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Credit: Int'l. Journal of Applied Ceramic Technology

The online version of ACerS’ International Journal of Applied Ceramic Technology has a new story that reveals many of the problems scientists and engineers face when designing the tips of missiles – called domes – used by primarily by the military, and the results of some interesting research on a new dome material. The gist of the paper in ACT is that a group of Saint-Gobain researchers have found that ultra pure α-alumina powder may provide a superior material for making these domes.

Consider, however, the problems that must be addressed in coming up with a dome material. Besides providing an aerodynamic leading surface for air-to-air and air-to-ground missiles, a missile dome shields an array of sensors that control various systems within the missile. Among other things, some of these sensors are used to detect a variety of electromagnetic radiation (e.g., in both visible and infrared ranges).

The best systems must be able to differentiate, for example, between the signature of the exhaust of a jet engine and the signature of decoy flares that a targeted airplane might jettison. Thus the domes have to be functionally transparent in a fairly wide range of the spectrum. Likewise, the shape and thickness of the domes must be such that they don’t distort any of the incoming electromagnetic radiation.

To complicate things even further, the dome must be able to withstand enormous mechanical stresses and thermal shocks. At the start of a missile launch, friction causes the outside leading surface of the front most portion of the dome to heat more rapidly than the rest. This hot front surface expands more than the cooler internal portions of the doom, and soon there is significant stress between the expanded and the unexpanded material. If the stress exceeds the mechanical strength of the material, the dome shatters.

Some materials work well at lower speeds, but new missiles will soon be rocketing at +Mach 4 levels.

Cost and ability to manufacture/machine are factors, too.

A commonly used material for domes is monocrystalline alumina (i.e., sapphire) and polycrystalline magnesium fluoride. Sapphire is costly, difficult to shape and prone to chipping. Large boules of sapphire must be drawn and then machined, which is an art in itself. A sapphire dome that has all of the required properties is typically 4-5 mm thick. The benefit of sapphire is that is a relative transparent material in the 0.25–5 μm range. However, transparency declines significantly at wavelengths higher than 5 μm.

The transparency of MgF₂ is more limited than sapphire, but does very well in the 2–5 μm range, It is much less expensive than sapphire. Unfortunately, MgF₂ isn’t a particularly rugged material.

So, the question is this: Is there some alternative material with good mechanical/thermo-mechanical strength and transparent in the key IR and visible ranges that is easy to work with and cheaper than sapphire, and a thermal shock resistance better than magnesium fluoride will be of use for this kind of application?

Guillaume Bernard-Granger, Christian Guizard and Nathalie Monchalin, from Saint-Gobain’s Laboratoire de Synthèse et Fonctionnalisation des Céramiques, say domes made of a dense and submicronic form α-alumina powder may be a good and relatively inexpensive alternative.

Comparative thermal shock resistance. Credit: Int'l. Journal of Applied Ceramic Technology

In brief, they were able to document that their alumina material has good transparency in the visible and mid-infrared ranges and can be formed using mold-and-sintering processes rather than complex and delicate machining. Just as importantly, a dome can be made with a thickness as low as 1 mm and still survive Mach 4 speeds.

Overall, this is just a great example of how advanced ceramics, using ultrapure feedstock refined at the submicron and nano levels is providing new, customizable solutions to engineering problems. Read the paper for the details.

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Micrographs of two single-phase FeTiO5 laminates with textured and untextured layers. Image (a), top, shows crack bifurcation within the textured layers. Image (b) shows tunnel cracks in the textured layer. Source: International Journal of Applied Ceramic Technology

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 Shanti, David Hovis, Michelle Seitz, John Montgomery, Donald Baskin and Katherine 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.

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Transmission electron microscope micrograph of calcium phosphate nanoparticles. Credit: ACerS

What may be a new and effective alternative to insulin injections is being reported in a paper (subscription required) to published in an online edition of ACerS’ International Journal of Applied Ceramic Technology. Researchers Willi Paul and Chandra Sharma, working in India, report favorable in vitro results from tests in which insulin was bound to nanoparticles of calcium phosphate.

Diabetes sufferers are always looking for an alternative to the standard multiple daily needle injections for delivery of their insulin. Some have sought relief from alternatives delivery–transport systems such as insulin pump systems and transdermal patches. An inhaled form was introduced in 2007 but pulled from the market by the maker after several problems were documented.

The holy grail for insulin therapy is is to find a satisfactory oral transport system. Among the benefits of the injection systems is that dosage and timing can be controlled, and the insulin isn’t altered by injection. Thus, any replacement system will have to meet these three minimum requirements.

One particularly difficult challenge for oral delivery methods to to have the insulin be attached to a carrier without fundamentally changing the insulin. Then, the system has to move the protein through the stomach intact so that it can later be absorbed in the gut. Several companies, including Oramed, are already working to bring one such product to market. The exact mechanisms these companies are using are uncertain and are closely held proprietary technologies, but it is assumed that these probably use some type of polymer system.

Now, however, it looks like Paul and Sharma may have devised a ceramic-based competitor (I recently wrote about another ceramic system that uses nanodiamonds to deliver insulin or genetic therapeutics). The method the duo uses is to first activate the phosphate group of the CaP. They then conjugate it with lauric acid and give it a chitosan spacer. Insulin is then loaded onto this substrate. Finally, the nanoparticles are given a coating of alginate that gives each particle a pH-dependent sustained release mechanism.

A battery of tests demonstrated that these ceramic particles had excellent biocompatibility with insulin:

Lauric acid conjugated CaP nanoparticles are highly compatible with insulin and the CaP–CH–LA system can deliver insulin in a sustained manner in the physiological pH of the intestine with no degradation or conformational changes of entrapped insulin. These nanoparticles are having a size distribution with majority of the particles <100 nm and have been demonstrated to be noncytotoxic. Because it is known that fatty acid complexation can improve uptake of particles across epithelium, the lauric acid conjugated CaP nanoparticles may be used as a carrier for delivering insulin orally.

While these results are exciting, much more work needs to be done, including in vivo tests. One issue that still needs to be documented is how quickly the CaP particles break down. Degradation within 24 hours would be ideal.

Paul and Sharma’s paper can be accessed via the ACT “Early View.”

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Credit: Theonlysilentbob

An upcoming issue of ACerS’ International Journal of Applied Ceramic Technology (available as an “Early View” paper, subscription required) will have an intriguing report from three researchers from the United Kingdom’s Imperial College and a colleague from Tectronics Ltd. regarding a process to render flue glass emissions into an environmentally safe form and, potentially, into useful ceramic applications such as tile and porcelain.

The underlying issue is what to do with the residues from the growing number of dual-purpose operations that extract energy from solid waste, such as municipal trash. These energy-from-waste plants are required to have complex and tightly regulated air pollution control systems to capture and treat the gas emissions they generate. These APC systems typically yield powder residues, that, if put in contact with water, can leach dangerous chemicals. In the U.K. and elsewhere, these dangerous powders currently are stored in special landfills or saltmines.

The researchers wondered if there was a way to make the residues safer. They took an idea already used with medical hazardous waste and nuclear waste material - DC plasma treatment - and tested how it might be applied the APC residues. It turned out that DC plasma, as expected, has the ability vitrify the powder and, in the process, destroy hazards, such as dioxins and furans. The vitrified product is a stable, inert, nonleachable and recyclable glass product. Heavy metals that had been present are volatilized and collected as a very low-volume (5–8 wt%) secondary APC residue in a bag house filter of the DC plasma plant.

The researchers then took the APC residues, mixed in glass-forming additives (SiO₂ and alumina) and subjected the material to DC plasma arc treatment. The resultant material was quenched into water to produce a sample with ∼5 mm particles. They then combined the resultant frit with bentonite (a binder) to make various glass-ceramic tiles using a standard powder pressing and sintering route. They found that their test tiles had physical properties comparable to commercially available ceramics such as porcelain and monoporosa, with high bulk density (2.4 g/cm3), low water absorption (<6%) and high flexural strength (∼60 MPa).

The researchers conclude that,

“DC thermal plasma treatment of APC residues is expected to become increasingly economically viable compared with alternative disposal options such as landfill that involves haulage, pretreatment, gate fees and landfill tax . . .

“Glass-ceramic tile production incorporating plasma treated APC residues could provide high quality ceramic tiles. Combined thermal plasma treatment and glass ceramic production from APC residues therefore represents an integrated solution to the management of this problematic waste.”

It should be noted that the commercial-scale plasma facilities are relatively new, and only five or six are in operation in the world. None, to my knowledge, are in the U.S. or Canada, although several are in the planning stages.

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