Pulsed Electric Current Sintering Research Articles

Pressure effects and grain growth kinetics in the consolidation of nanostructured fully stabilized zirconia by pulsed electric current sintering

The effect of applied uniaxial pressure on the densification and grain size of nanocrystalline cubic zirconia (c-YSZ) was investigated during sintering by the pulsed electric current sintering (PECS) method. The role of the pressure depended on temperature, being highly significant at lower temperatures and of little significance at higher temperatures. The kinetics of grain growth were determined under PECS conditions. Analysis of the results indicated a grain growth process that is retarded, probably due to the effect of the current on grain boundary energy or dopant segregation. The activation energy for grain growth of c-YSZ was determined as 252 ± 34 kJ mol −1, a value that is slightly smaller than reported values for microcrystalline samples.

ZrB 2–SiC composites prepared by reactive pulsed electric current sintering

Fully densified ZrB 2–20 vol% SiC composites were produced by reactive pulsed electric current sintering (PECS) of a powder mixture containing ZrH 2, B, SiC and B 4C within a total thermal cycle time of only 50 min. During the combined synthesis and sintering process, the ZrH 2 powder decomposed gradually from ZrH 2 into ZrH m and finally metal Zr that reacted with elemental B to form the ZrB 2 matrix. Reducing the ZrH 2 particle size by attritor milling significantly enhanced densification and allowed initiation of self-propagating high temperature synthesis (SHS) during PECS. The PECS grades exhibited a slightly textured structure, with ≤17% of the ZrB 2 grains oriented with their (0 0 1) planes perpendicular to the direction of pressure and DC current. Because of the ZrB 2 grain orientation, anisotropic mechanical properties were observed. Ceramics prepared from attritor milled powders and PECS with a pressure applied after 5 min upon reaching 1900 °C achieved excellent flexural strengths of 901–937 MPa. The hardness and fracture toughness were respectively 19.7–19.8 GPa and 4.0–4.7 MPa m 1/2 in the direction parallel and 20.2–21.3 GPa and 3.8–3.9 MPa m 1/2 in the direction perpendicular to the applied pressure.

Machining Properties of Insulating ZrO<sub>2</sub> Ceramics Using Porous Copper Electrodes

Insulating ceramics were considered as machinable material with the electrical discharge machining method using the assisting electrode method, which was proposed and advanced by the authors. The mechanical properties changed with the experimental parameters, including the physical characteristics of the workpiece and the tool electrode. In this research, sintered ZrO2-Y2O3 insulating ceramics were subjected to EDM. To investigate the effects of the porous tool electrode material on the removal rate, the porous copper cylinder material was fabricated by PECS (pulse electric current sintering) method. The relative density changed from 60 to 95 wt%. The suitable machining characteristics could be obtained with a relative density of 85% at the positive electrode polarity. The shape of electrodes was transcribed on the workpiece surface more precisely than the solid copper electrode.

Apatite type lanthanum silicate and composite anode half cells

Apatite type rare earth silicates are being extensively studied as electrolyte material for intermediate temperature solid oxide fuel cells (SOFC). In this paper we presents results on synthesis of Al and/or Fe-doped ATLS, the design of compatible anode materials, thermal expansion properties and co-sintering of half-cells from expansion matched materials using the advanced pulsed electric current sintering (PECS) technique. The issues related to the co-sintering of half cells have been addressed successfully by the combined use of nano powders and PECS.

Electrical discharge machining of ZrO 2 toughened WC composites

The interrelationships between the dispersion of the ZrO 2 phase and the electrical discharge machining (EDM) behaviour of WC based composites with 0, 5 or 10 vol% of ZrO 2 are investigated. Special attention was given to the homogeneity of the ultrafine WC – nanometric ZrO 2 powder based composites which were fully densified by means of pulsed electric current sintering (PECS). X-ray photoelectron spectroscopy (XPS) measurements revealed the presence of a nanometric WO 3 layer on the EDM surface, confirming oxidation as the major material removal mechanism (MRM). The surface roughness after the final EDM finishing regime was strongly influenced by the composite homogeneity and could be reduced down to 0.15 μm ( R a) on agglomerate-free composites. Residual stress measurements indicated that EDM did not introduce a significant amount of surface stresses, especially not after the final finishing regime. XRD measurements of EDM surfaces however indicated surface depletion of ZrO 2 by the formation of ZrC and W 2C during spark erosion.

Functionally graded boron carbide and aluminum composites with tubular geometries using pulsed electric current sintering

A functionally graded boron carbide (B4C) with precipitous property and microstructural gradients has been synthesized using pulsed electric current sintering (PECS) in tubular shape forms. During PECS the amorphous powders react and partially consolidate forming a density gradient radially across the tube walls. Modeling results support the presence of a large radial temperature gradient sufficient to produce B4C in smoothly varying densities from inside to outside diameters. This material and geometry is both novel and of particular use in applications requiring a cylinder of differing surface structures that transition smoothly into a metallic substructure or assembly.

Ultrafine Al 2O 3–B 4C composites consolidated by pulsed electric current sintering

Al 2O 3–60 vol.% B 4C composites with 0–5 wt% SiC addition were consolidated by pulsed electric current sintering (PECS) during 4 min at 1650 °C. The influence of the SiC additive on the densification behaviour, microstructure and mechanical properties of the Al 2O 3–B 4C composites were investigated. Microstructural analysis revealed the presence of Al 18B 4O 33 grains, formed by chemical reaction of Al 2O 3 with surface oxides on B 4C, in agreement with thermodynamic calculations. The Al 2O 3 grains were found to coarsen from 0.22 μm in the starting powder up to about 0.60 μm during PECS, whereas the size of the ∼0.6 μm B 4C grains hardly changed. The Al 2O 3 and B 4C grain size slightly decreased upon SiC addition. The addition of 3 or 5 wt% SiC hardly influenced the hardness and toughness of the Al 2O 3–B 4C composites, whereas the strength and stiffness were reduced. The composite with 1 wt% SiC combined a hardness of 26 GPa, an E-modulus of 448 GPa, a bending strength of 600 MPa and a fracture toughness of 6.2 MPa m 1/2. The influence of the loading condition and dwell time during PECS of the 5 wt% SiC doped composite was assessed.

Modified polyol-mediated synthesis and consolidation of Gd-doped ceria nanoparticles

A modification of the polyol-mediated synthesis method was developed to synthesize nanostructured gadolinia-doped ceria, Ce 0.9Gd 0.1O 2 − x . The oxide prepared by this method had uniform grains with a size of about 2 nm. The work demonstrated the critical role of water in the synthesis. A yield of up to 81% can be achieved. The as-synthesized Ce 0.9Gd 0.1O 2 − x nanoparticles are crystalline with a cubic fluorite type structure. The nanopowders were subsequently consolidated by a modified pulse electric current sintering (PECS) method to produce dense samples with an average size below 16 nm and a relative density of about 96%. The dense material is composed of isolated particles instead of agglomerates and provides an ideal sample for investigation of the size effect on the conductivity.

Preparation of Homogeneous Nb-Al Intermetallic Compound Sheet by Multi-Layered Rolling and Subsequent Heat Treatment

The intermetallic compound Nb3Al is widely investigated because of its high temperature strength, superior superconductivity and relatively small density. As Nb3Al has an extremely high melting point and lack of deformability, it is impossible to prepare it by using the conventional metallurgy. In this study, a Nb-Al intermetallic compound was prepared by multi-layered roll-bonding of elemental Nb and Al foils. The process consisted of the accumulative roll-bonding (ARB) for making a laminated Nb/Al sheet and the subsequent heat treatment promoting a solid-phase reaction in the laminated Nb/Al sheet. Accumulated foils were roll-bonded at 573 K. The rolling reduction at 1 pass was ~50%, and the final rolling reduction at 4 passes was ~94%. A pulsed electric current sintering (PECS) process was used for the subsequent heat treatment. The microstructures produced at each processing stage were characterized by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). A homogeneous intermetallic compound of Nb3Al could be obtained after the subsequent heat treatment for 1.8 ks at 873 K and for 0.9 ks at 1673 K.

Dielectric breakdown and thermal conductivity of textured alumina from platelets

A textured alumina was fabricated from anisotropic α-alumina particles (platelets) by heating at 1650°C for 15 min under applied pressure of 60 MPa using a PECS (Pulse Electric Current Sintering) technique. Subsequently, the thermal conductivity, mechanical properties and dielectric breakdown of the textured alumina were evaluated, are compared to those of conventional polycrystalline alumina and single crystal alumina. The results showed that the thermal conductivity of the textured alumina could be to exhibit almost the same dielectric breakdown as the conventional alumina, that is, the dielectric breakdown was 15 kV/mm and thermal conductivity was 36 W/mK, when the alumina was cut on the vertical plane in the load direction while sintering.

Influence of Si-Doping of Nano-Ni Dispersed Al<SUB>2</SUB>O<SUB>3</SUB> Composites on High-Temperature Oxidation

The oxidation kinetics of 5 vol% nano-Ni dispersed Al 2 O 3 with Si-doping (nano-Ni/Al 2 O 3 -Si) was studied in the present paper. The starting powder mixture was prepared by drying aqueous slurry consisting of alumina with nickel nitrate and Si (OCH 3 ) 4. The powder mixture was reduced at 600°C for 12 h in a stream of Ar-1%H 2 gas mixture. Nano-Ni/Al 2 O 3 -Si was densified by pulsed electric current sintering. The oxidation test was conducted at 1200―1350°C for 1―14 d in air. Oxidized zone consisted of Al 2 O 3 matrix and NiAl 2 O 4 as the oxidation product. Growth of the oxidized zone followed a parabolic manner. A thin NiAl 2 O 4 layer was also observed on the sample surface. Si-doping decreases growth rate of oxidized zone effectively at lower temperatures such 1200°C.

Fabrication of Mg Ferrite/permalloy Magnetic Nano-composites

Dense {Mg(Fe1-xMnx)2O4/53Fe-47Ni permalloy (0≤x≤0.4)} magnetic nano-composites consisting of spinel and alloy phases have been fabricated from N2-atomized metal particles and fine-powder mixtures of MgO/α-Fe2O3/MnO using a combination of pulsed electric current pressure sintering (PECPS) and hot isostatic pressing (HIP). Fe-Ni permalloy particles were coated with the mixed metal oxide powders homogeneously by passing these mixtures through a narrow gap (1 mm) under high shear compression in Ar. Mixed powder compacts of permalloy with the fine metal oxides were densified into the green bodies having relative densities of 83-85 % under isostatic high pressure of 1 GPa at room temperature. Then green powder compacts were pre-sintered at 823 K for 3 min under a uniaxial pressure of 100 MPa in Ar using a PECPS. The pre-sintered bodies composed of spinel ferrite and permalloy revealed the relative densities of 94-96 %; the mixed metal oxides changed into magnesium spinel ferrites Mg(Fe1-xMnx)2O4 during a short heating time. Then hot isostatic pressing (HIP: (1123 K/6 h/200 MPa-Ar)) was adopted to sinter the pre-sintered bodies into a near full density without an encapsulation, however, a sintering atmosphere control was performed using a ferrite muffle in order to maintain the both phase. Sintered nano-composites with the relative density more than 99.5 % have a saturation magnetization density ≥1 T and moderate permeability values of 30-35 at 10 MHz, which values have been expected from a material design.

Low-Temperature Formation of Cubic Th3P4-type Gadolinium and Holmium Sesquisulfides

The formation behavior of gadolinium and holmium sesquisulfides was examined by studying the sulfurization of their inorganic and organic salts such as nitrate, carbonates, oxalates, acetates and octanoate by carbon disulfide (CS2) gas. In previous studies, α-Gd2S3, which is low temperature phase, was produced by the sulfurization of the oxide at 1023 K. In the present study, single phase γ-Gd2S3, which is stable at relatively high temperature, was formed by the sulfurization of octanoate at 1073 K and oxalate at 873 K. It has also been found that gadolinium salts are thermally decomposed at temperatures higher than 500 K. The thermal decomposition leads to the formation of a gadolinium oxide via an oxycarbonate. In the case of holmium sesquisulfides, the sulfurization of holmium oxide and nitrate provide the mixture of δ-Ho2S3 and Ho2O2S impurity, while pure γ-Ho2S3 is exclusively formed from oxalates, acetates, or carbonates. These results reveal that the formation of oxycarbonate such as Gd2O2CO3 and Ho2O2CO3 play an important role in the formation of γ-phase. After sulfurization, the synthetic powder of γ-Ho2S3 was consolidated by pulse electric current sintering to investigate its high-temperature stability. Holmium sesquisulfide transformed from the γ-phase to the δ-phase at sintering temperatures above 1073 K while δ-phase was stable even at a high sintering temperature of 1773 K.

Thermoelectric Properties of Nanograined ZnO

Bulk ZnO with a grain size of 20 nm was successfully obtained by pulsed electric current sintering. The crystalline size was almost identical to that of the raw particles, because the sintering temperature was as low as 200°C. A pressure of 500 MPa effectively enhanced densification, leading to a relative density of >90% at 200°C. The small grain size led to a low thermal conductivity of 3 W/m K at room temperature, due to enhanced boundary scattering. The Seebeck coefficient was higher than that of micrograined ZnO with similar Ga doping (0.3 at.% Ga). However, the resistivity was increased by more than 1000 times. The temperature dependence of conductivity showed thermally activated conduction behavior, while that of micrograined ZnO exhibited metallic-like behavior. The thermoelectric properties suggest that a carrier trap in the nanograined ZnO hinders carrier transport. Surface modification of the ZnO nanoparticles by heat treatment in H2 resulted in observable photoluminescence which was quenched in the starting nanoparticles, and led to a decrease in the resistivity of the sintered bulk, which indicates that control of surface defects on the nanoparticles is crucial for enhancement of the thermoelectric properties of nanograined ZnO.

Highly thermal conductive metal/carbon composites by pulsed electric current sintering

Various composites with metals such as aluminum and copper, and carbons such as pitch based carbon fiber and carbon nanotube, and scale-like graphite have been prepared to realize high thermal conductive materials utilizing pulsed electric current sintering method. The composites have high thermal conductivity and one- or two-dimensional low thermal expansion coefficient.

Pulsed electric current, in situ synthesis and sintering of textured TiB 2 ceramics

Textured TiB 2 monolithic ceramics were prepared in a one step in situ synthesis and densification process during pulsed electric current sintering (PECS) using TiH 2 and amorphous B as raw materials. The Lotgering orientation factors of the (0 0 l) and ( h k 0) planes were measured to be 0.66 and 0.68, respectively. The crystallographically textured ceramics also display a clear microstructural anisotropy. The addition of SiC as a secondary phase was found to deteriorate the orientation in the TiB 2 ceramic due to grain growth inhibition.

Powder synthesis, processing and characterization of lanthanum silicates for SOFC application

Recent studies have shown that rare earth silicates exhibit higher oxygen conductivity at lower temperatures (500–800 °C) compared to traditional yttria-stabilized cubic zirconia. Among the rare earth silicates, doped lanthanum silicates are known to exhibit higher ionic conductivity. The difficulty in sinterability of these powders combined with the fact that the electrolyte material should be sintered onto a porous anode material in a single sintering step to produce SOFC half cells requires very fine starting powders with high sinterability at lower temperatures (1000–1200 °C). In this paper, we report on the synthesis of lanthanum silicates doped with aluminium or iron using the sol–gel process. Using this fairly simple and quick process, nanometer sized powders with an average grain size below 100 nm have been synthesised, while the intrinsic nanostructure could be maintained in the fully densified electrolyte material after pulsed electric current sintering (PECS) at 1100–1200 °C.

The Study on Carbon Nanofiber (CNF)‐Dispersed B4C Composites

Carbon nanofiber (CNF)‐dispersed B4C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B4C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σb) of ∼710 MPa, a Vickers hardness (Hv) of ∼36 GPa, a fracture toughness (KIC) of ∼7.9 MPa m1/2, and high‐temperature σb of 590 MPa at 1600°C in N2. Interfaces between the CNF and the B4C matrix were investigated using high‐resolution transmission electron microscopy, EDS, and electron energy‐loss spectroscopy.

Pulsed electric current sintering and characterization of ultrafine Al 2O 3–WC composites

Ultrafine Al 2O 3 powder mixtures with up to 80 vol% ultrafine WC were prepared and consolidated by pulsed electric current sintering (PECS) for 4 min at 1250–1750 °C under a mechanical pressure of 60 MPa. Fully dense composites with ≤40 vol% WC could be achieved when PECS at 1450 °C for 4 min, whereas 1650 °C was needed for the WC matrix composites. The microstructure, electrical conductivity and mechanical properties of the composites were assessed as a function of the WC content. Microstructural analysis showed that Al 2O 3 and WC grain growth was significantly suppressed due to secondary phase addition, resulting in ultrafine (<500 nm) microstructures. Excellent mechanical properties, combining a Vickers hardness of 25 GPa, fracture toughness of 4.8 MPa m 1/2 and flexural strength of 1000 MPa, as well as electrical conductivity of 1.09E+6 Ω −1 m −1 were obtained for the Al 2O 3–60 vol% WC composite.

Processing of ultrafine ZrO 2 toughened WC composites

The interrelationships between the dispersion of the secondary ZrO 2 phase and the material properties of WC-based composites with up to 10 vol% of ZrO 2 are investigated. The homogeneity of the ultrafine WC–nanometric ZrO 2 powder mixtures was optimized by means of multidirectional milling and bead milling. In an alternative route, zirconium butoxide was used as a ZrO 2 source. The composites were fully densified by means of pulsed electric current sintering (PECS), also known as spark plasma sintering, within a few minutes at 1700 °C allowing to maintain an ultrafine grained microstructure combining a hardness of 2600 kg/mm 2 with an indentation toughness of 6 MPa m 1/2. The ZrO 2 content and Y 2O 3 stabilization were found to strongly influence the mechanical properties and especially strength of the WC–ZrO 2 composites.