Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials VII, Volume 34, Issue 8 E-Book

Tatsuki Ohji

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Ceramic Engineering and Science Proceedings
• Sprache: Englisch

Seitenzahl: 310

Veröffentlichungsjahr: 2013

Beschreibung

Ceramic Engineering and Science Proceedings Volume 34, Issue 8 - Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials VII A collection of 20 papers from The American Ceramic Society's 37th International Conference on Advanced Ceramics and Composites, held in Daytona Beach, Florida, January 27-February 1, 2013. This issue includes papers presented in the 7th International Symposium on Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials and Systems (Symposium 8).

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

Introduction

Creation of Surface Geometric Structures by Thermal Micro-Lines Patterning Techniques

Abstract

Introduction

Experimental Procedure

Results and Discussion

Conclusions

References

Magnetoelectric Properties of La-Modified BiFeO3 Thin Films on Strontium Ruthenate (SrRuO3) Buffered Layer

Abstract

Introduction

Experimental

Results and Discussion

Conclusions

Acknowledgement

References

Properties of Pb(Zr, Ti)O3/CoFe2O4/Pb(Zr, Ti)O3 Layered Thin Films Prepared Via Chemical Solution Deposition

Abstract

Introduction

Experimental Details

Results and Discussion

Conclusions

References

Intelligent Processes Enable New Products in the Field of Non-Oxide Ceramics

Abstract

Introduction

Application Example: Heat Exchangers and Flow Reactors

Synthesis

Powder Processing

Forming

Sintering

Final Assembly

Summary

References

Fabricating Successful Ceramic Components Using Development Carrier Systems

Abstract

Introduction

Experimental Techniques

Results and Discussion

Conclusions

Acknowledgment

Reference List

Optimized Shaping Process for Transparent Spinel Ceramic

Abstract

Introduction

Material Preparation

Uniaxial Hydraulic Pressing

Thermal Processing

Sample Characterization

Summary and Outlook

References

Combustion Synthesis (SHS) of Complex Ceramic Materials

Abstract

Complex Multiphase Materials Prepared by Combustion Synthesis (SHS).

Complex Ceramics Processed from SHS-Derived Powders

Conclusions

Acknowledgment

References

Wear and Reactivity Studies of Melt Infiltrated Ceramic Matrix Composite

Abstract

Introduction

Procedure

Results

Discussion

Conclusion

Acknowledgments

References

Fabrication and Properties of High Thermal Conductivity Silicon Nitride

Abstract

Introduction

Factors Lowering Thermal Conductivity

High Thermal Conductivity Through Reaction Bonding and Post Sintering

Summary

References

Porous Silicon Carbide Derived from Polymer Blend

Abstract

1. Introduction

2. Experimental Procedure

3. Results and Discussion

4. Conclusion

References

Processing and Properties of Zirconia Toughened WC-Based Cermets

Abstract

Introduction

Experimental

Results and Discussion

Conclusion

Acknowledgements

References

Mechanism of the Carbothermal Synthesis of MgAl2O4-SiC Refractory Composite Powders by Forsterite, Alumina and Carbon Black

Abstract

Introduction

Experimental Procedure

Results and Discussion

Conclusions

Acknowledgement

References

Joining of Alumina by Polycarbosilane and Siloxane Including Phenyl Groups

Abstract

1. Introduction

2. Experimental Procedure

3. Results and Discussion

4. Conclusion

Acknowledgement

References

Microwave Joining of Alumina Using a Liquid Phase Sintered Alumina Insert

Abstract

Introduction

Experimental

Results and Discussion

Conclusions

References

Acknowledgement

Joining of Silicon Nitride Long Pipes Without Insert Material by Local Heating Technique

Abstract

Introduction

Experimental Procedure

Results and Discussion

Conclusion

Acknowledgment

References

Interfacial Characterization of Diffusion-Bonded Monolithic and Fiber-Bonded Silicon Carbide Ceramics

Abstract

Introducton

Experimental

Results and Discussion

Conclusion

Acknowledgements

References

Round Robin on Indentation Fracture Resistance of Silicon Carbide for Small Ceramic Products

Abstract

Introduction

Experimental Procedure

Results and Discussion

Conclusions

Acknowledgements

References

Numerical Analysis of Microstructural Fracture Behavior in Nano Composites Under HVEM

Abstract

Introduction

Experimental Test

Numerical Analysis

Conclusions

References

Commercialising University Research: Thoughts on the Challenges Based on Experience Gained in the Field of Ceramic Processing in the UK

Abstract

Introduction

Case Study 1: Soil Matric Potential

Case Study 2: Ceramic Foams

Case Study 3: Nanostructured Ceramics

Lessons Learned from Technologies Not Transferred

Licensing Versus Spin Out

Conclusions

References

Acknowledgement

Index

Advanced Processing andManufacturing Technologiesfor Structural andMultifunctional Materials VII

Cover Design: Wiley

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ISBN: 978-1-118-80773-6 ISSN: 0196-6219

Preface

The 7th International Symposium on Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials and Systems (APMT) was held during the 37th International Conference on Advanced Ceramics and Composites, in Daytona Beach, FL, January 27–February 1, 2013. The aim of this international symposium was to discuss global advances in the research and development of advanced processing and manufacturing technologies for a wide variety of non-oxide and oxide based structural ceramics, particulate and fiber reinforced composites, and multifunctional materials. A total of 56 papers, including invited talks, oral presentations, and posters, were presented from 15 countries (USA, Japan, Germany, China, Korea, UK, Switzerland, Canada, Estonia, India, Italy, Luxembourg, Serbia, and Malaysia). The speakers represented universities, industry, and research laboratories.

This issue contains 19 invited and contributed papers including 3 presented in Engineering Ceramics Summit of the Americas; all were peer reviewed according to The American Ceramic Society review process. The latest developments in processing and manufacturing technologies are covered, including green manufacturing, smart processing, advanced composite manufacturing, rapid processing, joining, machining, and net shape forming technologies. These papers discuss the most important aspects necessary for understanding and further development of processing and manufacturing of ceramic materials and systems.

The editors wish to extend their gratitude and appreciation to all the authors for their cooperation and contributions, to all the participants and session chairs for their time and efforts, and to all the reviewers for their valuable comments and suggestions. Financial support from the Engineering Ceramic Division and The American Ceramic Society is gratefully acknowledged. Thanks are due to the staff of the meetings and publication departments of The American Ceramic Society for their invaluable assistance.

We hope that this issue will serve as a useful reference for the researchers and technologists working in the field of interested in processing and manufacturing of ceramic materials and systems.

TATSUKI OHJI, Nagoya, Japan MRITYUNJAY SINGH, Cleveland, USA

Introduction

This issue of the Ceramic Engineering and Science Proceedings (CESP) is one of nine issues that has been published based on manuscripts submitted and approved for the proceedings of the 37th International Conference on Advanced Ceramics and Composites (ICACC), held January 27–February 1, 2013 in Daytona Beach, Florida. ICACC is the most prominent international meeting in the area of advanced structural, functional, and nanoscopic ceramics, composites, and other emerging ceramic materials and technologies. This prestigious conference has been organized by The American Ceramic Society’s (ACerS) Engineering Ceramics Division (ECD) since 1977.

The 37th ICACC hosted more than 1,000 attendees from 40 countries and approximately 800 presentations. The topics ranged from ceramic nanomaterials to structural reliability of ceramic components which demonstrated the linkage between materials science developments at the atomic level and macro level structural applications. Papers addressed material, model, and component development and investigated the interrelations between the processing, properties, and microstructure of ceramic materials.

The conference was organized into the following 19 symposia and sessions:

Symposium 1

Mechanical Behavior and Performance of Ceramics and Composites

Symposium 2

Advanced Ceramic Coatings for Structural, Environmental, and Functional Applications

Symposium 3

10th International Symposium on Solid Oxide Fuel Cells (SOFC): Materials, Science, and Technology

Symposium 4

Armor Ceramics

Symposium 5

Next Generation Bioceramics

Symposium 6

International Symposium on Ceramics for Electric Energy Generation, Storage, and Distribution

Symposium 7

7th International Symposium on Nanostructured Materials and Nanocomposites: Development and Applications

Symposium 8

7th International Symposium on Advanced Processing & Manufacturing Technologies for Structural & Multifunctional Materials and Systems (APMT)

Symposium 9

Porous Ceramics: Novel Developments and Applications

Symposium 10

Virtual Materials (Computational) Design and Ceramic Genome

Symposium 11

Next Generation Technologies for Innovative Surface Coatings

Symposium 12

Materials for Extreme Environments: Ultrahigh Temperature Ceramics (UHTCs) and Nanolaminated Ternary Carbides and Nitrides (MAX Phases)

Symposium 13

Advanced Ceramics and Composites for Sustainable Nuclear Energy and Fusion Energy

Focused Session 1

Geopolymers and Chemically Bonded Ceramics

Focused Session 2

Thermal Management Materials and Technologies

Focused Session 3

Nanomaterials for Sensing Applications

Focused Session 4

Advanced Ceramic Materials and Processing for Photonics and Energy

Special Session

Engineering Ceramics Summit of the Americas

Special Session

2nd Global Young Investigators Forum

The proceedings papers from this conference are published in the below nine issues of the 2013 CESP; Volume 34, Issues 2–10:

Mechanical Properties and Performance of Engineering Ceramics and Composites VIII, CESP Volume 34, Issue 2 (includes papers from Symposium 1)

Advanced Ceramic Coatings and Materials for Extreme Environments III, Volume 34, Issue 3 (includes papers from Symposia 2 and 11)

Advances in Solid Oxide Fuel Cells IX, CESP Volume 34, Issue 4 (includes papers from Symposium 3)

Advances in Ceramic Armor IX, CESP Volume 34, Issue 5 (includes papers from Symposium 4)

Advances in Bioceramics and Porous Ceramics VI, CESP Volume 34, Issue 6 (includes papers from Symposia 5 and 9)

Nanostructured Materials and Nanotechnology VII, CESP Volume 34, Issue 7 (includes papers from Symposium 7 and FS3)

Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials VII, CESP Volume 34, Issue 8 (includes papers from Symposium 8)

Ceramic Materials for Energy Applications III, CESP Volume 34, Issue 9 (includes papers from Symposia 6, 13, and FS4)

Developments in Strategic Materials and Computational Design IV, CESP Volume 34, Issue 10 (includes papers from Symposium 10 and 12 and from Focused Sessions 1 and 2)

The organization of the Daytona Beach meeting and the publication of these proceedings were possible thanks to the professional staff of ACerS and the tireless dedication of many ECD members. We would especially like to express our sincere thanks to the symposia organizers, session chairs, presenters and conference attendees, for their efforts and enthusiastic participation in the vibrant and cutting-edge conference.

ACerS and the ECD invite you to attend the 38th International Conference on Advanced Ceramics and Composites (http://www.ceramics.org/daytona2014) January 26–31, 2014 in Daytona Beach, Florida.

To purchase additional CESP issues as well as other ceramic publications, visit the ACerS-Wiley Publications home page at www.wiley.com/go/ceramics.

SOSHU KIRIHARA, Osaka University, Japan SUJANTO WIDJAJA, Corning Incorporated, USA

Volume Editors August 2013

CREATION OF SURFACE GEOMETRIC STRUCTURES BY THERMAL MICRO-LINES PATTERNING TECHNIQUES

Soshu Kirihara, Satoko Tasaki

Joining and Welding Research InstituteOsaka University11-1 Mihogaoka Ibaraki, Osaka 567-0047, Japan

Yusuke Itakura

Graduate School of EngineeringOsaka University2-1 Yamadaoka Suita, Osaka 565-0871, Japan

ABSTRACT

Thermal micro lines patterning techniques were newly developed as novel technologies to create geometrical intermetallics patterns for mechanical properties modulations of metal substrates. Pure copper particles were dispersed into the photo solidified liquid resins, and these slurries were spread on aluminum substrates. Micro patterns with fractal structures of Hilbert curves and dendritic lines were drawn and fixed by an ultra violet laser scanning. The formed patterns on the substrates were heated in an argon atmosphere, and the intermetallic or alloy phases with high hardness were created through reaction diffusions. The mechanical properties and surface stress distributions were measured and simulated by a tensile stress test and finite element method.

INTRODUCTION

Fractal geometries with self-similarity can be applied to modulate various flows in engineering fields [1,2]. Geometric networks with the fractal structure of intermetallic compounds patterned on light metals can strengthen whole materials efficiently by controlling surface stress distributions intentionally. In our research group, three dimensional metal and ceramics lattices with dendritic structures have been created and inserted into various matrices successfully to control stress and heat distributions [3,4]. Considering the next generation, mechanical properties enhancements by novel surface treatments will be expected to contribute novel materials processing of rare metals free. In this investigation, micro patterns composed of copper aluminide had been created on pure aluminum substrates by using a laser scanning stereolithography and a reaction diffusion joining. Microstructures and composite distributions in the vicinity of formed alloy and metal interfaces were observed and analyzed by using an electron microscope. Load dispersion abilities of the network were evaluated by using conventional mechanical tests and compared with simulated and visualized profiles by using a numerical analysis simulation.

EXPERIMENTAL PROCEDURE

Self-similar patterns of Hilbert curve with stage numbers 1, 2, 3 and 4 of the fractal line structures were designed by using a computer graphic application, and these graphic images were converted into the numerical data sets by computer software. These patterns of 25×25 mm in whole size were composed of arranged lines of 400 μm in width. These graphic models were transferred into the processing apparatus as operating data sets. Metal particles were patterned on a metal substrate by using a stereolithographic system. The pure copper particles of 50 μm in average diameter dispersed into a photo sensitive urethane resin at 60 volume percent. The mixed resin paste was spread with 100 μm in layer thickness on an aluminum substrate of 30×30×2 mm in size by using a mechanically moved knife edge as shown in Fig. 1. An ultraviolet laser beam of 355 nm in wavelength and 100 μm in beam spot was scanned on the resin surface according to the computer operation. A two dimensional solid pattern was obtained by a light induced photo polymerization. Figure 2 shows the appearance of the stereolithographic system. Subsequently, the dendiritic fractal patterns with the self-similarity of stage number 3 were adopted as the geometrical patterns. The patterns of 20×80 mm in whole size were composed of arranged lines of 400 and 8000 μm in width and length. The mixed resin paste with the pure copper particles was patterned on the aluminum specimen of 20×80×2 mm in size of the parallel part by using the stereolithography. After removing uncured resin by ultrasonic cleaning in ethanol solvent, the sample was heated to dewax the resin and create a self-similar pattern composed of copper aluminides at 600 °C above eutectic temperature for 4 hs of holding time in an argon atmosphere. Microstructures and composite distributions were observed by a scanning electron microscopy (SEM) and an energy dispersive X-ray spectroscopy (EDS), respectively. Stress distribution in the patterned material during uniaxial tension tests was simulated by a finite element method (FEM) calculation.

Figure 1. Schematically illustrated system configurations of a laser scanning stereolithography.

Figure 2. An appearance photograph of the used laser scanning stereolithography equipment.

RESULTS AND DISCUSSION

The copper aluminide micro pattern with the fractal structure of Hilbert curve was formed successfully on the substrate. Figure 3 shows the formed fractal polyline of the number 3 in fractal stage. The micrometer order geometric structure was composed of fine intermetallics lines of 450 μm in width. The part accuracy of these microline patterns was estimated as 10 % approximately. The copper aluminide composite was formed widely comparing with the designed line width, though the reaction diffusion between the copper and aluminum. The EDS measurement results suggested that copper concentrated in the micro network and formed intermetallic phase of CuAl2. Microscopic defects were not found in the formed intermetallics layers though the SEM observations. The smooth interfaces were obtained between copper aluminide and the aluminum substrate. During the heat treatment at the high temperature, eutectic reaction with liquid phase formation occurred between the pure copper particles and the pure aluminum substrate. After the solidification of molten alloys, the dual phase microstructure of the intermetallics and alloys composites can exhibit the higher mechanical strength. The stress distributions on the patterned surfaces were visualized for the Hilbert curve of stage number 3 through the numerical simulation as shown in Fig. 4. The required mechanical properties of Young’s modulus were defined along the compositional analysis and the phase identifications. The stress intensities concentrate into the vicinity of fixed edge and are distributed along the patterned lines and the corners with the higher hardness.

Figure 3. A copper aluminide Hilbert curve patterned on an aluminum substrate.

Figure 4. A stress distribution on the fractal pattern of intermetallics visualized by using FEM.

According to the designed dendritic model as shown in Fig. 5-(a), the real fractal pattern composed of the pure cupper dispersed urethane resin were created clearly as shown in Fig. 5-(b) by using the laser scanning stereolithography. The formed line width was measured as 400 μm. The part accuracies of geometric patterns were verified within 5 %. Through the heat treatments, the copper aluminide networks with the self similar patterns were formed by using the reaction diffusion joining as shown in Fig 5-(c). The mechanical properties of the patterned sample were measured through the tensile test as shown in Fig. 6. A small fracture crevasse is formed in the central position of the specimen perpendicularly to the intermetallics line connecting with two dendritic patterns. The tensile stresses were considered to be focused effectively by the both dendritic patterns and concentrated into the center connecting line. Through the dynamically monitoring for this artificial fracture source, real time materials life estimations will be realized effectively. The stress distributions on the patterned surface were simulated and visualized for the dendritic lines of stage number 3 through the FEM calculation as shown in Fig. 7. In the calculation process, the tensile strengths were loaded for the both edge of the specimen model. The red and blue colored areas indicate the higher and lower intensities of the bending stress, respectively. The stress intensities are concentrated into the vicinity of the center region and distributed along the patterned lines. The crossing points of the intermetalics lines with the higher hardness show the stress concentrations. The simulated and visualized results have good agreements with the measured results as shown in Fig. 6. The self-similar patterns can include the more numbers of sides and nodes in the limited area comparing with the periodic arrangements of the geometric polygon figures. Therefore, the dendritic fractal line patterns are considered to be able to disperse the mechanical stresses intentionally on the surface areas of the substrates.

Figure 5. The dendrite patterns formed by the stereolithography and reaction diffusion.

Figure 6. A tensile specimen with the dendritic fractal pattern after mechanical test.

Figure 7. A simulated and visualized stress distribution on the patterned test specimen.

CONCLUSIONS

Geometric networks with fractal structures of Hilbert curves and dendritic lines were created to modulate mechanical properties intentionally through computer aided designing and manufacturing. Microlines composed of urethane resin with copper particles dispersion were patterned successfully on pure aluminum substrates by using a stereolithography. The patterned samples were heated in an argon atmosphere to create intermetallics lines of copper aluminides through reaction diffusion between the copper and aluminum. Cracks and pores were not observed in welded interfaces by using a scanning electron microscopy. Mechanical stresses distributions along the formed intermetallics networks with were visualized and observed by numerical simulations and tensile tests. Investigated patterning techniques are considered to be efficient strengthening processes for light metals and alloys.

REFERENCES

1 Y. Kuan, J. Chang, S. M. Lee, S. R. Lee, Characterization of a Direct Methanol Fuel Cell Using Hilbert Curve Fractal Current Collectors, J. Power Source, 187, 112–122 (2009).

2 J. Yang, H. Bin, X. Zhang, Z. Liu, Fractal Scanning Path Generation and Control System for Selective Laser Sintering, Int. J. Mach. Tools Manuf., 43, 293–300 (2003).

3 S. Kirihara, Development of Photonic and Thermodynamic Crystals Conforming to Sustainability Conscious Materials Tectonics, WTI Trans. Ecolo. Env., 154, 103–114 (2011).

4 Y. Uehara, S. Tasaki, S. Kirihara, Fabrication of Hard Alloys Patterns with Fractal Structures on Light Metal Substrates through Reaction Diffusion, J. Smart Proc., 1, 186–189 (2012).

MAGNETOELECTRIC PROPERTIES OF La-MODIFIED BiFeO3 THIN FILMS ON STRONTIUM RUTHENATE (SrRuO3) BUFFERED LAYER

Regina C. Deusa, César R. Foschinib, José A. Varelac, Elson Longoc, Alexandre Z. Simõesa,c*

a Universidade Estadual Paulista- UNESP - Faculdade de Engenharia de Guaratinguetá, Av. Dr. Ariberto Pereira da Cunha, 333, Bairro Portal das Colinas, Zip-Code: 12516-410-Guaratinguetá-SP, Brazil, Phone [phone].

b Universidade Estadual Paulista, UNESP, Faculdade de Engenharia de Bauru, Dept. de Eng. Mecânica, Av. Eng. Luiz Edmundo C. Coube 14-01, Zip-Code: 17033-360, Bauru, SP, Brasil, Phone [phone]

c Universidade Estadual Paulista-UNESP- Instituto de Química - Laboratório Interdisciplinar em Cerâmica (LIEC), Rua Professor Francisco Degni s/n, Zip-Code: 14800-90-, Araraquara, SP, Brazil, Phone [phone].

*[email protected]

ABSTRACT

This paper focus on the magnetoelectric coupling (ME) at room temperature in lanthanum modified bismuth ferrite thin film (BLFO) deposited on SrRuO3-buffered Pt/TiO2/SiO2/Si (100) substrates by the soft chemical method. BLFO film was coherently grown at a temperature of 500°C. The magnetoelectric coefficient measurement was performed to evidence magnetoelectric coupling behavior. Room temperature magnetic coercive field indicates that the film is magnetically soft. The maximum magnetoelectric coefficient in the longitudinal direction was close to 12 V/cmOe. Dielectric permittivity and dielectric loss demonstrated only slight dispersion with frequency due the less two-dimensional stress in the plane of the film. The spontaneous polarization of the film was 25 μC/cm2. The film has a piezoelectric coefficient, d33, equal to 85 pm/V and a weak pulse width dependence indicating intrinsic ferroelectricity. Retention measurement showed no decay of polarization while piezoelectric response was greatly improved by the conductor electrode. Polarization reversal was investigated by applying dc voltage through a conductive tip during the area scanning. We observed that various types of domain behavior such as 71° and 180° domain switchings, and pinned domain formation occurred.

INTRODUCTION

Multiferroics materials1,2 exhibit several ferroic (or antiferroic) orders simultaneously and can be for instance ferroelectric and ferroelastic3 or ferroelectric and ferro- or ferrimagnetic4,5. The concomitance of several order parameters in a given material is attractive for storage applications as it offers the possibility to store twice as much information in a given memory cell volume, thereby providing an exponential increase in storage density6. Ferromagnetic and ferroelectric order parameters are widely used to store binary information in MRAMs7 and FeRAMs8, respectively but, unfortunately, ferroelectric ferromagnets (or ferrimagnets) are very scarce and the quest for a material with both large finite polarization and magnetization at room temperature is still in progress. To reach this goal, a first goal is to obtain materials with magnetoelectric coupling. Among all known multiferroics, the only compound that satisfies these criteria is bismuth ferrite (BFO). First synthesized in the late 1950s9, BFO was shown to be a G-type antiferromagnet with a Néel temperature of 630 K by Kiselev et al10. Later, Sosnowska et al. showed that the magnetic order of bulk BFO is not strictly collinear and that a cycloidal modulation with a period of 62 nm is present11. The magnetoelectric (ME) coefficient αME=dE/dH=dV/ (tdH) is the most critical indicator for the magnetoelectric coupling properties in multiferroic materials, where V is the induced magnetoelectric voltage, H is the exciting ac magnetic field, and t is the thickness of the sample used for measuring V across the laminate12. In previous work, we grow bismuth ferrite thin films on strontium ruthenate layer in which the conducting oxide was used as both the bottom electrode and the buffer layer13. We have observed an improvement in the crystal growth and electric properties of ferroelectric oxide. Wang et al.14 reported the considerable enhancement in the leakage, ferro/piezoelectric, and magnetic properties of the epitaxial bismuth ferrite films with conductive strontium ruthenate electrodes. Of particular interest, the SRO electrode is promising in inducing the preferred orientation and improving the film/electrode interface of ferroelectric films. Previous studies suggested that the fatigue endurance can be effectively suppressed by La doping15. To our knowledge, few reports are available on the magnetoelectric coefficient dependence on dc bias magnetic field of lanthanum modified bismuth ferrite thin film (BLFO) grown in oxide electrode. In this way, the magnetic and dielectric properties of BLFO thin films crystallized by the polymeric precursor method was investigated. The main focus was to evaluate the role exerted by the SRO bottom electrode on crystal structure and electrical properties of BLFO ferroic material.

EXPERIMENTAL

The SRO and BLFO thin films were prepared using the polymeric precursor method, as described elsewhere16. The bottom electrode thin films were spin coated on Pt/Ti/SiO2/Si (100) substrates by a commercial spinner operating at 5000 revolutions/min for 30 s (spin coater KW-4B, Chemat Technology). Each layer was pre-fired at 400°C for 2 h in a conventional oven. After the pre-firing, each layer was crystallized in a microwave furnace at 700 °C for 10 minutes. Using the same procedure, the BLFO thin films were deposited by spinning the precursor solution on the desired substrates. Through this process, we have obtained thickness values of about 150 nm (5 layers) for the bottom electrode and around 300 nm for BLFO (10 layers), reached by repeating the spin-coating and heating treatment cycles. The microwave furnace used to crystallize the SRO electrode was a simple domestic model similar to that described in literature16. Phase analysis of the films were performed at room temperature by X-ray diffraction (XRD) using a Bragg- Brentano diffractometer (Rigaku 2000) and CuKα radiation. A PHI-5702 multifunction X-ray photoelectron spectrometer (XPS) was used, working with an Al-Kα X-ray source of 29.35 eV passing energy. The pressure in the chamber during the experiments was about 4.5.10−9 Torr. Calibration of binding energy scale was controlled using the O1’s line, which appears in the photoelectron spectra of the as grown samples. The electrical properties of Pt/BLFO/SRO/Pt/Ti/SiO2/Si (100) capacitor structure were measured. The upper electrodes of Pt for the electrical measurements were prepared by evaporation through a shadow mask with a 0.2 mm2 dot area. Dielectric and ferroelectric properties of the capacitor were measured using HP 4192A impedance/gain phase analyzer and a Radiant Technology RT6000 A in a virtual ground mode, respectively. The magnetoelectric coefficient measurements in BLFO films were attained in a dynamic lock-in technique. The dc magnetic bias field was produced by an electromagnet (Cenco Instruments J type). The time-varying dc field was achieved by a programmable dc power supply (Phillips PM2810 60 V/5 A/60 W). To measure the dc magnetic field, a Hall probe was employed. Magnetization measurements were done by using a vibrating-sample magnetometer (VSM) from Quantum Design™. The magnetoelectric signal was measured by using a lock-in amplifier (EG&G model 5210) with input resistance and capacitance of 100 MΩ and 25 pF, respectively. Piezoelectric measurements were carried out using a setup based on an atomic force microscope in a Multimode Scanning Probe Microscope with Nanoscope IV controller (Veeco FPP-100). In our experiments, piezoresponse images of the films were acquired in ambient air by applying a small ac voltage with an amplitude of 2.5 V (peak to peak) and a frequency of 10 kHz while scanning the film surface. To apply the external voltage we used a standard gold coated Si3N4 cantilever with a spring constant of 0.09 N/m. The probing tip, with an apex radius of about 20 nm, was in mechanical contact with the uncoated film surface during the measurements. Cantilever vibration was detected using a conventional lock-in technique.

RESULTS AND DISCUSSION

XRD Phase Analysis

Figure 1 shows the XRD pattern of BLFO deposited on SRO electrode. The polycrystalline film exhibits a pure perovskite phase. Furthermore, except for the Si (100) and Pt (111) peaks, no peaks of impure phases such as Bi2Fe4O9 and Bi46Fe2O72 were spotted so pure phase BiFeO3 films were obtained by the soft chemical method. The polycrystalline nature of the film can be attributed to the differences in nucleation energy between the ferroelectromagnetic material and the oxide electrode. The insertion of SRO can avoid a possible interfacial reaction between platinum and bismuth which can lead to undesired electrical properties. Therefore, the substitution of metallic electrodes based on noble metals like platinum by conductive oxides is an alternative to reach better electrical properties caused by the high oxygen affinity of these electrodes17.

Figure 1. X-ray diffraction of BLFO thin film deposited by the polymeric precursor method on SRO electrode and annealed at 500°C in static air for 2 h.

XPS Analysis

In order to identify the chemical bonding of BFO thin films, we performed XPS studies. The spectrum expanded from 700 to 745 eV is also shown Figure 2. The 3/2 and 1/2 spin-orbit doublet components of the Fe 2p photoemission located at 711.1 and 724.6 eV, respectively were identified as Fe3+. No Fe2+ and Fe were found. The XPS results show that BFO thin films annealed at 500°C for 2 hours has a single phase with a Fe3+ valence state, consistent with XRD result shown in Figure 1. The oxidation state of Fe was purely 3+, which was advantageous for producing BFO film with low leakage18.

Figure 2. XPS analysis of BLFO thin films deposited on SRO electrode and annealed at 500°C in static air for 2 h.

Dielectric Characterization

Figure 3 displayed frequency-dependent dielectric behavior of BLFO film annealed at 500 °C. The dielectric measurements were carried out at room temperature as a function of frequency in the range of 10 kHz - 1 MHz. It is easy to see that the film possess small dielectric dispersion at low frequency since at which the dielectric constants decrease slightly with the frequency. It means that the films has good interface between the BLFO film and SRO bottom electrode. As shown, the dielectric constant shows very little dispersion with frequency indicating that our films possess low defect concentrations at the interface film-substrate. The low dispersion of the dielectric constant and the absence of any relaxation peak in tan δ indicate that both, interfacial polarization of the Maxwell Wagner type and a polarization produced by the electrode barrier can be neglected in the film19. The dielectric constant and dissipation factor, at 100 kHz, were found to be 122 and 0.05, respectively. The BLFO film had higher relative dielectric permittivity when compared with those previously reported in ceramics or films20–25. The observed improvement of dielectric permittivity may be associated with less structural disorder and less two-dimensional stress in the plane of the film. The tan δ values of the BLFO films keeps unchanged with increasing frequency suggesting that the films exhibited low defect concentration.

Figure 3. Dielectric permittivity and dielectric loss spectra of BLFO thin films deposited by the polymeric precursor method and annealed at 500°C for 2 hours as function of frequency.

Ferroelectric Characterization

Figure 4. (a) Remanent polarization -electric field hysteresis loops, (b) Pulsed polarization (ΔP) as a function of an applied voltage and (c) Pulse width dependence of switched polarization (ΔP) in the range from 1 μs to 1 ms, (d) Retention characteristics for BLFO thin film deposited on SRO electrode and annealed at 500°C in static air for 2 h.

Magnetic Characterization

The magnetolectric coefficient versus dc bias magnetic field in the longitudinal and transversal directions reveals hysteretic behavior, as observed in the magnetic field cycles shown in Figures 5a and 5b. The maximum magnetoelectric coefficient of 12 V/cmOe in the longitudinal direction is much larger than that previously reported for thin films as high as 3 V/cmOe in the same direction at zero fields29. This is a consequence of the antiferromagnetic axis of BLFO which rotates through the crystal with an incommensurate long-wavelength period of ~ 620 Å30,31. Early reports showed that the spiral spin structure leads to a cancellation of any macroscopic magnetization and would inhibit the observation of the linear magnetoelectric effect32. Significant magnetization (~ 0.5μB/unitcell) and a strong magnetoelectric coupling have been observed in epitaxial thin films, suggesting that the spiral spin structure could be suppressed33–35.

Figure 5. The magnetoelectric coefficient dependence on dc bias magnetic field for BLFO thin films deposited by the polymeric precursor method and annealed at 500° C for 2 hours at a 7 kHz ac magnetic field at room temperature. (a) Longitudinal and (b) Transversal.

Magnetization (M) versus field (H) loops were recorded at 300 K (Figure 6). The magnetization for the film was observed with a magnetic field of 2.5 emu/g. A weak ferromagnetic response was noted, although enhanced magnetization was observed as compared to bulk specimens. Gehring36 and Goodenough et al37 suggested that statistical distribution of Fe3+ and Ti4+ ions in the octahedra or creation of lattice defects might lead to bulk magnetization and weak ferromagnetism. The appearance of weak ferromagnetism in this compound may be attributed to either the canting of the antiferromagnetically ordered spins by a structural distortion38 or the breakdown of the balance between the antiparallel sublattice magnetization of Fe3+ due to metal ion substitution with a different valence39,40. As can be seen, the magnetization of our film linearly increases with the applied magnetic field. This behavior is characteristic of antiferromagnets and can be changed by introducing a small fraction of rare-earth additives. That will change the magnetic structure of pure bismuth ferrite. A change in the magnetic properties in this case is explained not only by different magnetic moments and ion radii of lanthanum (the La atom does not possess any intrinsic magnetic moment), but also by the anisotropy of the magnetic moments of lanthanum ions. Further studies are required to understand the magnetic behaviour of this compound. The low coercive magnetic fields of BLFO film is indicative of its magnetically soft nature and suitablility for device applications.

Figure 6. Field dependencies of the magnetization obtained for BLFO films deposited by the soft chemical method and annealed at 500° C for 2 hours.

As can be seen in Figure 7, when films are magnetically poled, initially there is an enhancement in saturation polarization, as expected. However, as the magnetic field increases, the polarization value drops. This may be attributed to disturbance created in grain alignment due the magnetic field. The random orientation of grains can lead to cancellation of polarization values. In that case the reduction in overall Ps value is expected.

Figure 7. Effect of magnetic poling on saturation polarization (Ps) of BLFO films deposited by the soft chemical method and annealed at 500° C for 2 hours.

Piezoresponse Analysis

The domain structures observed in the film by piezoelectric force microscopy (PFM) was illustrated in Figure 8. The out-of-plane (OP) and in-plane (IP) piezoresponse images of the as-grown films after applying a bias of −12V, on an area of 2 μm × 2 μm, and then an opposite bias of + 12V in the central 1 μm × 1 μm area were employed. For comparison the topography of the film was also analysed (Figure 8a