Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals

Summary

A viable technique for the formation of strontium titanate bicrystals at high pressure and fast heating rate via the spark plasma sintering apparatus is developed.

Abstract

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with one twist grain boundary. This apparatus utilizes high uniaxial pressure and a pulsed direct current for rapid consolidation of material. Diffusion bonding of strontium titanate bicrystals without fracture, in a spark plasma sintering apparatus, is possible at high pressures due to the unusual temperature dependent plasticity behavior of strontium titanate. We demonstrate a method for the successful formation of bicrystals at accelerated time scales and lower temperatures in a spark plasma sintering apparatus compared to bicrystals formed by conventional diffusion bonding parameters. Bond quality was verified by scanning electron microscopy. A clean and atomically abrupt interface containing no secondary phases was observed using transmission electron microscopy techniques. Local changes in bonding across the boundary was characterized by simultaneous scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy.

Introduction

Spark plasma sintering (SPS) is a technique in which application of high uniaxial pressure and pulsed direct current leads to the rapid densification of powder compacts¹. This technique also leads to the successful formation of composite structures from various materials, including silicon nitride/silicon carbide, zirconium boride/silicon carbide, or silicon carbide, with no additional sintering aids required^2,3,4,5. The synthesis of these composite structures by conventional hot-pressing had been challenging in the p....

Protocol

1. Sample Preparation of Single Crystal Strontium Titanate

NOTE: Single crystal STO is supplied with a (100) surface polished to a mirror finish.

1. Section STO into 5x5 mm² pieces using diamond wire saw.
2. Ultrasonically clean samples at 50-60 Hz consecutively in baths of acetone, isopropanol, and methanol for fifteen minutes each.
3. Remove STO from methanol bath to immediately place on hot plate held at a temperature of 200 °C. Heating the sample after cleaning prevents formation of evaporation spotting from the alcohol.
4. Place samples for ten minutes in buffered hydrofluoric acid (pH=4), ....

Results

Bonding temperature, time, and misorientation angle were all altered to determine optimum parameters needed for the maximum possible bonded interface fraction of the STO bicrystal (Table 1). The interface was considered 'bonded' when the grain boundary was not visible during SEM imaging (Figure 2a). A 'non-bonded' interface was exhibited when a dark image contrast or voids were present at the boundary location (Figure 2b). Dark image cont.......

Discussion

The bonding temperature of 1,200 °C was chosen to maximize diffusion as small changes in temperature can greatly impact the kinetics of all diffusion bonding mechanisms. A temperature of 1,200 °C is outside the brittle-ductile transition temperature range of STO. However, the sample underwent brittle fracture at this temperature. The catastrophic failure of the STO bicrystal was not unexpected as STO has ~ 0.5% ductility at 1,200 °C. Also, the sample was held at a pressure of 140 MPa throughout the heating.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Strontium titanate single crystal (100)	MTI Corporation	STOa101005S1-JP
Buffered oxide etch, hyrofluoric acid 6:1	JT Baker	MBI 1178-03
Scanning electron microscope (SEM)	FEI	Model: 430 NanoSEM
SPS apparatus	Sumitomo Coal Mining Co	Model: Dr. Sinter 5000 SPS Apparatus
High Temperature Furnace	Thermolyne	Model: 41600
Ultrasonic Cleaner	Bransonic	Model: 221
Mechanical polisher	Allied High Tech Products	15-2100-TEM
Diamond lapping film	3M	660XV	1 μm to 9 μm Grit Size
Diamond lapping film	3M	661X	0.5 μm to 0.1 μm Grit Size
Colloidal silica	Allied High Tech Products	180-20000	0.05 μm Grit Size
Sputter coater	QuorumTech	Model: Q150RES
Focused ion beam (FIB) instrument	FEI	Model: Scios dual-beamed focused ion beam (FIB) instrument
Nanomill TEM specimen preparation system	Fischione Instruments	Model: 1040
Transmission electron microscope (TEM)	JEOL	Model: JEM2500 SE
Scanning transmission electron microscope (STEM)	FEI	Model: TEAM 0.5