Advanced Processing and Manufacturing Technologies for Nanostructured and Multifunctional Materials III, Volume 37, Issue 5

Tatsuki Ohji, Mrityunjay Singh, Michael Halbig, Kyoung Il Moon, Tatsuki Ohji, Mrityunjay Singh, Michael Halbig, Kyoung Il Moon

• Publisher: Wiley-American Ceramic Society
• Year: 2017
• Print ISBN: 9781119321705
• eBook ISBN: 9781119321729
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

This issue contains 9 papers from The American Ceramic Society's 40th International Conference on Advanced Ceramics and Composites, held in Daytona Beach, Florida, January 24-29, 2016. This issue includes papers presented in the 10th International Symposium on Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials and Systems (Symposium 8), Additive Manufacturing and 3D Printing Technologies (Focused Session 4), and Field Assisted Sintering (Focused Session 5).

Field Assisted Sintering

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FLASH SINTERING OF ALUMINA AND ITS MICROSTRUCTURAL EVOLUTION

Mattia Biesuz and Vincenzo M. Sglavo

Department of Industrial Engineering, University of Trento 38123 Trento, Italy

ABSTRACT

Alumina 99.8% pure was Flash Sintered under different fields and currents in the range of 500–1500 V/cm and 2– 6mA/mm², respectively. It was shown that the considered material can be sintered down to 900°C and that the current density is controlling the shrinkage upon sintering. The analysis of the microstructure evolution pointed out that the porosity-grain boundary separation is enhanced by increasing the current.

INTRODUCTION

Field Assisted Sintering (FAS) represents a very promising route for consolidating ceramics at reduced time and temperature, with potential consistent decrease in energetic and environmental costs. Among FAS techniques, Flash Sintering (FS) allows to reduce, drastically, the sintering time to few seconds.

Flash Sintering has been studied in recent years on several semiconductive (YSZ^1–5, GDC⁶, SiC⁷, ZrO₂-Al₂O₃ composites⁸) and conductive (MnCo₂O₄⁹, LSCF¹⁰) ceramics. Other insulating materials like high purity alumina and doped alumina have been tested and it has been shown that impurities play a crucial role upon field assisted sintering behavior^11,12. In particular, it has been pointed out that the presence of different chemicals can reduce the onset temperature for FS.

Previous works have also shown that the application of an electric field may have a significant effect on the grain growth kinetics. The conclusions are anyway quite controversial and in some cases a retardation in grain growth was observed^1,21,22 while in others an enhance growth kinetic was observed under the effect of high field²².

In the present work reactive alumina 99.8% pure was tested under different combinations of voltage/current limit with the purpose of identifying correlations between the microstructural features and the imposed electrical parameter. Moreover, the grain growth behavior upon conventional and field assisted sintering was studied.

EXPERIMENTAL PROCEDURE

α-alumina (Almatis CT3000SG, D50 = 0.5 µm) with nominal composition (wt%) Ak₂O₃ 99.8, MgO 0.04, Na₂O 0.03, Fe₂O₃ 0.015, SiO₂ 0.015, CaO 0.015 was used in the present work. The powder was shaped in dog-bone-like pellets by uniaxial pressing at 120 MPa with the addition of 6 wt% of water as binder. The cross section of the central part of the dog-bone green sample was 3×3÷4 mm². The samples were connected by two platinum wires to a DC power supply (Glassman EW series, 5 kV-120 mA) and to a multimeter (Keithley 2100), which allows to measure second-by-second the applied current and voltage. A drop of platinum paste was applied on the electrode for improving the electrical contact.

The samples were sintered in an alumina dilatometer (Linesis L-75) with a constant heating rate of 20°C/min and target temperature of 1450°C. A load of 0.8 N was applied to the sample by the dilatometer measuring rod. Once the sample reached 300°C, the power supply was opened and the system started to work under voltage control (250–1500 V/cm). Then, if the current limit was reached (2–6 mA/mm²), the current was let to flow for 2 min and finally the system was shut down.

For comparison, some samples were also conventionally sintered in the temperature range 1550–1700°C for different times (2–8 h) using a constant heating rate of 15°C/min.

The fracture surface of the samples (manually produced in the gage section at similar distance from the two electrodes) was analyzed by SEM (Jeol JSM 5500). The grain size was estimated by the linear intercept method. For each sample 12 measurements were taken.

RESULTS AND DISCUSSION

The dilatometric plots recorded during the FAS are reported in Fig 1. The dog-bone samples can be “flash sintered” here with fields higher than 500 V/cm. The sintering temperature is found to be definitely lower if compared with the results previously published on the same material with cylindrical samples¹². This is probably due to an improved metal-alumina electrical contact, which accounts for a reduced electrode resistance. Moreover, the onset temperature measured here under “flash sintering” conditions is lower than that found by Cologna et al. for 0.25 wt% MgO-doped alumina using a similar experimental set-up¹¹. It is very likely that the presence of different chemicals and impurities plays an important role on FS behavior. One possibility is that the presence of aliovalent elements may act as electron source/donor or may promote the formation of different lattice defects^13–16.

The dilatometric plots in Fig 1 show that the sintering temperature is drastically reduced down below 1000°C when fields in excess to 1000 V/cm are applied. In this case, the shrinkage occurs abruptly just after the current limit is reached. Conversely, under voltage equal to 750 V/cm or less, densification starts before the current limit is achieved and two main steps can be pointed out, the first c...

Table of contents

• Cover
• Titlepage
• Copyright
• Preface
• Introduction
• Field Assisted Sintering
• Advanced Processing and Manufacturing
• Additive Manufacturing
• Advanced Materials and Innovative Processing Ideas for the Industrial Root Technology
• Author Index
• EULA