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Harper International has been awarded a contract to supply an advanced thermal processing system for the sintering of nuclear fuel pellets. Sintering is the crucial final step in the refinement of nuclear fuels before they can be used in nuclear power plants.

Typically, nuclear fuel is initially produced in a powder form, but needs to be converted into pellets before being assembled into rods or special bundles of rods. The powder is mixed with a binder and the pressed into a pellet shape. The sintering stage fires the pellets at high temperature, often in special atmospheres to induce reduction or other  chemical reactions to and remove the binder. Sintered pellets are a hard, dense solid with few pores.

Nuclear fuel sintering systems must meet several critical requirements such as hydrogen gas atmosphere with controlled dew point in the 1700–1800°C temperature range, as well as temperature uniformity, and safety control systems. The company says it has advanced furnace of this type in operation on four continents.

Harper International hasn’t put all of its eggs in the nuclear basket. It has a diversified energy strategy that has it working on silicon production, solar cells, wind energy and thermal processing of advanced materials for energy storage systems. Many of these advanced materials have been commercialized with process development and process optimization assistance labs at the Harper Technology Center in Buffalo, N.Y.

Here is a generic video illustrating a sintered pellet.

Post-sintering grain size, with electric field (right) and without (right). Credit: NC State, Hans Conrad and Di Yang

Two months ago, I wrote about how North Carolina State University’s Hans Conrad had apparently discovered that sintered ceramic materials could be deformed and shaped by applying an electric field. According to Conrad, the field interact swith the charges at the grain boundaries and make it easier for the crystals to slide against each other along these boundaries.

Now, Conrad has been again been tinkering with electric fields and ceramics, this time targeting the sintering process itself, and, once more, apparently has come up with some startling conclusions.

In brief, Conrad and his research team introduced an 60 Hz alternating current electric field during sintering of materials made of zirconia. Compared to normally sintered zirconia, the grain size of the ceramics fired under the influence of this electric field was reduced by 63 percent. They were also able to eliminate porosity in the material at 1,250°C rather than the expected 1,500°C.

In their experiments, Conrad’s team created grains with a diameter of 134 nm compared to the 360 nm diameter grains produced using conventional sintering methods.

The team also found that similar but less pronounced effects could be caused by a DC electric field (porosity eliminated at 1,400°C, grain diameter of 217 nm). Both AC and DC fields were 13.9 volts/cm.

“We found that the use of a small electric field - with a current of only six-tenths to eight-tenths of an amp per centimeter squared - can result in improved sintering rates with much finer grain size,” Conrad says. In other words, ceramics manufacturers can make their products more quickly and cheaply by using an inexpensive electric field - and make their product stronger as well.

“You don’t use much energy, and you put it right at the atomic site where it is needed - rather than using more energy to create higher temperatures in a kiln, which is less efficient,” Conrad says. “If you want to make a strong ceramic, you want to eliminate porosity and keep the grain size as small as possible. And you want to do it at the lowest cost - which means using the smallest amount of energy and doing it at the lowest temperature at the fastest rate possible. Using an electric field achieves all of these goals.”

The phenomenon is discussed in “Enhanced sintering rate of zirconia (3Y-TZP) by application of a small AC electric field,” which will be published in a forthcoming issue of letters-oriented journal, Scripta Materialia. The paper’s lead author actually is Di Yang, a senior research associate at NC State who works with Conrad.

For Conrad and Yang, the next steps are to guage how the electric field’s frequency and strength affect the outcomes, and also to test the electric field on other ceramic materials.

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In this video, Dinesh Agrawal, professor of materials and director the Penn State’s Microwave Processing and Engineering Center, provides an overview of the growing use of microwaves to make a variety of products and materials at faster rates, cheaper processing costs and, often, with improved properties. As Agrawal notes in a paper written for the Bulletin of the American Ceramic Society, the speed and efficiency of microwave technology makes the process “ecologically friendly.”

Microwaves can cut processing times by 90 percent, enhance sintering and reaction kinetics (providing much finer and uniform microstructures) and often create new materials not possible with conventional methods.

Want to catch up on the cutting edge of sintering? Then check out the new edition of the Journal of the American Ceramics Society (subscription required). The July JACerS has a special section on Advances in Sintering Science and Technology edited by Rajendra Bordia (University of Wisconsin) and Eugene Olevsky (San Diego State University).

The section contains 19 of the key papers presented at the International Sintering Conference held at in November 2008. According to Bordia and Olevsky, that conference became the largest specialized sintering forum in history that included over 203 presentations from 30 countries.

“[Sintering 2008] addressed the latest advances achieved in the sintering processes for the fabrication of powder-based materials in terms of fundamental understanding, technological issues and industrial applications. The conference has demonstrated a significant progress that has been made in multiscale modeling of densification and microstructure development, better understanding of the processing of complex systems (multilayered, composites, and reactive systems). In sintering technology, innovative approaches like field-assisted sintering (also known as spark plasma sintering) gain more attention of the materials processing community. Another very timely and well-represented topic was sintering and microstructure development in nanostructured materials,” they write.

Here are the paper titles:

• Particle Rearrangement and Pore Space Coarsening During Solid-State Sintering
• Evolution of Sintering Anisotropy Using a 2D Finite Difference Method
• Discussion of Nonconventional Effects in Solid-State Sintering of Cemented Carbides
• Linearization of Master Sintering Curve
• Master Sintering Curve Formulated from Constitutive Models
• Densification of Powder Compact Containing Large and Small Pores
• An Analysis of Four Different Approaches to Predict and Control Sintering
• Effect of Different Particle Size Distributions on Solid-State Sintering: A Microscopic Simulation Approach (p
• Evolution of Defects During Sintering: Discrete Element Simulations
• Verification, Performance, Validation, and Modifications to the SOVS Continuum Constitutive Model in a Nonlinear Large-Deformation Finite Element Code
• Three-Dimensional Solar Cell Finite-Element Sintering Simulation
• Hot Isostatic Pressing of Transparent Nd:YAG Ceramics
• Microstructural Evolution During Sintering with Control of the Interface StructureA Review on the Sintering and Microstructure Development of Transparent Spinel (MgAl2O4)
• An Experimental Measurement of Effective Diffusion Distance for the Sintering of Ceramics
• Uniaxial Freezing, Freeze-Drying, and Anodization for Aligned Pore Structure in Dye-Sensitized Solar Cells
• Investigation of the Sintering of Heterogeneous Powder Systems by Synchrotron Microtomography and Discrete Element Simulation
• Nickel-Boron Nanolayer-Coated Boron Carbide Pressureless Sintering
• Effect of Varying Displacement Rates on the Densification of Nanostructured Zirconia by Current Activation

Jingzhe Pan’s predictive sintering technique starts with a model of green compacted ceramic (left) and then projects an anticipated post-sintering dimensions (mesh, right) in comparison to pre-sintering cross section (outside shape). The distortion is caused by heterogeneous density in the green body. The figure shows two predictions, one made by Pan’s technique (solid line) which requires only the densification data. The other (dashed line) used a constitutive law which is difficult and expensive to obtain experimentally. Credit: Univ. of Leicester.

[This post has drawn a lot of attention, and we have updated it with the assistance of Professor Pan] A group of engineers at the University of Leicester in the United Kingdom, led by ACerS member Jingzhe Pan, believe they’ve made a critical breakthrough for improving sintering processes. The group describes their new approach as one that “removes trial and error” in the manufacture of ceramics, and achieves significant time and money savings by using new modeling techniques. Pan describes current sintering approaches as being too inefficient:

“Manufacturing advanced ceramics, even in this era of ‘precision’ techniques, is still very much a ‘trial and error’ process . . . [During sintering], materials are essentially re-packed more closely, such that overall volume decreases, whilst the density increases. Ceramics are intrinsically brittle making post-production alterations in dimensions very difficult. Failure to accurately estimate the final dimensions of ceramic parts, therefore, leads to a waste of materials, time and money.”

Pan notes that predicting change in dimension during sintering using the traditional finite element method requires extensive data on the materials being use, but getting this data can be difficult and expensive. He explains to the Bulletin that,

“Before our work, people thought that a ‘constitutive law’ is always needed to predict sintering deformation. The constitutive law is difficult, time consuming and expensive to obtain experimentally because the measurement requires applying force to the sample during sintering. This is why computer modeling has not been widely used by the ceramic industry.”

His group, instead, discovered that the constitutive law is not always necessary.

“We developed a method to use only the densification data – density as function of time – to predict the sintering deformation. Such data can be obtained by free sintering of small samples with no need to apply force. “[Using this data,] our computer software can predict changes in dimensions, even before production begins. This method does not depend on the physical properties of any one ceramic material. Direct comparison between our predictions with experimental measurements independently obtained by Bouvard’s group at Grenoble and Blanchart’s group at Limoges shows that the method works for both high purity alumina and low purity clays. Our method simply uses densification data from the small sample of the material and extrapolates the data, such that it can be applied to larger quantities used in manufacturing. It can thus, be applied to a wide range of ceramics,” he says.

He warns that his method is invalid for pressure assisted sintering, such as sinter forging or hot isostatic pressing. Pan acknowledges that his system is not quite ready for prime time, and the human interfaces needs to be simplified and redesigned before it can be marketed and installed in manufacturing settings. The group is also working on getting the system to apply to a broader range of industrial products.

Computer model (left) in comparison with experiment (right). High purity alumina powder compact - comparison between predicted (dashed line) and measured (solid line) profiles. The outer frame shows the initial shape of the section. The experimental measurement was done by H.G. Kim, O. Gilla, P. Doremus and D. Bouvard at the Institut National Polytechnique De Grenoble, France.

Computer model (left) in comparison with experiment (right). Low purity clay compact - comparison between predicted (dashed line) and measured (solid line) profiles. The outer frame shows the initial shape of the section. The experimental measurement was done by Magali Barriere and Philippe Blanchart at Ecole Nationale Supérieure de Céramique Industrielle, France.