Field-assisted sintering technology/Spark plasma sintering is a low voltage, direct current (DC) pulsed current activated, pressure-assisted sintering, and synthesis technique, which has been widely applied for materials processing in the recent years. After a description of its working principles and historical background, mechanical, thermal, electrical effects in FAST/SPS are presented along with the role of atmosphere. A selection of successful materials development including refractory materials, nanocrystalline functional ceramics, graded, and non-equilibrium materials is then discussed. Finally, technological aspects (advanced tool concepts, temperature measurement, finite element simulations) are covered.

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Field-Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

Spark-plasma sintering (SPS) provides accelerated densification and, in many cases, limited grain growth compared to regular hot pressing and sintering. Possible mechanisms of this enhancement of the consolidation in SPS versus conventional techniques of powder processing are identified. The consolidation enhancing factors are categorized with respect to their thermal and nonthermal nature. This paper analyses the influence of a major factor of thermal nature: high heating rates. The interplay of three mechanisms of material transport during SPS is considered: surface diffusion, grain-boundary diffusion, and power-law creep. It is shown that high heating rates reduce the duration of densification-noncontributing surface diffusion, this favors powder systems’ sinterability and the densification is intensified by grain-boundary diffusion. Modeling indicates that, besides the acceleration of densification, high heating rates diminish grain growth. The impacts of high heating rates are dependent on particle sizes. Besides SPS, the obtained results are applicable to the broad spectrum of powder consolidation techniques which involve high heating rates. The conducted experiments on SPS of an aluminum alloy powder confirm the model predictions of the impact of heating rates and initial grain sizes on the shrinkage rates during the electric current-assisted consolidation. It is noted, that this study considers only one of many possible mechanisms of the consolidation enhancement during SPS, which should stimulate further efforts on the modeling of field-assisted powder processing.

Consolidation enhancement in spark-plasma sintering: Impact of high heating rates

In this paper, we present the results concerning the Pt surface modification of nickel oxide thin films deposited by dc reactive magnetron sputtering. Pt very thin overlayers with a thickness of about 3 and 5 nm have been sputtered on the top of NiO samples. The surface structure and morphology of the samples have been analysed by X-ray diffractometer (XRD) and by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The electrical responses of the NiO-based sensors towards different H2 concentration (500–5000 ppm) have been also considered. The Pt-modified NiO samples showed an enhancement of the response towards H2 as compared to the unmodified NiO sample. The thickness of the Pt thin layers seems also an important parameter in determining the properties of the NiO films as H2 sensors.

Enhancement of H2 sensing properties of NiO-based thin films with a Pt surface modification

Spark plasma sintering (SPS) parameters including pressure, temperature, holding time and heating rate were considered and optimized to investigate the sintering process of bulk copper. The relationship between the sintering parameters and the microstructure of the bulk compact were obtained and studied. It was concluded that large-size ultrafine-grained copper with an average grain size less than 2.2 μm and a relative density greater than 96% could be prepared by the SPS process with initial pressure of 1 MPa, holding pressure of 50 MPa, sintering temperature of 750 °C, holding time of 6 min and heating rate of 80 °C/min. The production process improved the mechanical properties.

Ultrafine-grained copper prepared by spark plasma sintering process

Effect of crystallographic orientations on electrical properties of sputter-deposited nickel oxide thin films

Cubic nickel oxide (bunsenite) films with a preferred (200) orientation and an average grain size of 15 nm were deposited on Si(111) substrate by a pulse ultrasonic spray pyrolysis method. Aqueous ethanol solutions of nickel nitrate were ultrasonically nebulized and thermally deposited on Si(111) using an aerosol generator, which was operated at 2.5 MHz. The resulting films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy. Highly crystalline NiO films with (200) orientation were obtained at 360 °C substrate temperature from 0.05 M solution and 5 l min −1 carrier gas flow rate. All grains were seen to be elongated along one direction by AFM. The ratio (64.58%) of the oxygen adsorbed to the total O1s intensity indicated that the film deposited by this method was a good candidate for electrochromic films.

Wen-Zhong Wang

Papers

Preparation and characterization of highly oriented NiO(200) films by a pulse ultrasonic spray pyrolysis method