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The current issue of the International Journal of Applied Ceramic Technology reports on a Northwestern University group’s work related to using improved gelcasting techniques that allow new possibilities for manufacturing of certain laminates.

Gelcasting, a technique perfected at the Oak Ridge National Lab, is a method used to create large, near-net-shape ceramic and metal components with complex shapes from low-viscosity slurries composed of powders suspended in a liquid binder system. The components begin to be solidified when a chemical initiator is added to the slurry. This starts the formation of a polymer gel network. The slurry is quickly poured into a molds and allowed to dry. The additives and binders are burned out before sintering.

The team of Noah O. Shanti, David B. Hovis, Michelle E. Seitz, John K. Montgomery, Donald M. Baskin and Katherine T. Faber describe the use of more flexible gelcasting system – thermoreversible gelcasting – that allows more opportunities to manipulate the materials during the molding stage, something they found useful, for example, in toughening laminates.

The advantage of TRG over traditional gelcasting is that it is not time constrained (as long as the slurry temperature is kept above the transition temperature). Lamination is possible during gelcasting by adding successive layers of slurries selected because the properties and interfaces being sought. The teams describes concepts of tailoring the porosity and texture of the layers to, for example, strengthen the final laminate material by crack deflection, crack bifurcation and taking advantage of residual compressive stresses.

In one case, they describe the using the enhanced manipulation time to introduce a magnetic field to the materials during casting. The magnetic field aligns ceramic particles and allows the development of highly textured microstructures. The would not be possible using traditional gelcasting techniques because of the relatively brief window of opportunity before solidification begins.

The groups notes that while the use of TRG requires a good understanding of polymer chemistry, physics, and slurry rheology, not to mention drying and sintering kinetics, they predict the technique will find much use in applications ranging from strong bioceramic materials to solid oxide fuel cells.

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

[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.