Pure cubic phase of LLZO was obtained with the addition of 1.2-1.8 wt.% Al2O3. The pellet sintered at 1150 °C with 1.5 wt.% content of Al2O3 had a maximum relative density of 99.8 % with total ionic conductivity of 5.7×10−4 S/cm at room temperature. Al2O3 can stabilize the cubic LLZO phase which promotes the transformation of LLZO from tetragonal to cubic phase.

Development of an Mg2Si Unileg Thermoelectric Module Using Durable Sb-Doped Mg2Si Legs

Mg2Si unileg structure thermoelectric (TE) modules, which are composed only of n-type Mg2Si legs, were fabricated using Sb-doped Mg2Si. The Mg2Si TE legs used in our module were fabricated by a plasma-activated sintering method using material produced from molten commercial doped polycrystalline Mg2Si, and, at the same time, nickel electrodes were formed on the Mg2Si using a monobloc plasma-activated sintering technique. The source material used for our legs has a ZT value of 0.77 at 862 K. The TE modules, which have dimensions of 21 mm × 30 mm × 16 mm, were composed of ten legs that were connected in series electrically using nickel terminals, and the dimensions of a single leg were 4.0 mm × 4.0 mm × 10 mm. From evaluations of the measured output characteristics of the modules, it appeared that the electrical resistance of the wiring that is used to connect each leg considerably affects the power output of the unileg module. Thus, we attempted to reduce the wiring resistance of the module and fabricated a module using copper terminals. The observed values of the open-circuit voltage and output power of the Sb-doped Mg2Si unileg module were 496 mV and 1211 mW at ΔT = 531 K (hot side: 873 K; cool side: 342 K).

Fabrication of large sintered pellets of Sb-doped n-type Mg2Si using a plasma activated sintering method

Sb is known to be the most stable n-type dopant in Mg2Si, whereas the reproducibility and sintering scalability of Sb-doped Mg2Si has proved to be difficult. We have developed a metallic binder for the Sb-doped Mg2Si sintering process. A remarkable benefit of the binder is that it enables us to reproducibly fabricate large sintered Sb-doped Mg2Si pellets up to 30mm in diameter with no internal cracks. For binder contents of 3–7wt%, the observed power factor and thermal conductivity were identical to that of conventional Sb-doped samples, while excess binder degraded the TE properties. The binder mixture also plays a part in increasing the yield in the production of TE legs, up to 82% for 5wt% Ni incorporation, as well as improving the hardness of the sintered sample. Incorporation of 5wt% Ni binder affected an increase in the ZT value up to 0.97.

Power Generation Characteristics of Mg2Si Uni-Leg Thermoelectric Generator

Mg2Si thermoelectric (TE) elements were fabricated by a plasma-activated sintering method using a commercial polycrystalline n-type Mg2Si source produced by the Union Material Co., Ltd. This material typically has a ZT value of ∼0.6. A monobloc plasma-activated sintering technique was used to form Ni electrodes on the TE elements. The dimensions of a single element were 4.0 mm × 4.0 mm × 10 mm, and these were used to construct a TE module comprising nine elements connected in series. To reduce the electrical and thermal contact resistance of the module, each part of the module, i.e., the elements, terminals, and insulating plates, was joined using a Ag-based brazing alloy. In addition, to maintain the temperature difference between the top and bottom of the module, a thermal insulation board was installed in it. The observed values of open-circuit voltage (VOC) and output power (P) of a uni-leg structure module were 594 mV and 543 mW, respectively, at a maximum ΔT = 500 K.

Fabrication and characterizations of new nanocomposite WC/Al 2O 3 materials by room temperature ball milling and subsequent consolidation

Reactant material powders of pure WO 3, Al and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces which are generated by the balls. After 43 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Al and WO 3 particles to form a product of nanocrystalline mixture of Al 2O 3 and W powders. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball milling (360 ks), the nanocrystalline WC grains (5 nm) are embedded into the fine matrix of Al 2O 3 to form fine nanocomposite powders (less than 1 μm in diameter) of WC–32 at.% Al 2O 3 material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both the consolidated samples have been investigated. A model for fabrication of new refractory nanocomposite WC/Al 2O 3 materials at room temperature is proposed.

PAS consolidating behavior of mechanically alloyed nanocrystalline NiAl intermetallic compound

In this study, nanocrystalline NiAl intermetallic compound was obtained by mechanical alloying and PAS (plasma activated sintering method). Nanocrystalline NiAl powder was fabricated after 30 hr of milling with 2 wt.% stearic acid added as a PCA (process control agent) to the Ni-50at%Al composition. The grain size of the nanocrystalline NiAl powder was about 10 nm. Nanocrystalline NiAl powder was consolidated at 1000°C, 1100°C, 1200°C and 1300°C for 2 min with 30 MPa compressive force. The surface morphology of the NiAl consolidated at 1300°C was very regular and dense, above 96% of theoretical density (5.9 g/cm3). Al4C3 was observed in the NiAl consolidated at 1300°C by TEM analysis. It is thought that the carbons came from the stearic acid during the MA process and the graphite mold during the PAS process. The grain size of the NiAl consolidated at 1300°C did not increase but the grain shape became flat due to compressive force.

燃焼合成法で作製しプラズマ活性化焼結したＴｉＡｌ‐Ｍｎ材の酸化特性

The combustion synthesis of the material mainly composed of γ-TiAl as a promising candidate of a light-weight high-temperature structural material is carried out. Moreover, the plasma activated sintering method is adopted as a means to make high density material, and the validity of the combination of this method and the combustion synthesis process is investigated. The oxidation is deteriorated though Mn is added to improve mechanical properties of TiAl. Then, the TiAl-Mn material manufactured by the combustion synthesis process is sintered by the plasma activated sintering method. The oxidation examination is carried out, and the feature of the combustion synthesis process is investigated compared with the conventional production process. The material manufactured by the combustion synthesis process put out the result more than the equal when made by a conventional arc melting method. Moreover, without using a very high-level process such as HIP, the good result with the sample which was able to be sintered in simple plasma activated sintering method is obtained. The usage of the combustion synthesis process can be expected to extend further.

Synthesizing of nanocomposite WC/MgO powders by mechanical solid-state reduction and subsequent plasma-activated sintering

A ceramic/ceramic nanocomposite powder of WC/MgO has been fabricated by high-energy ball milling a mixture of elemental Mg and powders of C with WO3 under an argon gas atmosphere at room temperature. During the early stage of milling (at 1.8 ks), the WO3 and C powders are embedded into the soft matrix of Mg (the reducing agent) particles to form coarse composite powders of the reactant materials. Increasing the milling time (to 22 ks) leads to the formation of fresh active surfaces of Mg, which have a high reducing potential and react with the WO3 in a typical oxidation/reduction reaction. At the end of this stage (at 43 ks), the Mg powders are oxidized to MgO, whereas the WO3 is reduced completely to metallic W. During the last stage of milling (86 to 173 ks), a solid-state reaction takes place between W and the unreacted C powders to yield nanocomposite WC/MgO particles. This end-product was consolidated in vacuum at 1963 K with a pressure of 19.6 to 38.2 MPa for 0.3 ks, using a plasma-activated sintering (PAS) method. The sintered sample is fully dense (above 99.5 pct of the theoretical density) and contains nanocrystalline grains of less than 50 nm in diameter. This fine grain structure offers an opportunity for the composite material to combine high values of two opposite properties, i.e., hardness and fracture toughness (Kc), of 15 GPa and 14 MPa √m, respectively. Here, we propose this nanocomposite material for a wide range of industrial applications, including tips for cutting tools and tips for oil drilling equipment.

Structure and properties of nanocrystalline TiC full-density bulk alloy consolidated from mechanically reacted powders

High-energy ball milling has been successfully employed for synthesizing nanocrystalline powders of Ti 44C 56. The milling procedure involves milling of elemental Ti and C powders at room temperature in an argon gas atmosphere. The progress of the mechanically induced solid state reaction was monitored by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy and/or high resolution transmission electron microscopy at several stages of the milling time. A single phase of NaCl-type TiC is obtained after 22 ks of milling time. No free Ti and/or C crystals could be detected at this stage of milling. Increasing the milling time leads to a decrease in the grain size of the powders to <50 nm in diameter after 40 ks of milling time. Towards the end of the milling processing time (720 ks) the powders possess excellent morphological characteristics, such as a homogeneous shape (spherical-like morphology) with fine and smooth surface relief and uniform size (<1 μm in diameter). The lattice parameter of this end-product was calculated to be 0.4326 nm. In addition, the powders at this final stage of milling consist of nanocrystalline grains (<5 nm in diameter) with cell-like morphology. In order to determine some physical and mechanical properties of the synthesized TiC material, different samples at different milling times were consolidated into bulk samples, using a plasma activated sintering method. The powders of the final-product (720 ks) has a density of 5.21 g/cm 3, being nearly the theoretical density of TiC. In addition, this bulk sample maintains its unique structure characteristics with nanocrystalline grains of <60 nm in diameter. On the basis of the results of the present study, the ball-milling technique accompanied with plasma activated sintering can provide powerful tools for the fabrication of nanocrystalline TiC bulk alloys with unique and advanced properties.

Fabrication of nanocrystalline WC and nanocomposite WC–MgO refractory materials at room temperature

Reactant material powders of pure WO3, Mg and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces generated by the balls. After 86 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Mg and WO3 particles to form a product of nanocrystalline mixture of MgO and W. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball-milling (173 ks), the nanocrystalline MgO grains (10 nm) are embedded into the fine matrix of WC to form fine nanocomposite powders (1 μm in diameter) of WC–18% MgO material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. In addition, pure nanocrystalline WC powders (7 nm in diameter) obtained by removing the MgO from the milled powders, using a simple leaching technique have been also consolidated by the same consolidation technique. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both consolidated samples have been investigated. A model for fabrication of refractory nanocrystalline WC and nanocomposite WC–18% MgO materials at room temperature is proposed.

Mechanical solid state mixing for synthesizing of SiC p/Al nanocomposites

High-energy ball milling was successfully employed to fabricate the metal matrix composite (MMC) of Al reinforced with SiC particulate (SiC p) with distinct nanocrystalline characteristics. The powders of the end-products (86 ks of milling time) of the mechanically-mixed nanocomposites SiC x Al 100− x ( x=2, 5, 7, 10 vol. %) that have spherical-like morphology (less than 0.5 μm in diameter) were consolidated into bulk compacts, using a plasma activated sintering method. Density measurements show that this consolidation step leads to the formation of fully dense bulk samples. Moreover, the consolidated SiC x Al 100− x samples reveal a nanocrystalline structure with a homogeneous reinforcement distribution of SiC p in the Al matrix. Reactive products, i.e. Al 4C 3 and Si, could not be detected on the interfaces between the reinforcement and the matrix in both mechanically mixed powders and consolidated samples. The mechanically mixed powders and consolidated compacts have been characterized by means of X-ray diffraction, scanning and transmission electron microscopes, optical metallography and chemical analyses. The hardness and some mechanical properties of SiC p/Al have been determined as a function of SiC p content.

Ag Coated SiC/YBCO Composite Prepared by the Plasma Activated Sintering Method

Abstract YBCO/Ag coated SiC composites containing 0.5, 1.0 and 3.0 wt% whisker were prepared by the conventional plasma activated sintering method (PAS) at 973-1,103 K. SiC whisker was covered with Ag thin film by electroless coating method to suppress interfacial reaction between superconductor and SiC whisker. It was shown that superconducting transition occured at 90 K in the specimen with 3 wt% of Ag coated SiC whisker.

Plasma Activated Sintering for Consolidation of Mechanically Reacted TiN Powder.

Nanocrystalline titanium nitride (Ti56N44) powder with an average grain size of 5 nm has been synthesized by ball-milling elemental Ti powder under flow of nitrogen gas at room temperature. During the first stage of the reactive ball milling time (0 ks-3.6 ks), the metallic Ti particles agglomerate to form powder particles with larger diameter. At the second stage (3.6 ks-22 ks), these agglomerated particles are disintegrated to form smaller particles. The disintegrated particles which have fresh surfaces begin to react with nitrogen (milling atmosphere) during the third stage of milling (22 ks-86 ks) to form TiN powder coexisting with unreacted metallic Ti powder. Towards the end of milling (86 ks-173 ks), a single phase of nanocrystalline TiN with NaCI-structure was obtained. The powder of this end product has spherical like morphology with an average particle size of about 0.4 μm in diameter. A consolidation procedure using a plasma activated sintering method has been employed to consolidate the powder particles at the several stages of the reactive ball milling. The density measurements of the consolidated samples show that after 86 ks to 173 ks of the reactive ball milling time, the compacted samples are fully dense. The results have shown also that the consolidated TiN compacts still maintain their unique nanocrystalline properties with an average grain size of about 65 run. The hardness and some mechanical properties of the consolidated TiN compacts have been determined as a function of the reactive ball milling time.

Consolidation of Ball-milled (Al-TM)-SiC (TM; Ti,Mo and Cu) Composite Powders by Plasma Activated.

Elemental powders of Al (79 vol.%), TM (TM; Ti, Mo and Cu) (1 vol.%) and J3-SiC (20 vol.%) compound were mechanically alloyed using ball-milled technique. The end-product which is a composite Al-SiC-TM nanocrystalline powders was consolidated into compact (20 mm in diameter) using the plasma activated sintering method. The as-milled and as consolidated samples were characterized by means of X-ray diffraction, transmission electron microscopy, scanning electron microscopy and chemical analysis. Moreover, the hardness of the compacted sample was determined using a Vickers indenter with a load of 10 kg, and found to be 2.27 GPa. In addition, the density of the consolidated sample was determined by Archimedes' principle using water immersion and found to be 3×103 kg.m-3. The effect of the TM elements on the mechanical properties (Poisson's ratio, Young's modulus and shear modulus) of Al-SiC bulk composite has been studied.

Formation of Refractory WC Compound by Mechanical Solid State Reduction.

A new process for preparation of nanocrystalline WC powder at room temperature is reported. In our process, a mixture of WO3, Mg and graphite powders are mechanically reacted under an argon gas atmosphere using a high energy ball mill. After few kiloseconds of milling (11 ks), numerous fresh surfaces of the powder are created by the repeated impact and shear forces of the balls. After 86 ks, the fresh Mg particles that have high reducing potential are oxidized, whereas the WO3 is reduced completely to a metallic W powder. In parallel, a solid state reaction between W and the unreacted graphite powders takes place to yield a fine powder of WC. At the final stage of milling (173 ks), composite WC/MgO powder particles is obtained. The MgO is removed from the matrix of WC using a leaching technique. The residual WC powder was then consolidated in vacuum at 1963 K with a pressure of 19.6 - 38.2 MPa for 0.3 ks, using a plasma activated sintering method. No binding material was used during the consolidation procedure. Under these consolidation conditions, the obtained sample is fully dense (>99.5 % theoretical density) with nanoscale grain size of about 25 nm in diameter. The elastic properties and the hardness of the consolidated sample have been determined. A model for the formation of WC at room temperature is proposed.

Fabrication of Titanium Blocks from Commercial Pure Titanium Powder by a Plasma Activated Sintering Method

Fabrication of Titanium Blocks from Commercial Pure Titanium Powder by a Plasma Activated Sintering Method