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The image on the left is 3Y-TZP sintered at 1,500°C. The image on the right is of 3Y-TZP, which had a 60 Hertz AC electric field applied to it followed by sintering at 1,250°C. Credit: Hans Conrad, NC State University.

Last year we reported on a number of papers that were published in 2010 on electric field assisted sintering, including a few by Hans Conrad at North Carolina State University. According to press releases, the application of AC or DC fields during sintering was effective in reducing processing temperatures necessary for elimination of porosity, while simultaneously reducing grain growth. The effect is athermal, i.e., essentially independent of heating rate and sintering temperature.

Two mechanisms for the effect have been proposed to explain the suppression of grain growth. One possibility is the grain boundary energy (the driving force) is reduced because of an interaction of the electric field with the space charge. The other is that there may be Joule heating at the grain boundaries.

A Rapid Communication (doi: 10:111/j.1551-2916.2011.04823.x) in the August 2011 Journal of The American Ceramic Society by Hans Conrad presents an analytical approach to testing the feasibility of the grain boundary energy reduction mechanism.

Yttria-stabilized zirconia (3Y-TZP) samples were sintered under electric field at temperatures between 800°C and 1500°C. (Process parameters are detailed in referenced papers). Grain size was measured using SEM.

Starting with the equation for grain growth, which relates change in grain size over a time interval to an activation energy term and to the driving force. Field-adjusted values are substituted in for the variables. The mathematics eventually leads to a value for the space charge potential, which the paper states “is in reasonable accord with predictions, calculations, and measurements of the potential in oxides.”

The authors conclude that the “agreement of the calculated values of the space charge potential and the grain boundary energy with theoretical considerations and actual measurements provides support [for the theory] that the mechanism for the retardation of grain growth” is related to an electric field-induced reduction of the grain boundary energy through an interaction between the applied field and the space charge.

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Credit: Science/AAAS/UTD NanoTech Institute

Recently I wrote about Israeli-based TorTech, which says it will soon be manufacturing carbon nanotube fiber yarns that it claims to be stronger than Kevlar, yet still flexible and lightweight. While Tor-Tech plans to implement the technology for military use, spinning nano yarns has potential for many applications.

MIT’s Technology Review reported on research out of the University of Texas at Dallas, demonstrating a way to spin yarn out of nanotubes infused with nano powders. According to the article, the researchers have used the method to make strips of yarn that have a wide array of properties, allowing them to function in diverse applications, such as a battery electrodes, superconducting materials and self-cleaning yarns.

These fibers could solve a big practical problem: Powders without form are difficult to use, yet they are a very important component to functional materials because they have very high surface area. Sintering and binding are complicated processes typically used to give form to powder materials.

Ray Baughman, director of the MacDiarmid NanoTech Institute at the University of Texas at Dallas, says nano yarns should make it easier to work with a wide range of powdered materials. “You can take almost any powder and make a sewable, knittable, knotable, braidable yarn,” he tells Technology Review.

According to the article:

The researchers start by growing a forest of vertically aligned carbon nanotubes in a chemical reactor. Then they drag a roller over the nanotubes, which separate from the surface and get tangled up in a long, stretchy ribbon — a so-called nanotube web . . . The researchers spray the surface of the web with the powder and then twist it into a yarn. The powder is confined inside the spirals of the nanotube web. “When you wash it, almost all the powder is retained,” he says. The resulting yarns can be 95 to 99 percent powder by weight.

Baughman’s group used a mixture of powdered boron and magnesium to make superconducting yarns by a simple process. The conventional process for making superconducting wires involves packing the powders in copper tubes and heating and drawing them tens of times to stretch them into wires. But the superconducting yarns are heated just once to anneal the powders and form a superconducting thread.

Some applications in which Baughman has applied the technology includes battery-electrode fabric using lithium-iron-phosphate powders. Since the fabric is about 99 percent active material, he thinks it could be used to make less-heavy batteries. The yarns could also be used to produce material for structural manufacturing.

On the NanoTech Institute’s website, researchers note, “We have spun carbon nanotube composite fibers at a hundred times the prior-art rate, and obtained fibers that pound-per-pound have twice the strength and stiffness and 70 times the toughness of strong steel wire. In addition to other functionalities, we have used these fibers for both electrical energy transmission and sensor devices in electronic textiles.”

First, its important to find appropriate markets. “Right now it’s more sensible to talk about batteries, not airplane wings, because of the tonnage [of materials] required,” says Baughman.

The powers can be applied via spraying techniques or even using inject printing A video demonstrating titanium dioxide powder being sprayed onto a moving nanotube web can be viewed on MIT’s website here.

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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.

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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 interacts with 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 gauge 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.

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