Salvatore Grasso

Electric current activated/assisted sintering (ECAS): a review of patents 1906?2008

Abstract The electric current activated/assisted sintering (ECAS) is an ever growing class of versatile techniques for sintering particulate materials. Despite the tremendous advances over the last two decades in ECASed materials and products there is a lack of comprehensive reviews on ECAS apparatuses and methods. This paper fills the gap by tracing the progress of ECAS technology from 1906 to 2008 and surveys 642 ECAS patents published over more than a century. It is found that the ECAS technology was pioneered by Bloxam (1906 GB Patent No. 9020) who developed the first resistive sintering apparatus. The patents were searched by keywords or by cross-links and were withdrawn from the Japanese Patent Office (342 patents), the United States Patent and Trademark Office (175 patents), the Chinese State Intellectual Property Office of P.R.C. (69 patents) and the World Intellectual Property Organization (12 patents). A subset of 119 (out of 642) ECAS patents on methods and apparatuses was selected and described in detail with respect to their fundamental concepts, physical principles and importance in either present ECAS apparatuses or future ECAS technologies for enhancing efficiency, reliability, repeatability, controllability and productivity. The paper is divided into two parts, the first deals with the basic concepts, features and definitions of basic ECAS and the second analyzes the auxiliary devices/peripherals. The basic ECAS is classified with reference to discharge time (fast and ultrafast ECAS). The fundamental principles and definitions of ECAS are outlined in accordance with the scientific and patent literature.

Review of graphene–ceramic matrix composites

Abstract Graphene has remarkable mechanical properties, which makes it potentially a good reinforcement in ceramic composites. It also has unique electrical and thermal properties, which makes it an attractive filler for producing multifunctional ceramics for a wide range of applications. In the past few years, relatively little attention has been focused on graphene ceramic matrix composites (GCMC) in comparison to polymer composites. This review gives a comprehensive overview on the state of the art of GCMC, including materials synthesis, densification and characterisation. The published literature allows us to define the critical steps for processing GCMC, and identify its influence on the multifunctional and mechanical properties of the composites. Finally, the potential future applications and current research trends in GCMC are presented.

Relation between microstructure, properties and spark plasma sintering (SPS) parameters of pure ultrafine WC powder

Abstract A combined experimental/numerical methodology is developed to fully consolidate pure ultrafine WC powder under a current-control mode. Three applied currents, 1900, 2100 and 2700 A, and a constant pressure of 20 MPa were employed as process conditions. The developed spark plasma sintering (SPS) finite-element model includes a moving-mesh technique to account for the contact resistance change due to sintering shrinkage and punch sliding. The effects of the heating rate on the microstructure and hardness were investigated in detail along the sample radius from both experimental and modeling points of view. The maximum hardness (2700 HV10) was achieved for a current of 1900 A at the core sample, while the maximum densification was achieved for 2100 and 2700 A. A direct relationship between the compact microstructure and both the sintering temperature and the heating rate was established.

Review of flash sintering: materials, mechanisms and modelling

ABSTRACT Flash sintering (FS) is an energy efficient sintering technique involving electrical Joule heating, which allows very rapid densification (<60 s) of particulate materials. Since the first publication on flash-sintered zirconia (3YSZ) in 2010, it has been intensively researched and applied to a wide range of materials. Going back more than a century ago, we have found a close similarity between FS of oxides and Nernst glowers developed in 1897. This review provides a comprehensive overview of FS and is based on a literature survey consisting of 88 papers and seven patents. It correlates processing parameters (i.e. electric field magnitude, current density, waveforms (AC, DC) and frequency, furnace temperature, electrode materials/configuration, externally applied pressure and sintering atmosphere) with microstructures and densification mechanisms. Theorised mechanisms driving the rapid densification are substantiated by modelling work, advanced in situ analysis techniques and by established theories applied to electric current assisted/activated sintering techniques. The possibility of applying FS to a wider range of materials and its implementation in industrial scale processes are discussed. GRAPHICAL ABSTRACT Abbreviations: ECAS: Electric Current Assisted/Assisted Sintering; FS: flash sintering; SPS: spark plasma sintering; FSPS: flash spark plasma sintering; HIP: hot isostatic press; HP: hot press; TF: furnace temperature; TOnset: onset temperature; E: electrical field; TS: sample temperature; TSoft: softening temperature for glasses; Tm: melting temperature; PTC or NTC: positive or negative temperature coefficient of electrical resistance; IS: impedance spectroscopy; OES: optical emission spectroscopy; AES: atomic emission spectroscopy; DBS: Dog-bone shape, L: length, W: width, T: thickness, D: diameter, H: height, R: rectangular, CS: cross-section; P: particle size; C: crystallite size; 3YSZ: 3 mol-% yttria-stabilised zirconia; I: ion, H: hole, V: vacancy, E: electron, P: proton

Large ZT enhancement in hot forged nanostructured p-type Bi0.5Sb1.5Te3 bulk alloys

Bi2Te3 based alloys play a dominant role in commercial applications in the fields of thermoelectric energy generation and solid state cooling. By combining the densification of nanostructured powders followed by a two-step hot forging process, hierarchical nanostructured p-type Bi0.5Sb1.5Te3 alloys with good preferred orientation were successfully fabricated. The Seebeck coefficient in the direction perpendicular to the pressing force, which is highly anisotropic, is much greater than that of the material sintered via one-step sintering. Meanwhile, the nanostructure and crystal defects produced during hot forging also contribute to both higher Seebeck coefficient, and lower thermal conductivity due to more effective and preferential scattering of phonons than electrons. As a result, a 50% enhancement of ZT value (from 1 to above 1.5) in the orientated, hierarchical, nanostructured alloys was obtained.

Toughened and machinable glass matrix composites reinforced with graphene and graphene-oxide nano platelets

Abstract The processing conditions for preparing well dispersed silica–graphene nanoplatelets and silica–graphene oxide nanoplatelets (GONP) composites were optimized using powder and colloidal processing routes. Fully dense silica–GONP composites with up to 2.5 vol% loading were consolidated using spark plasma sintering. The GONP aligned perpendicularly to the applied pressure during sintering. The fracture toughness of the composites increased linearly with increasing concentration of GONP and reached a value of ∼0.9 MPa m1/2 for 2.5 vol% loading. Various toughening mechanisms including GONP necking, GONP pull-out, crack bridging, crack deflection and crack branching were observed. GONP decreased the hardness and brittleness index (BI) of the composites by ∼30 and ∼50% respectively. The decrease in BI makes silica–GONP composites machinable compared to pure silica. When compared to silica–Carbon nanotube composites, silica–GONP composites show better process-ability and enhanced mechanical properties.

Physical and mechanical properties of highly textured polycrystalline Nb4AlC3 ceramic

Abstract Highly textured polycrystalline Nb4AlC3 ceramic was fabricated by slip casting in a strong magnetic field followed by spark plasma sintering. Its Lotgering orientation factor was determined on the textured top and side surfaces as f(00l) ∼1.0 and f(hk0)=0.36, respectively. This ceramic showed layered microstructure at the scales ranging from nanometers to millimeters. The as-prepared ceramic had excellent anisotropic physical properties. Along the c-axis direction, it showed higher hardness, bending strength, and fracture toughness of 7.0 GPa, 881 MPa and 14.1 MPa m1/2, respectively, whereas higher values of electrical conductivity (0.81×106 Ω−1 m−1), thermal conductivity (21.20 W m−1 K−1) and Young’s modulus (365 GPa) were obtained along the a- or b-axis direction.

Enhanced thermoelectric performance of porous magnesium tin silicide prepared using pressure-less spark plasma sintering

Magnesium tin silicide based thermoelectrics contain earth abundant and non-toxic elements, and have the potential to replace established commercial thermoelectrics for energy conversion applications. In this work, porosity was used as a means to improve their thermoelectric properties. Compared to dense samples of Sb doped Mg2Si0.5Sn0.5 with a maximum zT of 1.39 at 663 K, porous samples (37% porosity) prepared by a pressure-less spark plasma sintering technique showed significantly lower thermal conductivity and higher Seebeck coefficient, resulting in an increased maximum zT of 1.63 at 615 K. The possible origins of the enhanced Seebeck coefficient can be attributed to a change of carrier concentration and modification of the band structure, produced by microstructural engineering of the surface composition and particle–particle contacts.

Attempts to synthesise quaternary MAX phases (Zr,M)2AlC and Zr2(Al,A)C as a way to approach Zr2AlC

Despite having never been synthesized, the MAX phase Zr2AlC attracts a lot of interest owing to its foreseen properties. A possible way to circumvent this obstacle is to stabilize Zr2AlC by partially substituting one of its constituting elements. Here we report on attempts to synthesise quaternary MAX phases (Zr,M)2AlC and Zr2(Al,A)C where M = Cr, Ti or Mo and A = S, As, Sn, Sb and Pb. We were notably able to produce Zr2(Al0.2Sn0.8)C, Zr2(Al0.35Pb0.65)C, and Zr2(Al0.3Sb0.7)C, with the latter representing the first antimony-based MAX phase ever reported. Impact Statement: Numerous syntheses of Zr2AlC derived compositions were attempted. Zr2(Al0.2Sn0.8)C, Zr2(Al0.35Pb0.65)C and Zr2(Al0.3Sb0.7)C were notably produced and reported for the first time.

Reverse boundary layer capacitor model in glass/ceramic composites for energy storage applications

Reverse boundary layer capacitor (RBLC) configuration model, where the grain boundary has a higher electrical conductivity than the grain, is proposed in glass/ceramic composites for dielectric energy storage applications. By introducing glass additives as grain boundaries with electrical conductivity higher than ceramic grains, the steady electric field across grains can be larger than grain boundaries as desired due to the conductivity difference. The breakdown field is thus expected to increase in the RBLC-type brick wall model because of the field distribution. The equivalent circuit, grain boundary conductivity dependence of energy density, low-loss frequency range of the RBLC model are discussed. The simulation results suggest that the RBLC approach has advantages in overall energy density, compared with normal insulating glass phase composites.