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Bridging the Macro to Micro Gap
December 31, 1969 |Estimated reading time: 5 minutes
By Arthur Chait, EoPlex Technologies Inc.
Nanotechnology promises to revolutionize everything someday. A Google search on "nanotechnology" returns over 12 million citations. While nano gets all the attention, the rest of us have to design and manufacture miniature devices using current technology. One solution is a high-volume forming method derived from print layering, using specialized sets of proprietary materials.
At the macro end of the spectrum, we use methods like machining, assembly, and automation to build everything from automobiles to iPods. In the miniature range, we use MEMS, lithography-electroforming-replication (LIGA), semiconductor processes, etching, micro-molding, embossing, and other techniques to create devices.
Unfortunately, there is a gap in the miniature scale, especially when we need to build devices with multiple materials (Figure 1). MEMS and related methods work well with a single material, like silicon, and some techniques can handle two materials. However, few technologies deal in several materials, with complex geometries, in small parts.
Basic assembly is still used to manufacture small, multi-material devices. Unfortunately, even with automation, assembly has significant limitations. Most significantly, it requires low-cost labor. A good example is the tiny motor used in cell phones for vibration mode. These motors are about 4 × 8 mm and are one of the smallest devices made in high volume and assembled by hand (Figure 2).
In many ways, only little progress has been made in manufacturing devices like these. After Richard Feynman's famous lecture, "There's Plenty of Room at the Bottom" in 1959, he offered a prize for a motor 1/64th-inch square. The award money of $1,000, equivalent to about $7,000 today, was won by Bill McClelland. His original motor is still on display at Cal Tech.
Feynman happily awarded the prize but was disappointed that no new technology was created. McClelland was an excellent engineer with great patience. He used tools like toothpicks, pins, a microscope, and a single-hair brush to create his motor with a conventional design. This was a tour de force of craftsmanship, but it was not new. Fifty years later, cell phone motors are made in a way that is not too different from 1959.
Assembly has additional limitations and, even with low-cost labor, some parts are still too expensive and others cannot be made at all. Other technologies are available for small, complex devices and often these can be used alone or in combination to fill the gap. However, there are designs where none of the technologies available will work in a cost-effective manner.
For example, suppose that we want to manufacture a portable fuel cell small enough for consumer products but powerful enough for applications like laptop computers and emergency radios. A fuel cell using hydrogen gas is an attractive option, but carrying hydrogen is not practical. We either would carry a compressed tank of gas, which is heavy, dangerous and presents a refueling challenge, or look at exotic schemes like metal hydrides that are almost as bad. An attractive alternative would be to extract hydrogen from a fuel like alcohol, which is relatively safe and readily available. This is easily done in a chemical reactor called a hydrogen reformer; however, for a small portable application, we need to miniaturize the reactor.
It is easy to design a small hydrogen reformer, but it is exceedingly difficult to manufacture one at low cost. The problems become obvious when we look at what it would take to build a reformer of any size:
- • The body of the device would be ceramic for chemical and thermal resistance; • The interior would have a labyrinth of channels to carry the alcohol to the reaction chambers; • Reaction chambers would be ceramic and lined with catalysts to extract the hydrogen; • There would be a tiny metal heating coil wound around the reaction chamber; • Micro ducts would carry away the hydrogen, water vapor, and carbon dioxide produced by the reaction; • The outside of the reformer would have electrical contacts and connections for inputs of alcohol and air, as well as outputs of hydrogen and exhaust.
A structure like this would require at least 5 different materials, e.g., ceramics, catalysts, conductors, the heating element, and connectors.
This is just one example of the type of device that engineers struggle with in the miniature scale. Others include energy harvesters, specialized RF components, and micro fluidic pumps. One solution is a high-volume forming method derived from print layering, using specialized sets of proprietary materials. Here, large sheets of images are deposited with proprietary "inks." The materials decompose on heat treating to leave behind ceramic; metals; dielectrics; conductors; and even voids, channels, and open spaces. Parts are built up by combining hundreds of these layers to create thousands of parts at once, then sintering them to finished structures.
The process can be explained by way of the hydrogen reformer discussed earlier. Computer modeling allows engineers to scan the reformer in slices, or layers. Enough scans ensure that all the features within the part have the required detail. This resembles the way an MRI or CAT scan slices images of the human body for diagnosis. A single scan does not look like much, but when all the layers are stacked together, the full 3-D structure is formed.
With less-complex parts like cell phone antennae, it typically takes only about 30 images to achieve the required resolution. For a hydrogen reformer, 300 scans could be necessary. Once the proper number of images is determined, the computer data helps define the required printing tools. These include printing plates, masks, screens, stencils, transfer rollers, etc., depending on the printing methods.
Inks, the consistency of toothpaste, are assembled so that no materials are conflicting in the structure. Matching the properties of the different inks is vital. These inks can deposit ceramics, glasses, dielectrics, piezoelectrics, conductors, metals, and polymers. They must be able to be processed in the same heat treating and atmosphere processing required. Materials scientists have many tricks available to make dissimilar materials work together, a "critical mass" limits possibilities. For example, two materials with widely different expansion coefficients require careful design and modification to prevent them from splitting apart in heat treating. Some material combinations cannot be processed together. For example, sintering tin and tungsten powders together is not possible, since one requires a low temperature process and the other requires very high temperatures.
ConclusionHigh-volume printing of material inks is one method to create miniaturized devices that enable new applications macro devices could not facilitate. Along with cell antennae, reformers, and energy harvesters, other applications could bring electronics and mechanical elements together in the miniature scale to bring about new generations of product and technology. SMT
Arthur Chait, CEO of EoPlex Technologies Inc., may be contacted at (650) 298-6501; achait@eoplex.com; www.eoplex.com.