Until now, bulk measurements that produced average strength values had shown that catalytically grown tubes are weaker than comparable structures grown by arc discharge. The new measurement technique, based on mechanical resonance, makes it possible to produce data on individual tubes with well-characterized microstructure, resulting in better information to predict performance.

The measurement technique is based on the mechanical resonance induced by an oscillating electrical voltage in a transmission electron microscope to measure the comparative bending strength of the tiny carbon tubes.

Strength measurements could be correlated to observed point defects, which could help materials scientists select the best variety of nanotube for an application. A point defect occurs when a missing or added atom results in a five- or seven-sided structure rather than a hexagonal one.

Point defects have big implications on nanotube applications, said Z.L. Wang, a Georgia Tech professor of materials science and director of the Center for Nanoscience and Nanotechnology. The high strength, light weight, and conductive properties of multiwalled carbon nanotubes make them candidates for applications as diverse as ultralight composites and low-power field emission displays. Both catalytically grown tubes and carbon arcs have unique advantages. Catalytically grown nanotubes may offer advantages for ultralightweight composites, in which point defects could help interlock the tubes to prevent pulling out of the finished part. However, those defects could cause problems in electronic applications, such as field emission electrodes where current flow could cause uneven heating in narrow regions.

The measurement technique begins by gluing a single nanotube, a few hundredths of a micron in diameter and five to 20 microns long, to a golden ball in a specially prepared transmission electron microscope sample holder. The tube is aligned near another gold ball and an oscillating voltage is applied. Adjusting the frequency of the voltage allows the researchers to induce a mechanical resonance in the tube that can be observed and measured. By knowing the outer diameter, inner diameter, length, and density of the nanotube under study, the bending modulus can be determined from the frequency at which the tube resonates. The oscillation can be observed in a transmission electron microscope, allowing the strength to be correlated to visible defects.

Wang said the technique can be applied to any fibers or wire-like materials, such as silicon and SiC nanowires, for mechanical property measurements. He said the measurement technique can improve theoretical modeling, making the models much more practical in determining applications. He believes that the technique could be an advantage in space technology, where composites reinforced with carbon nanotubes can reduce weight by a factor of five to 10, while increasing the strength by the same factor compared to a conventional carbon fiber matrix.

The client was Peregrine Industries, a Deerfield Beach, Fla., supplier of equipment used to heat swimming pools. Its goal was to replace conventional metal heat exchangers with a plastic version to reduce size and weight, and to resist corrosion. Peregrine chose a liquid crystal polymer, or LCP, that was filled with a proprietary carbonaceous material.

The more thin fins a heat exchanger has, the greater its heat transfer will be. For its polymer heat exchanger, Peregrine designed a part containing 150 fins, ranging in thickness from 0.024 to 0.062 inch. Dimensions were given to 3 Dimensional Engineering for production of a master by stereolithography.

The contractor took the master and reinforced liquid crystal polymer to the Ralph S. Alberts Co., a custom molder in Montoursville, Pa. Scott Hay, president of 3 Dimensional Engineering, requested an epoxy injection mold to form as many prototype parts as possible of the reinforced polymer.

More than 300 highly detailed heat exchanger prototypes (front) were produced from a cast epoxy tool, shown here in two halves.

The large number of thin ribs in the new design would make the part difficult to mold in an epoxy mold, which would have to withstand high temperatures and pressures, according to Mike Downs, injection-molding supervisor of Alberts Co. The company began evaluating rapid tooling epoxies that could withstand the high temperatures and pressures needed to shoot the parts. It decided to cast the tools of an aluminum-filled epoxy called Cast-IT 2000 from Ciba Specialty Chemicals' performance polymers division (now renamed Vantico) in East Lansing, Mich.

Alberts Co. moldmakers found that Cast-IT 2000 had a combination of strength, high glass transition temperature, and thermal properties required for the injection molds that would produce the heat exchanger prototypes. The epoxy could also be polished to achieve a metal-like finish, and had low shrinkage and dimensional stability to mold accurate parts.

Once the tooling material was selected, moldmaking began with the master preparation. All surfaces on the stereolithographic model were smoothed to a fine finish. Then, several coats of sealer and release agent were applied to prevent mold sticking in the many thin, ribbed sections. Next, the molds were cast, incorporating several aluminum inserts. The tools were poured at room temperature on a vibrating table to eliminate voids at the rib edges. Then, REN RP 4037 R/H tooling epoxy was poured behind the Cast-IT 2000 to form back structure support. The molds were post-cured in an oven. Finally, a flash gate was installed to optimize mold filling.

To run the parts, molds were heated to 190°F in an oven and mounted in a 150-ton press. Several parts were shot at different temperatures and pressures, with best results achieved using a melt temperature of 760°F and an injection pressure of 18,500 psi. Parts were produced in a cycle time of four minutes, with 30 seconds for part injection and curing and three and a half minutes for cooling. The Alberts Co. produced 300 parts with no failure in the epoxy mold.

The assembly designer also had a special texturing requirement, which was different from the standard glass, matte, or orange-peel options that are typically applied by airbrush. In this case, a process known as Prototex, developed by the Akron Metal Etching Co. in Akron, Ohio, would be used to apply texturing.

Lee Eisinger, president of Akron Metal Etching and inventor of the Prototex texturing process, saw a potential problem with DSM Somos series resins because their polypropylene-like flexibility might cause the part to reject the texture.

So far, it appears his concerns were unfounded. Eisinger was able to successfully apply the requested Akron Metal Etching E204 pattern with a depth of 0.0015 inch to the three-part mirror assembly consisting of rear, middle, and bezel components.

Akron Metal Etching's process makes it possible to achieve intricate designs, including text and logos. The texturing process involves applying a blue photopolymer onto the surface of the part in a layer thickness that varies from 0.0005 to 0.012 inch.

A rearview automotive mirror prototype has been textured to give the look of a production part.

Once the pieces are dried, the texture is applied by one of a number of methods, depending on the selected grain, configuration, and thickness required. In this case, the pattern, which was a very fine texture, was applied by a film-based system, Eisinger said. Further blending of seams and splices for uniformity and total coverage to deter any pattern anomaly is then completed. After they have been covered, the parts are exposed to ultraviolet light curing cycles. The film is then removed and the part is placed in a chemical bath, which etches away any unexposed polymer. The texture is actually built up in relief.

"Once Lee finished and the parts were returned, we made silicone rubber tooling and cast several polyure-thane prototypes—which have an appearance just like injection-molded parts," said LeMaster.

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