CHARACTERIZATION OF HYDROXYAPATITE COATING  
ON TUBULAR SURFACES



PAVITHRA RAMANARAYANAN

University of Rochester
Department of Chemical Engineering

OPT 407: Practical Electron Microscopy
Spring 2010

INDEX

ABSTRACT

INTRODUCTION

COATING PROCEDURE: ELECTRODEPOSITION

RESULTS AND DISCUSSION

CONCLUSION


 ABSTRACT


In this project, we investigate the coating characteristics of Hydroxyapatite (HAP) on to tubular surfaces by varying parameters like current density, deposition time and substrate material. Micrographs of the coating were obtained which showed the growth of HAP crystals on the substrate. Also composition analysis was performed which established that the crystals were indeed hydroxyapatite
. Also different electron microscopy techniques like: Back-scattered electron detector, Variable pressure mode detector etc were used to obtain micrographs. Sample was also sputter-coated with Gold to reduce ‘charging’ during image collection.
 
                                                                                                       

  INTRODUCTION

HYDROXYAPATITE

Hydroxyapatite is a mineral that is found in human bones and teeth and has a chemical formula of Ca10(PO4)6(OH)2. It is mostly white in color and has a rod-shaped hexagonal prism like structure when grown on the right substrate under the right conditions. It has a density of 3.156 g/cm3 and a  molecular weight of 502.31.

Hydroxyapatite coated on to metal/metal alloy surfaces has diverse applications. It can be used as a dental/orthopedic implant as it has high osteo-integrity. Ti alloys and Co alloys have been used as substrates to create such prosthetics. On the other hand, metal/metal alloy coated with hydroxyapatite can also be modeled into an Intermediate Temperature Solid oxide Fuel Cell. With a suitable cathode, fuel cell made out of hydroxyapatite as the electrolyte membrane have been shown to have conductivities several folds higher than other fuel cells, thanks to its high proton conductivity at a low temperature of 200oC.

TUBULAR SUBSTRATE: SIGNIFICANCE

Research has always been focused on using flat sheets of substrate material for HAP coating for both biomedical implants and for fuel cells. Using a tubular substrate can have several advantages in both cases. Coating will be more effective given the larger surface area. Packing and osteo-integrity in the human system will be much much easier owing to the design advantages and as for a fuel cell, efficiency increases manifold due to increased hydrogen transfer. 

Shifting our focus to a tubular fuel cell, a typical fuel cell has an anode, cathode and a electrolyte membrane to conduct protons for the electrochemical reaction that would generate energy. The HAP-coated metal/metal alloy which I am trying to develop will serve as the anode for the electrochemical reaction wherein HAP is the electrolyte membrane.


   Figure 1: TUBULAR FUEL CELL: SINGLE UNIT

Also tubular fuel cells have a shorter start-up time. Design is a huge advantage for fuel cell application too, as cylindrical bundles of fuel cells are easier to scale up.


  FIGURE 2:  TUBULAR FUEL CELL: ENTIRE UNIT

COATING PROCEDURE: ELECTRODEPOSITION

Hydroxyapatite is coated on the substrates using electrodeposition method. Electrolyte solution is placed in a beaker. A Pt plate is used as anode and the substrate used is the cathode. Different electrodeposition potentials are used to vary the current density (80mA through 150mA). Also deposition time is varied between 5 minutes and 20 minutes to find the optimum time for best crystal growth. Two substrates were investigated: Stainless steel and Titanium. Rods of 0.25” diameter and 2” length were used. Subsequent secondary layer hydrothermal growth step was done for after optimizing time and current density.

RESULTS AND DISCUSSION

The following are the micrographs obtained after investigating the effect of each variable.

Figures 3 through 5 are micrographs obtained after varying the different operational parameters and optimizing them. These are the images of crystals that have exhibited best growth and morphology.

 

FIGURE 3: MICROGRAPHS OF 150mA SAMPLE


FIGURE 4: MICROGRAPHS OF SAMPLE AFTER 20min GROWTH

FIGURE 5: MICROGRAPHS OF STAINLESS STEEL (left) & TITANIUM (right)
                                                                                       


The following are micrographs obtained under various SEM conditions.

FIGURE 6: VARIABLE PRESSURE MODE


FIGURE 7: BACK-SCATTERED ELECTRON DETECTOR



FIGURE 8: GOLD-COATED SAMPLES

Some of the images were colored using Adobe Photoshop to make them look attractive.

FIGURE 9: COLORIZED IMAGES


COMPOSITIONAL ANALYSIS 

EDAX was used to qualitative and quantitative analysis of the samples and the following were the results obtained.

 FIGURE 10: QUALITATIVE COMPOSITION DATA


  FIGURE 11: QUANTITATIVE DATA FROM EDAX

ELECTRON FLIGHT SIMULATION

Electron Flight Simulations at 30kV was obtained. This shows the depth of penetration of the beam and the interaction volume. Higher the beam strength, greater is the penetration depth. Hence the deeper of the penetration volume, as observed. The figure on the right shows the interaction volume along with X-rays that are emitted.


                                 

                                                               FIGURE 12: ELECTRON FLIGHT SIMULATION

CONCLUSION


SUMMARY

Hydroxyapatite crystals grew into expected hexagonal rods at high current density and higher reaction time, namely 150mA and 20min. Charging is seen in some of the micrographs which can be eliminated by coating, reducing beam strength, scanning speed or aperture. Morphology and compositional analysis through EDAX also establish the fact that I have achieved fine coating.

ACKNOWLEDGEMENTS

My sincere gratitude goes to Professor Brian McIntyre who kept me motivated throughout the project and guided me in all respects. I would like to thank the TA Andreas Liapis for his constant assistance in the laboratory. I also extend my gratitude to my lab-mate Keith Savino, who has always been ready to help me out. Above all, I have to thank my mom, Meena Ramanarayanan, who has always handled my stress and ill-temper with all the patience she could!

REFERENCES

1.    http://en.wikipedia.org/wiki/Hydroxyapatite

2.    http://americanhistory.si.edu/fuelcells/images/sofc2.jpg

3.    http://www.acumentrics.com/Collateral/Images/English-US/fuel-cell-system-schematic.jpg

4.    Liu, Dongxia, and Matthew Z. Yates. "Microstructural Engineering of Hydroxyapatite Membranes to Enhance Proton Conductivity."

 

 

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