Plasma Activated Sintering Research Articles

Panoscopic approach for high-performance Te-doped skutterudite

One-step plasma-activated sintering (OS-PAS) fabrication of single-phase high-performance CoSb3-based skutterudite thermoelectric material with a hierarchical structure on a time scale of a few minutes is first reported here. The formation mechanism of the CoSb3 phase and the effects of the current and pressure fields on the phase transformation and microstructure evolution are studied in the one-step PAS process. The application of the panoscopic approach to this system and its effect on the transport properties are investigated. The results show that the hierarchical structure forms during the formation of the skutterudite phase under the effects of both current and sintering pressure. The samples fabricated by the OS-PAS technique have defined hierarchical structures, which scatter phonons more intensely over a broader range of frequencies and significantly reduce the lattice thermal conductivity. High-performance bulk Te-doped skutterudite with the maximum ZT of 1.1 at 820 K for the composition CoSb2.875Te0.125 was obtained. Such high ZT values rival those obtained from single filled skutterudites. This newly developed OS-PAS technique enhances the thermoelectric performance, dramatically shortens the synthesis period and provides a facile method for obtaining hierarchical thermoelectric materials on a large scale.

Effect of composition fluctuation on structural and electrical properties of BZT-xBCT ceramics prepared by Plasma Activated Sintering

Lead-free [Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3, BZT-xBCT] ceramics were prepared by Plasma Activated Sintering (PAS). The effect of composition fluctuation on structural and electrical properties of BZT-xBCT ceramics was investigated. It was found PAS can obtain compact BZT-xBCT ceramics and BZT-0.50BCT ceramic possessed a MPB structure consisting of rhombohedral and tetragonal phases. Impedance spectroscopy confirmed two contributions from grains and grain boundaries, and the ceramics showed typical characteristic of diffuse phase transition. As compared with the BZT-xBCT ceramics with the pseudo-cubic phase, PAS-ed BZT-0.50BCT ceramic exhibited better properties of d33=127 pC/N, kp=25.1%, εr=2614, εm=6655, and 2Pr=25.2μC/cm2 due to the existence of the MPB structure.

Nonmagnetic In Substituted CuFe1–xInxS2 Solid Solution Thermoelectric

CuFeS2 is an environmentally friendly n-type thermoelectric material composed of earth-abundant, inexpensive and nontoxic elements. However, its rather undistinguished electronic properties combined with a high thermal conductivity lead to a low thermoelectric performance. In this work, an attempt is made to reduce the lattice thermal conductivity of CuFeS2 by In substituting on the Fe site. A series of CuFe1–xInxS2 (x = 0–0.08) compounds was synthesized by vacuum melting combined with the plasma activated sintering (PAS) process, and the effect of substituting In atoms on the band structure and thermoelectric properties of CuFeS2 has been investigated. The results show that the solubility limit of In in CuFeS2 is more than 8%. For the In content of 0.08, the lattice thermal conductivity of room temperature and at 630 K was reduced by 60% and 37%, respectively, indicating that substituting In for Fe is an effective method to reduce the lattice thermal conductivity of CuFeS2. A single parabolic band model ...

Thermoelectric properties of Bi2Te2.7Se0.3 nanocomposites embedded with MgO nanoparticles

Bi2Te2.7Se0.3 bulk materials containing x vol% MgO nanoparticles (average particle size ≈ 100 nm, x = 0, 0.5, 1.0, 1.5) were synthesized by using high-energy ball milling and plasma- activated sintering (PAS) without any special process for nanoparticle dispersion. A microstructure investigation using a scanning electron microscope (SEM) confirmed that MgO nanoparticles were properly dispersed in the Bi2Te2.7Se0.3 matrix and that the grain size was smaller in MgO-containing samples due to suppressed grain growth. The resistivity and the maximum Seebeck coefficient of Bi2Te2.7Se0.3 increased with increasing MgO content whereas the thermal conductivity decreased in the measurement temperature range of 298 K - 573 K. As a result, the maximum dimensionless figure of merit, ZTmax, increased about 8.5% in this study, from 0.806 for pristine Bi2Te2.7Se0.3 to 0.875 when x = 1.5. The ZTmax was observed to shift to lower temperature, the electron concentration to decrease, and the electron mobility to increase with increasing x, which were explained using a hypothesis that the TeBi antisite defect concentration decreased as the MgO content increased. In summary, the addition of MgO nanoparticles has been shown to be a simple and effective method to improve the low-temperature thermoelectric properties of n-type Bi2Te3 materials.

Structural, dielectric and ferroelectric properties of lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramics prepared by Plasma Activated Sintering

Lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) ceramics were prepared by Plasma Activated Sintering (PAS). The influence of PAS sintering temperature on the crystalline phase, microstructure, and, dielectric and ferroelectric properties of BCZT ceramics were studied. The phase structure of BCZT ceramics first changed from rhombohedral phase to the coexistence of rhombohedral and tetragonal phases and then to tetragonal phase as the sintering temperature increased. Microstructural characterization of BCZT ceramics indicated that PAS can obtain a compact microstructure at lower temperatures of 1150–1300°C compared with that from common pressureless sintering. The BCZT ceramics showed different degrees of diffuseness with increased temperature, and the diffuseness exponents C are all approximately on the order of 105°C. The dielectric and ferroelectric properties of BCZT ceramics were enhanced with increased sintering temperature. BCZT ceramics sintered at 1250°C exhibited optimum properties of room-temperature εr=2863, εm=6650, and 2Pr=25.24μC/cm2, resulting from the relatively higher tetragonal phase content of the MPB between tetragonal and rhombohedral phases together with a compact microstructure.

Fabrication and mechanical behavior of porous Cu via chemical de-alloying of Cu25Fe75 alloys

We report on a study of the synthesis and deformation of bulk porous Cu processed via plasma activated sintering (PAS) of Cu-Fe precursor alloys. The process used in our work involved both low temperature (600–900 °C) and low pressure (50 MPa) followed by chemical de-alloying of Cu-Fe in a solution of 5 wt% H2SO4. Our results show that when this approach is applied to a Cu-75 at.% Fe alloy, de-alloying of the active Fe phase leads to the formation of micron-sized 1–4 μm pores and approximately 73 vol% porosity. Our results show that the microstructure of the porous Cu principally depends on the morphology of the precursor Cu25Fe75. Moreover, studies of the relationship between the microstructure and compressive deformation behavior of porous Cu reveal that the mechanical strength increases with decreasing pore size in the linear elastic and plateau regions, but decreases with decreasing pore size in the densification region. The stress level corresponding to the plateau stage in the range of 16.11–18.11 MPa corresponding to a strain of 30%.

Rapid diffusion bonding of WC-Co cemented carbide to 40Cr steel with Ni interlayer: Effect of surface roughness and interlayer thickness

WC-Co cemented carbides were rapidly diffusion bonded to 40Cr steels with pure Ni as interlayers by utilizing plasma activated sintering (PAS). The bonding was carried out at 750°C for 13min under a pressure of 40MPa. It was found that the roughness of the initial surfaces still plays an important effect on the microstructure and mechanical behavior of the joints diffusion bonded by PAS irrespective of the electric current applied during bonding. The adoption of smoother original surfaces was significantly favorable to eliminate the interfacial interstices and microvoids. Correspondingly, the shear strength of the diffusion bonded joints increased with decreasing surface roughness. Additionally, the effect of interlayer thickness on the shear strength of the joints was also evaluated, and the results showed that the strength decreased sharply when thicker interlayer was employed. A maximum value of shear strength, 293.07MPa, was obtained when the original surfaces was ground with P1200 grit SiC paper and at the same time 50µm thick interlayer was used. In this case, the fracture initiated and run predominantly along the bonding interfaces instead of in the WC-Co substrate.

Toward High‐Thermoelectric‐Performance Large‐Size Nanostructured BiSbTe Alloys via Optimization of Sintering‐Temperature Distribution

High thermoelectric performance of mechanically robust p‐type Bi2Te3‐based materials prepared by melt spinning (MS) combined with plasma‐activated sintering (PAS) method can be obtained with small, laboratory grown samples. However, large‐size samples are required for commercial applications. Here, large‐size p‐type Bi2Te3‐based ingots with 30, 40, and 60 mm in diameter are produced by MS‐PAS, and the influence of temperature distribution during the sintering process on the composition and thermoelectric properties is systematically studied for the first time. Room‐temperature scanning Seebeck Microprobe results show that the large‐size ingot is inhomogeneous, induced by ellipsoidal‐shape‐distributed temperature field during the sintering process, which is verified by finite‐element analysis. Although some temperature differences are unavoidable in the sintering process, homogeneity and mechanical properties of ingots can be improved by appropriately extending the sintering time and design of graphite die. Samples cut from ingots attain the peak ZT value of 1.15 at 373 K, about 17% enhancement over commercial zone‐melted samples. Moreover, the compressive and bending strengths are improved by several times as well. It is important to ascertain that large‐size p‐type Bi2Te3‐based thermoelectric materials with high thermoelectric performance can be fabricated by MS‐PAS.

Microstructure and thermoelectric properties of Sb doped Hf0.25Zr0.75NiSn Half-Heusler compounds with improved carrier mobility

Half-Heusler (HH) semiconductor alloys are being widely investigated due to their promising potential for thermoelectric (TE) power generation applications. Sb is an effective doping element for n-type ZrNiSn half-Heuslers alloys. HH thermoelectric materials Hf0.25Zr0.75NiSn1−xSbx (0 ≤ x ≤ 0.03) were synthesized by induction melting combined with plasma activated sintering (PAS) technique. X-ray diffraction concluded that single-phase HH compounds without compositional segregations were obtained. Presence of bended lamellar structures was revealed by the FESEM. Sb doping significantly enhanced the electrical conductivity, power factor and carrier concentration of the alloys. An increase in the carrier mobility was also observed. Consequently, optimum values of 4.36 × 10−3 W/mK2 and 4.7 × 1020 cm−3 were achieved for power factor and carrier concentration, respectively. As a result, a ZT value of 0.83 at 923 K was obtained which is about 67% improvement compared to the un-doped sample.

Synthesis of ultra-fine hafnium carbide powders combining the methods of liquid precursor conversion and plasma activated sintering

Ultra-fine hafnium carbide (HfC) powders were synthesized using a novel method combining liquid precursor conversion and plasma activated sintering (PAS). Solution-based processing was used to achieve a fine-scale mixing of the reactants, and further treatment by PAS allowed fast formation of HfC. We investigated the effect of the type of acid used during the liquid precursor conversion on the synthesized powders, where mixtures were prepared using salicylic acid, citric acid, or a combination of these. The results show that pure HfC powders (with an average particle sizes of 350nm) were obtained at a relatively low temperature (1550°C) using a HfOCl2·8H2O precursor with the mixed acids. The oxygen content of the synthesized powders was only 0.97wt%. The type of acid had a significant effect on the synthesis product. When using only citric acid, the temperature required to produce pure hafnium carbide increased to 1700°C. In the case of a salicylic acid precursor, pure HfC was not obtained, even at a synthesis temperature of 1700°C.

Phase Segregation and Superior Thermoelectric Properties of Mg2Si(1-x)Sb(x) (0 ≤ x ≤ 0.025) Prepared by Ultrafast Self-Propagating High-Temperature Synthesis.

A series of Sb-doped Mg2Si(1-x)Sb(x) compounds with the Sb content x within 0 ≤ x ≤ 0.025 were prepared by self-propagating high-temperature synthesis (SHS) combined with plasma activated sintering (PAS) method in less than 20 min. Thermodynamic parameters of the SHS process, such as adiabatic temperature, ignition temperature, combustion temperature, and propagation speed of the combustion wave, were determined for the first time. Nanoprecipitates were observed for the samples doped with Sb. Thermoelectric properties were characterized in the temperature range of 300-875 K. With the increasing content of Sb, the electrical conductivity σ rises markedly while the Seebeck coefficient α decreases, which is attributed to the increase in carrier concentration. The carrier mobility μ(H) decreases slightly with the increasing carrier concentration but remains larger than the Sb-doped samples prepared by other methods, which is ascribed to the self-purification process associated with the SHS synthesis. In spite of the increasing electrical conductivity with the increasing Sb content x, the overall thermal conductivity κ decreases on account of a significantly falled lattice thermal conductivity κ(L) due to the strong point defect scattering on Sb impurities and possibly enhanced interface scattering on nanoprecipitates. As a result, the sample with x = 0.02 achieves the thermoelectric figure of merit ZT ∼ 0.65 at 873 K, one of the highest values for the Sb-doped binary Mg2Si compounds investigated so far. A subsequent annealing treatment on the sample with x = 0.02 at 773 K for 7 days has resulted in no noticeble changes in the thermoelectric transport properties, indicating an excellent thermal stability of the compounds prepared by the SHS method. Therefore, SHS method can serve as an effective alternative fabrication route to synthesize Mg-Si based themoelectrics and some other functional materials due to the resulting high performance, perfect thermal stability, and feasible production in large scale for commercial application.

In situ Investigation of Titanium Powder Microwave Sintering by Synchrotron Radiation Computed Tomography

In this study, synchrotron radiation computed tomography was applied to investigate the mechanisms of titanium powder microwave sintering in situ. On the basis of reconstructed images, we observed that the sintering described in this study differs from conventional sintering in terms of particle smoothing, rounding, and short-term growth. Contacted particles were also isolated. The kinetic curves of sintering neck growth and particle surface area were obtained and compared with those of other microwave-sintered metals to examine the interaction mechanisms between mass and microwave fields. Results show that sintering neck growth accelerated from the intermediate period; however, this finding is inconsistent with that of aluminum powder microwave sintering described in previous work. The free surface areas of the particles were also quantitatively analyzed. In addition to the eddy current loss in metal particles, other heating mechanisms, including dielectric loss, interfacial polarization effect, and local plasma-activated sintering, contributed to sintering neck growth. Thermal and non-thermal effects possibly accelerated the sintering neck growth of titanium. This study provides a useful reference of further research on interaction mechanisms between mass and microwave fields during microwave sintering.

SiC 粒子を添加した熱電変換材料 Mg<sub>2</sub>Si の高靱化

Thermoelectric conversion technology is expected as one of the next generation power-generating technologies. Among thermoelectric materials (TE), Magnesium silicide (Mg2Si) has advantages such as lightweight, low environmental load, abundance of elements, and high power generation efficiency at the temperature range from 600 to 900K. For automotive applications, TE modules are exposed to external force such as mechanical vibration and thermal stress in severe thermal environment. Among mechanical properties, improvement of fracture toughness is essential because that of Mg2Si is about 1.2 MPa√𝑚. In this study, we fabricated the SiC particle-dispersed Mg2Si by the Plasma Activated Sintering (PAS) method in order to increase toughness. The Young's modulus was measured by the ultrasonic pulse-echo method and the fracture toughness was measured by the indentation fracture (IF) method. SiC particles are located on the grain boundary of the Mg2Si. The Young's modulus of the composite increases with the amount of the SiC particles. The fracture toughness of 1.53 MPa√𝑚 is obtained for 5vol%, 2-3µm SiC addition and 1.68 MPa√𝑚 is obtained for 5vol%, 35nm SiC addition. Crack deflection at interface between Mg2Si/SiC acts efficiently.

Accelerated Bonding of Magnesium and Aluminum with a CuNi/Ag/CuNi Sandwich Interlayer by Plasma-Activated Sintering

Plasma-activated sintering (PAS) has been applied, for the first time, to join magnesium and aluminum using a CuNi/Ag/CuNi sandwich structural interlayer. A cleaning effect and high efficient plasma heating mode in PAS have contributed to forming a strong interfacial diffusion bond under low temperature 673 K (400 °C) and short dwell time (0.6 ks). The designed interlayer provides a diffusion barrier effect and an enhanced physical contact between the interfaces. Strong bonding has been achieved without forming the brittle Mg-Al intermetallics.

Microstructure and mechanical performance of Cu-SnO2-rGO based composites prepared by plasma activated sintering

A novel chemical technique combined with unique plasma activated sintering (PAS) was utilized to prepare consolidated copper matrix composites (CMCs) by adding Cu-SnO2-rGO layered micro powders as reinforced fillers into Cu matrix. The repeating Cu-SnO2-rGO structure was composed of inner dispersed reduced graphene oxide (rGO), SnO2 as intermedia and outer Cu coating. SnO2 was introduced to the surface of rGO sheets in order to prevent the graphene aggregation with SnO2 serving as spacer and to provide enough active sites for subsequent Cu deposition. This process can guarantee rGO sheets to sufficiently disperse and Cu nanoparticles to tightly and uniformly anchor on each layer of rGO by means of the SnO2 active sites as well as strictly control the reduction speed of Cu2+. The complete cover of Cu nanoparticles on rGO sheets thoroughly avoids direct contact among rGO layers. Hence, the repeating structure can simultaneously solve the wettability problem between rGO and Cu matrix as well as improve the bonding strength between rGO and Cu matrix at the well-bonded Cu-SnO2-rGO interface. The isolated rGO can effectively hinder the glide of dislocation at Cu-rGO interface and support the applied loads. Finally, the compressive strength of CMCs was enhanced when the strengthening efficiency reached up to 41.

Improvement of Thermoelectric Properties of Bi0.4Sb1.6Te3 with Addition of Nanoscale Zinc Oxide Particles

Bulk Bi0.4Sb1.6Te3 materials with ZnO nanoinclusions have been fabricated by mechanical alloying and plasma-activated sintering. The effects of ZnO addition on the microstructure and thermoelectric performance of the Bi0.4Sb1.6Te3 materials have been studied in detail. Scanning electron microscopy (SEM) observation confirms that the ZnO particles distribute in the Bi0.4Sb1.6Te3 matrix homogeneously until the content is over 0.5 wt.%. The resistivity and Seebeck coefficient of the samples increase slightly with the ZnO content, while the thermal conductivity is reduced effectively by addition of ZnO nanoparticles due to extra phonon scattering at ZnO nanoinclusions, and a maximum ZT of 1.21 is achieved in the sample with 0.3 wt.% ZnO at 373 K.

Enhanced Thermoelectric Properties of La-Doped ZrNiSn Half-Heusler Compound

The effect of La doping on ZrNiSn half-Heusler (HH) compound has been studied to explore the composition variation and structural modifications for improvement of its thermoelectric performance. A series of LaxZr1−xNiSn (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03) alloys were prepared by induction melting combined with plasma-activated sintering. Structural analysis using x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) confirmed the resulting material to be a composite of HH, NiZr, and La3Sn4-type phases. The volume fraction for the phases other than HH ranged from 1.5% to 25% with increasing La content, as estimated by Rietveld analysis. The solubility of La in ZrNiSn is estimated to be 1.5%. Point defects may play a significant role in carrier and phonon transport. Interestingly, the thermoelectric transport properties exhibited a considerable increase in electrical conductivity σ with La doping and a significant drop in thermal conductivity κ, leading to a thermoelectric figure of merit (ZT) of 0.53 at 923 K, representing an improvement of about 37% compared with the undoped sample.

Effect of Cr on Microstructure, Mechanical Properties, and Wear Behavior of In Situ 20 wt.%Al2O3/Fe-25Al Composites

In order to improve the room temperature ductility and high temperature strength of Fe-25Al alloys, in situ 20 wt.%Al2O3/Fe-25Al composites with 0, 1, 2, 3, 4, and 5 at.% Cr element contents (0-6.70 wt.%) were prepared by mechanical alloying inducing self-propagating reaction with subsequent plasma-activated sintering. Microstructures, room temperature hardness, flexural strength, fracture toughness, and compression property and wear behavior at mid-high temperatures of the sintered samples were tested and analyzed. The results showed that all the composites with Cr element addition had good microstructure with fine grain size and high relative density. The flexural strength and fracture toughness increased first and then decreased with increasing content of Cr. Especially, the composites with 3 at.% Cr had the highest flexural strength, highest fracture toughness, and best compressive properties during 298-1073 K, and the main fracture mechanism changed from brittle fracture at room temperature to plastic deformation and pullout of the second phase at 673 K. With Cr content increasing, the friction coefficients decreased and the wear rates increased because of decreasing hardness. The composites with 3 at.% Cr had excellent wear properties with lower friction coefficient and wear rate.

Enhanced power factor of Mg2Si0.3Sn0.7 synthesized by a non-equilibrium rapid solidification method

Sb-doped Mg 2 Si 0.3 Sn 0.7 was respectively prepared by melt spinning (MS) and by a two-step solid-state reaction (SSR), followed by plasma-activated sintering (PAS). The nonequilibrium MS technique greatly improves the structural homogeneity by rapidly quenching the homogeneous melt. As a consequence, the carrier mobility and electrical conductivity is much enhanced, leading to a record power factor of 5.18 × 10 −3 W m −1 K −2 at 600 K, a 15% improvement over the SSR–PAS sample, and a high figure of merit ZT ≈ 1.30 at 750 K.

Ultra-fast synthesis and thermoelectric properties of Te doped skutterudites

The self-propagating-high-temperature-synthesis (SHS) technique is applied here for the first time to synthesize CoSb3 thermoelectric materials. Mixtures of Co and Sb powders were compacted into pellets which were ignited from one end. A single-phase skutterudite material was obtained in a very short period of time using the SHS process which is maintained by the heat released from the chemical reaction of Co with Sb. Thermodynamic parameters and kinetics of the SHS reaction are investigated. The ignition temperature, adiabatic temperature, and the propagation speed of the combustion wave in the synthesis of CoSb3 are 723 K, 861 K, and 1.25 mm s−1, respectively. Using the SHS technique followed by Plasma Activated Sintering (PAS), we synthesized high performance bulk skutterudites of composition CoSb2.85 Te0.15 with a ZT of 0.98 at 820 K, one of the highest ZT values for an unfilled form of skutterudites. Compared with the samples synthesized by the traditional methods, the synthesis time is shortened from the typical several days to less than 20 minutes. Our work opens a new avenue for ultra-fast, low cost, mass production fabrication of skutterudite-based materials, which may also be universally applicable for the synthesis of other thermoelectric materials.