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The Effect of Microwave Energy on Sintering

Doctoral work by Raghu Thridandapani
by

Raghunath Rao Thridandapani

on 31 March 2011

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Transcript of The Effect of Microwave Energy on Sintering

The Effect of Microwave Energy on Sintering Thank you Raghu R. Thridandapani Ph.D. Defense
Materials Science and Engineering W. H. Sutton, Ceramic Bulletin (1988)
W. H. Sutton, Mat. Res. Soc. Smp. Proc. (269), 1992 Motivation Sintering A phenomena of developing bonds between powdered particles. Non-densifiying mechanisms Densifiying mechanisms MRS, v. 124 (1990), v. 189 (1992), v. 269 (1992), v. 347 (1994), v. 430 (1996
Ceramic Transactions v. 21 (1991), v. 36 (1993), v. 59 (1995), v. 80 (1997), v. 111 (2000).
Microwave World Congress, III (2003), IV (2005) Experimental Procedure M. N. Rahaman, “Ceramic Processing and Sintering” 2nd Edition, CRC Press (2003)
S. L. Kang, “Sintering, grain growth and microstructure” Elsevier (2005) Forming Technique Conventional Heating Microwave Heating Background
Goals and objectives
Experimental procedure
Results and discussion
Summary
Future work Presentation Outline Microwave Hybrid Dilatometer Results: Densification Behavior Microwave sintered samples showed higher density at lower temperatures.

Is this due to an inaccuracy in temperature measurement ? Expansion of Alumina with microwave heating supports the accuracy in temperature measurement. Temperature Validation Three different experimental methods were used to study the sintering behavior of 8YZ

Isothermal sintering (most common)
Constant heating rate
Master sintering curve
Reduced sintering temperatures with comparable microstructures. Microstructural Analysis Acknowledgments The motivation for this project was to develop a microwave sintering method to fabricate IMFs. Historical Perspective Applications Microwave sintering has been used for low temperature processing J. P. Cheng et al., Mat. Res. Innovat. 1, 44-52 1997)
J. M. Moore, Ph.D dissertation, University of Florida (1999)
K.H. Brosnan, et al., J. Am. Ceram. Soc. 86, p.1307 (2003)
R. R. Thridandapani et al., J. Nucl. Mater. 384 [2] 153-157 (2009 Goals Reproduce the enhanced densification with microwave sintering, eliminating the possibility of inaccuracy in temperature measurement.

Determine which of the terms, if any, in the flux equation is leading to enhanced densification. 1. Conduct a thorough literature review in the area of microwave sintering.

2. Develop a microwave dilatometer to validate temperature measurements and monitor the microwave sintering process.

3. Evaluate the microwave sintering of 8 mol % Y2 O3 -stabilized ZrO2 relative to conventional sintering with isothermal and non-isothermal methods.

4. Examine the microstructural differences (if any) due to microwave sintering.



5. Investigate the variation in measured activation energies under the influence of an electric field.

6. Provide some insight into the role of microwave energy on sintering. Objectives for This Work Uniaxial pressing Isostatic pressing Distribution of forces 46 % dense (theoretically) isostatically pressed pellets were used for this work. Furnace Dilatometer The sample was ~3 mm from the thermocouple. Sintering experiments were performed in both Singlemode and Multimode applicators. The electric field intensity per mode is higher in singlemode than multimode. Software developed using LABVIEW Modified design: Singlemode applicator Front View of Microwave Dilatometer Used to validate temperature measurements.


Sintering experiments were performed under different magnitudes of electric field. Funding Agency Research Team and MSE Dept. Dr. Clark
Ms. Folz
Dr. Folgar Mr. Kulp
Ms. Mellodge
Dr. Mahmoud Microwave field has no effect on thermocouple. Effect of Electric Field on Densification Electric field enhances densification, be it oscillating or static.


These findings substantiate the literature reports and satisfies the first goal set for this study. Role of Driving Force for Conventional Sintering Densification is represented by Role of Driving Force on Sintering The driving force due to the electric field is comparable to the surface area driving force. The oscillating electric field produces equivalent forces. This should not result in net flux. Role of Driving Force for
Microwave Sintering Role of Transport Coefficient The attempt frequency and inter-atomic distance are constant for microwave and conventional sintering. Isothermal Method Conventional sintering Microwave sintering The activation energy decreased from 437 for conventional sintering to 214 kJ/mol for microwave sintering. Conventional sintering Non-isothermal Method:
Constant Rate Heating Microwave sintering The activation energy decreased from 500 kJ/mol for conventional sintering to 199 kJ/mol for microwave sintering. The activation energy decreased from 500 kJ/mol for conventional sintering to 200 kJ/mol for microwave sintering. Non-isothermal Method:
Master Sintering Curve Swaroop et al, Acta Materialia 53 (2005) 4975
Mayo, International Materials Reviews 41 (1996) 85 The electric field is decreasing the activation energy for sintering. Regarding first goal:

Microwave dilatometer was custom designed and built.

Enhanced densification with microwave sintering are not due to inaccuracy in temperature measurement.

Microstructural features observed for microwave sintering were similar to conventionally sintered samples, but were observed at lower temperatures. Driving force due to the A.C. field is considered to be symmetrical, but should be confirmed through a probabilistic study.

There is a possibility of new driving force from thermal gradients that needs to be examined by future investigators.

Direct microwave sintering experiments need to be expanded to include inert atmospheres.

Potential improvement in engineering aspect of microwave dilatometer include upgrading both hardware and software for minimizing human error.

A study of master sintering curves as a function of particle size would benefit the fuel fabrication industry. The microwave energy is 100 times lower than thermal energy.

Microwave photon (10 times lower) does not have enough energy for creating defects.

Microwave is accelarating the grain boundary diffusion through preferential heating.

This is altering the transport mechanism from VD to GD. Discussion Regarding second goal:


Electric field component of microwave increased the rate of densification.

It is likely that the electric field is activating grain boundary diffusion resulting in a major transport of ions along grain boundaries.

In microwave heating, as in conventional heating, reduction in surface free energy still appears to be the major driving force for sintering. Sintering: Transport Mechanism Are inaccurate temperature measurements responsible for observed enhancement in microwave sintering? Are any of these terms being affected by the microwave energy?

(or) Sintering Equation Related to Goal 1: Related to Goal 2: 5 It is not the terms in the flux equation that are effected by microwave energy.

The relative high temperatures along the grain boundaries are activating diffusion.

It is temperature distribution that is leading to enhanced microwave sintering and is not due to inaccuracy in temperature measurement.
Conventional furnace Microwave furnace Thermocouple Setup Comparison of Activation Energies Electric Field Effect on Activation Energy Summary Future Work Why is Q Decreasing with Increasing E? Discussion Summary . . The hardware and software were specifically developed for this study The thermocouple "averages" the temperature of grain and grain boundary.

The surface area of grain boundary is ~ 36 % (for a 10 nm width)

For a temperature of 1100 C the grain boundary would be at 1600 C than grain (800 C). 0 0 0 Reproduce the enhanced densification with microwave sintering, eliminating the possibility of inaccuracy in temperature measurement Multimode Applicator Singlemode Applicator
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