Controlled relative humidity storage for high toughness and strength of binderless green pellets

Significance 

Finding quantitative relations between the crack length, the material’s inherent resistance to crack growth, and the stress at which the crack propagates at high speed to cause structural failure are well established in dense ceramics. However, little effort has been made to develop equations focusing on fracture mechanics for green ceramics. Such equation would come in handy as they would help improve the green strength and toughness of ceramics thereby, ensuring minimized chipping and breakage during manufacture. Up to date, it has been more challenging to develop fracture models for unsintered bodies than for dense ceramics due to the inhomogeneity of the microstructure and of the uncertainty of the mechanism of bonding between particles in green ceramics. A few modeling attempts have  been made on binderless powder compacts since assumptions tend to be simpler but these computational models assume only contact flattening due to the high pressure of dry pressing and Van der Waals attraction between particles whose interfaces must be broken for the crack to propagate. Unfortunately, it is wrong to assume these are the only forces holding the particles together.

A team of researchers led by Professor W. Roger Cannon from the Idaho National Laboratory, developed an equation to predict fracture toughness of green powder compacts. They purposed crack tip toughness  is due only to the rupture of flattened interparticle contacts  but that closure forces along the crack surface supplied by the meniscus from moisture are significant, as well. Their work is now published in Journal of American Ceramic Society.

The research team initiated their experiments by preparing the pellets from cerium dioxide powder to be utilized in the fracture studies. The powders were pressed without binder and  stored for a specific period in five different jars of varying relative humidity. Afterward, the moisture content was measured.   Then pellets were randomized and tested using the standard diametral compression test to determine the fracture strengths.

The authors observed that storing the pellets at high relative humidity for an extended period of time led to fracture strength more than double those stored at lower relative humidity. However, it was seen that at lower relative humidity, there was no noteworthy increase in fracture strength with increased relative humidity which was in accordance with the predictive model. The lower strength at low relative humidity was considered to be either due to insufficient capillary and surface forces as predicted by the model or may have also been related to the insufficiency in the adsorbed moisture to form the bridging meniscuses.

W. Roger Cannon and colleagues have presented for the first time a novel equation for predicting fracture toughness of green compacts. This equation is the sum of Kendall’s equation for contact flattening between particles combined with Van der Waals attraction and the crack bridging equation based on capillary and surface tension forces of the meniscus developed along the crack. Their work presents significant development that upon application would help minimize chipping and breakage of green ceramics during processing.

Controlled relative humidity storage for high toughness and strength of binderless green pellets. Advances in Engineering

Controlled relative humidity storage for high toughness and strength of binderless green pellets-Advances in Engineering

About the author

Dr Roger Cannon is a professor emeritus of the Department of Materials Science and Engineering at Rutgers the State University of New Jersey.  Research reported here was performed after retirement from Rutgers University while consulting with Idaho National Laboratory.  Dr. Cannon has a Ph.D. from Stanford University and was a member of the research staff of the Department of Materials Science at MIT prior to becoming a member of the faculty at Rutgers University.

About the author

Rita Hoggan is a Nuclear Fuels Development Research Engineer who works at the Materials and Fuels Complex, at the Idaho National Laboratory, to perform research and development of nuclear fuels from fuel design, development, and fabrication through post irradiation analysis. Rita believes that advancements in nuclear materials will help to change the world’s energy future by increasing the safety and longevity of the current nuclear fleet and advancing the progress toward new nuclear systems.
Rita has supported fabrication and characterization of accident tolerant fuel concepts that are set to revolutionize current light water reactor fuel technology. Rita’s recent investigations on fuel/clad interactions and thermal stability of new fuel types has provided valuable information for fuel design and development. Rita has received several achievement awards for her work at the INL, is trained in sintering of ceramics, and metallographic techniques and interpretation. Rita holds a Master’s and Bachelor’s degree in Nuclear Science and Engineering and Applied Physics from Idaho State University and Brigham Young University, respectively.

 

About the author

Arnold Erickson is an R & D scientist/engineer in advanced materials development and processing characterization.  He has studied pyro-processing of rare earth oxides, silicon carbide joining, graphite oxidation, fossil energy materials, aluminum-oxy-nitride materials, low level mixed waste encapsulation, and thermal and cold plasma chemical conversion processes.  He received his BS in Chemistry from Idaho State University.

About the author

Mr. Forsmann is currently the lead operator for sample preparation in the Advanced Materials Lab at the Center for Advanced Energy Studies (CAES). He assists and collaborates with researchers, and graduate students, using one of kind testing equipment and making metallurgical samples of tested materials for microscopic analysis and destructive testing.

Reference

W. Roger Cannon, Rita E. Hoggan, Arnold Erickson, Bryan Forsmann, Robert C. O’Brien, Paul A. Lessing. Controlled relative humidity storage for high toughness and strength of binderless green pellets. Journal of American Ceramic Society. 2017; volume 100: pages 4442–4449

 

Go To Journal of American Ceramic Society

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