Characterization of Zirconia coated Carbonyl Iron
Particles using Electron Microscopy

 

Karan Mehrotra

University of Rochester
Department of Mechanical Engineering

MSC 507: Practical Electron Microscopy
Spring 2011


Introduction

  1. Background: The Technology
  2. Latest Developments
  3. Project Objectives
  4. Materials & Methods
Results & Discussions
  1. Secondary Electron (SE) Imaging
  2. Backscattered Electron (BSE) Imaging
  3. EDS & Elemental Area Mapping
  4. Focused Ion Beam (FIB) Milling
  5. Transmission Electron Microscopy (TEM)
  6. Stereo Pairs - 3D Anaglyphs
Summary
  1. Conclusions
  2. Suggestions for Future Work

References and Acknowledgements

  1. References
  2. Acknowledgements
  3. Comments


 

BACKGROUND AND INTRODUCTION

 

1. Background: The Technology

Optics manufacturing was revolutionized with the invention of Magnetorheological Finishing (MRF) [1]. It is a novel and deterministic polishing technology for producing complex optics with figure accuracy <50 nm and surface roughness <1 nm. MR fluids are the key element of MRF technology. In general, MR fluids consist of uniformly dispersed noncolloidal magnetic particles, e.g., CI, in a carrier fluid. MRF was commercialized in 1997 by QED Technologies and is considered to be an excellent, deterministic process for finishing

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Figure 1: The MRF Technology: A schematic(on the left)and an actual machine QED Q22-Y (on the right)

MRF is a subaperture polishing process. For a conventional MRF setup, the MR fluid is pumped through a delivery system and ejected through a nozzle in the form of a ribbon onto a rotating vertical wheel. The ribbon stiffens upon passing into a region with a high magnetic field in the vicinity of the workpiece. The MRF removal function is characterized by a D-shaped polishing spot in the zone of contact between the ribbon and the workpiece, and the material removal rate is determined by the time of contact (e.g., dwell time) as well as other process and workpiece parameters. Temperature of the MR fluid is controlled by a chiller normally set to ~20 C.


 

2. Latest Developments

More recently [2], zirconia-coated carbonyl-iron (CI) particle based magnetorheological (MR) fluid was developed at the LLE, University of Rochester using a sol-gel synthesis in kilogram quantities. Zirconia (ZrO2) is a hard polishing abrasive used in conventional polishing of hard and soft glasses. The coating layer is ~75 nm thick, faceted in surface structure, and well adhered. The zirconia coated magnetic CI particles (ranging typically in the ~0.5 to 2.0 m range) show long-term stability against aqueous corrosion. "Free" nanocrystalline zirconia polishing abrasives are also cogenerated in the coating process, resulting in an abrasive-charged powder for MRF.
This work revelaed that the zirconia-coated CI-based MR fluid was designed, prepared, and circulated in an experimental MRF platform for a period of nearly three weeks with no signs of degradation/corrosion. As a part of testing, a variety of optical glasses spanning a range of hardness values were tested, as well as several polycrystalline optical ceramics.

 

3. Project Objectives

This project is an attempt to use electron microscopy and other related techniques to characterize these coated particles specifically, their morphology, size and surface properties.

 

4. Materials & Methods

Two types of CI particles were tested for this project, (a) Uncoated CI HQ particles (commercially available from BASF) and (b) Zirconia coated CI particles (produced at the LLE). All samples were in powder form and were milled using mortar and pestle before mounting on stubs for SEM. Sample preparation for the TEM involved the use of Isopropyl suspensions of the milled powder . Additionally, two other samples were prepared in form of pucks by mixing CI particles powder with conductive epoxy powder under controlled pressure and temperature. The pucks were polished using the MR Fluid STM in order to polish the particles and obtain their cross sections.

Several techniques were used to examine these particles. These included using various imaging modes on the SEM, bright field imaging using the TEM, EDS analysis and FIB. One more technique, the generation of 3-D anaglyphs using stereo pair images in Adobe Photoshop CS5 was also used.




 

Results & Discussions

This section discusses the techniques used in this project to study these particles and characterize them. . The coated CI particles (for SEM) were sputtered (in the Denton sputtering system) with gold for 25 s using a 15 mA current to make them conductive and avoid charging and beam deflection during imaging. The uncoated particles did not require sputter coating.

 

1. Secondary Electron (SE) Imaging

Two detectors, InLens and SE2 were used in collecting the following micrographs. Figures 2 (a) and (b) depict the uncoated CI particles at a working distance (wd) of 10 mm and a voltage of 10 kV using both the detectors.

 

Figure 2: (a)Using the InLens detector and, (b) Using the SE 2 detector

Figures 3 (a) and (b) depict the Zirconia coated CI particles using these detectors. Again a wd of 10 mm and a voltage of 10 kV was used to obtain these micrographs. The difference in surface morphology between the coated and the uncoated particles is clearly revealed in these micrographs. Also notice the free zirconia in these images. This is an essential component to effective and superior polishing .

 

Figure 3: (a)Using the InLens detector and, (b) Using the SE 2 detector

Figures 4 (a) and (b) show the cross section of these particles (from the polished epoxy pucks). Spots of 15 min duration each were taken using a standard MR fluid on the conductive epoxy pucks. The micrograph for the coated CI particle shows a thin (~50 nm) and uniform Zirconia coating.

 

Figure 4: (a)Uncoated CI HQ particles and, (b)Zirconia coated particles

Figures 5 (a),(b),(c) and (d) show the comaprison of the two SE detectors at a short working distance of 3.5 mm. Clearly, the SE 2 detector does not image well at this wd in spite of the Faraday cage on it.

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Figure 5: CI particles at a wd of 3.5 mm and voltage of 10 kV.(a) and (c):Uncoated & coated particles using InLens. Figs(b)and(d): Uncoated & coated particles using the SE 2 detector

 

3. Backscattered Electron (BSE) Imaging

Micrographs obtained using BSD do not show much surface morphology but are a fair indication of the coating and free Zirconia present around and on the coated particles.

 

 

Figure 6: (a)Coated CI using the BSD (b) Uncoated CI (c) Coated CI at a low magnification

 

3. EDS & Elemental Area Mapping

The EDS analysis was performed on uncoated & coated particles both in the SEM and TEM (only coated). Additionally, elemental area maps were also generated. These are useful in recognizing the presence of a certain element over the specific chosen area.

 

Figure 7: EDS analysis of uncoated CI particles (a)Area used for analysis (b) EDS plot of the center particle(c) Image generated using the area mapping software [correlates with the SEM image] and the maps of the elements present in the imaged area. Aluminum is present from the stub and Carbon from the mounting tape used

 

Figure 8: EDS analysis of coated CI particles (a)Area used for analysis (b) EDS plot (c) Image generated using the area mapping software [correlates with the SEM image] and the maps of the elements present in the imaged area.

 

Figure 9: EDS analysis of coated CI particles on the TEM in the STEM mode (a)STEM HAADF detector image with the area selected for analysis highlighted (b)the EDS plot and (c)maps of the elements present .

 

4. Focused Ion Beam (FIB) Milling

FIB was employed to attempt to reveal the cross section of the coated particles without having to embed them in epoxy and then polish the puck. A current of 50 pA was used for 80 s employing 4 passes. A clear cross section was not revealed.

 

Figure 10: (a) The milled particle imaged in SEM mode (b) Milled particle at a higher magnification

 

5. Transmission Electron Microscopy (TEM)

TEM was used to determine any lattice structure that might be present in the coating thus indicating if it was crystalline or amorphous. The specimens were made by drop casting a suspension of particles in isopropyl alcohol on a carbon-coated copper grid. The results do not reveal any lattice structure in the coating.

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Figure 11: Bright Field TEM Images

 

6. Stereo Pairs 3D Anaglyphs

Two images are obtained 3.50 apart from one another after setting the eucentric point at a wd of ~5mm. The images were combined using Adobe Photoshop CS5.

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Figure 12: 3D Anaglyphs




 

Summary

 

1. Conclusions

The project provided significant insight into the Zirconia coated particles - its surface morphology and thickness, the distribution of free zirconia within the sample and an idea of the nature of the coating deposited using this sol-gel technique. Focused Ion Beam milling, due to its very high precision is definitely one technique that will be essential in better characterizing the coated particles. Better results using FIB were not possible for this project due to time constraints. Also, TEM micrographs revealed that the coating on these particles is probably amorphous in nature due to the lack of presence of crystal lattice at significantly high magnifications.
Overall, Electron Microscopy proved to be a very effective means to characterize these particles and study their properties.

 

2. Suggestions for future work

When conducting FIB milling on the coated particles, Pt should be deposited on them to ensure better results. The corrected milling process might be employed to reveal the native Iron Oxide layer on the uncoated particles.
A more detailed analysis may be required to determine if the coating on the particles is indeed amorphous. This should include collection of diffraction patterns at various regions of the particle under observation on the TEM.



 

REFERENCES AND ACKNOWLEDGEMENTS

 

1. References

1. S. D. Jacobs et al., Magnetorheological finishing: A deterministic process for optics manufacturing, in Optical Fabrication and Testing (SPIE, Tokyo, Japan, 1995), Vol. 2576, pp. 372382

2. Shafrir et al., Zirconia-coated carbonyl-iron-particle-based magnetorheological fluid for polishing optical glasses and ceramics, 10 December 2009 / Vol. 48, No. 35 / Applied Optics

 

2. Acknowledgements

A special thanks to Mr. Brian McIntyre for the wonderful time throughout the semester and more importantly his teaching prowess, assistance at all times and patience; all of which were critical for the successful completion of this project.
Also, thank you to Zachary Lapin, the Teaching Assistant for the course for his patience when I was just starting out with the microscope. Lastly, many thanks to Sivan Salzman and Prof. Steve Jacobs for their technical guidance throughout the project.


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