Scientists Build Microdevices With Magnetic Manipulation

December 2004
By Robert K. Ackerman
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North or south may point the way to a new generation of small systems.

Researchers at a national laboratory have discovered a way to construct microelectrical systems using magnetic fields to arrange internal structures. The technology already is opening the door to breakthroughs in sensor and magnetic identification systems, and yet-undiscovered capabilities such as realistic artificial limbs and more esoteric applications may lie on the horizon.

The focal point of this research is the properties of deposits formed of particles in a polymeric matrix. In effect, scientists are embedding particles in liquid polymer and are arranging them in patterns using magnetic fields. As the polymer dries to a hardened state, the particles set in patterns that define the function of each composite structure. This can affect how they conduct electricity or generate magnetic fields, depending on the types of embedded particles.

As the researchers developed new methods of establishing patterns in the base materials, they discovered new properties and additional applications emerged. Each success paved the way for further breakthroughs to the point where the scientists who have pioneered the process now are considering how to exploit the optimum performance levels that they believe they have attained.

James E. Martin, who is on the technical staff at Sandia National Laboratories, New Mexico, and heads a team conducting this research, explains that the technology emerged from a series of successive developments. The process begins with a fluid that can be solidified, and this can be any kind of polymeric resin—even wax. Magnetic particles are introduced into the fluid. Sometimes they are treated so that they are oxide-free and can conduct current.

If these particles are randomly dispersed throughout the solidifying polymer, little can be achieved in terms of effect. However, when researchers applied magnetic fields to place the particles in particular patterns, new capabilities began to emerge. For example, if engineers want to run as much electrical current as possible through the particle-embedded polymeric material, then a particular pattern of particles should provide optimum results. This is where magnetic manipulation comes in. Scientists used magnetic fields to arrange the particles in chains that run along the direction of the desired flow of electrical current. In effect, the particles became little wires within the material, and their conductivity improved several orders of magnitude.

This first step opened up a host of possibilities, and it also unveiled some undiscovered properties. One of the first target applications was sensors. Martin relates that, after fabrication, researchers applied a small amount of pressure to the field-structure composite at the end of the particle chain. By applying various measured loads to the chain, researchers could change the conductivity of the composite by as much as 12 orders of magnitude.

The effectiveness of a conventional strain sensor based on electrical resistance is measured by the change in resistance relative to the strain on the sample. A formula for quantifying this effectiveness defines typical strain sensor systems as having a gauge value of 10, although some systems are measured as high as in the hundreds. Martin relates that the laboratory’s field-structure composite sensor has a gauge value of 1014, or 100,000,000,000,000—an 11 or 12 order-of-magnitude improvement.

Following up on the first strain experiments, researchers exposed thin forms of these same materials to chemical vapors. They used inert substances such as xylene, which penetrated the polymer and diffused quickly. It swelled the polymer, which broke up the particle chains slightly, and produced a measurable effect in the current flow—the resistance changed by 10 orders of magnitude.

Martin explains that this opens the door to constructing a whole family of chemical sensors fabricated of different polymers. Each polymer embedded with these particle chains would react differently to different chemical substances, so a group of sensors would generate a level of selectivity by their differential response.

Another application of this technology would be a magnetic watermark. Extremely small particles in a polymeric film would be structured differently in various regions of the film. While still liquid, a thin layer of this composite could be spread out on an identification or credit card, for example, and the particles would be exposed to a complicated magnetic field. When the film dries, its fixed pattern of magnetism would be unique, and this would become apparent in a magnetic field such as from a scanner.

Other options focus on the post-manufacture composite material itself. Martin suggests that one alternative would be to use a flexible material—an elastomer—as the basis for embedding the particles. Engineers could build an elastomeric worm embedded with these particles that would hang vertically with a weight attached to the bottom. When a magnetic field is applied to this composite worm, it would contract by 100 times as much as a piezo-electric material. So, Martin continues, it could be used as a long, large-strain actuator. “Any time you need something to actuate, you could use magnetic fields and these magnetic particle elastomers and obtain these relatively large strain effects.”

One potential application would be robot muscles. Magnetic fields exerted on an elastomer construct could be varied or cycled to obtain different degrees of actuation. So, the composite muscle could “flex” according to variations in the magnetic field.

Researchers also are making the particle chains out of conductive magnetic particles. These are embedded in an elastomeric worm, which then is stretched using a weight until it barely conducts electricity. Then, when a magnetic field is applied, the worm contracts and it conducts much more current—as much as a 100,000-fold increase in conductivity.

The same principle that is applicable to robots could be used for prosthetic arms and hands. The elastomeric robot muscle system could control the structure of the prosthesis, and conductive magnetic particle elastomers could comprise the hand’s fingertips. As the elastomer fingertips touch an object, the physical resistance that they encounter registers as a change in electrical conductivity, and the system would respond accordingly. These soft-strain sensors would allow a robot claw—or a prosthetic hand—to pick up an egg without accidentally crushing it. The “fingers” would apply only the amount of pressure necessary to grip the egg.

All of these applications work using magnetic fields that are very small and that do not interfere in everyday activities, Martin adds. The field required to activate the elastomeric worm is only one-tenth of a tesla, or 1,000 gauss, which is too small to affect conventional magnetic storage devices such as legacy floppy disks or even credit cards. And, the fields can be localized.

These spectacular results that researchers are demonstrating have been achieved by structuring the particles using a uniaxial magnetic field, or one that runs in only one linear direction. However, when scientists decided to combine magnetic fields during the fabrication process, they opened up another magnitude of potential applications.

Martin relates that researchers applied two more magnetic fields at cross axes to the original uniaxial field in a three-dimensional, XYZ-axis pattern. All three fields were generated at substantially different frequencies to avoid interfering with one another. Instead of forming chains among the particles, these combined fields formed much more complicated structures that actually resembled isotropic networks, Martin notes. The particles that assemble in the polymer conduct electricity well in all three directions. So the material’s properties can be optimized in all three.

But, even more possibilities unfold when two of the three frequencies are close to one another. By generating one field at 200 hertz and another at 201 hertz—while keeping the third farther away at about 450 hertz—the first two magnetic fields will beat against one another creating a heterodyne effect. Martin describes this as “a little bit like what you see if you look down into a washing machine.” The particles form sheetlike structures in one direction, and then they disintegrate and form sheetlike structures along a new direction.

“Suppose you had a lot of people in a large field standing in a checkerboard pattern,” he analogizes. “Now, suppose that everybody is facing north, and everyone is told to place their right hand on the right shoulder of the person in front of them. From above, you would see lines of stripes running north-south.

“Now suddenly everybody is told to drop their right arm and take their left arm and put it on the person alongside them. Looking down, you would see stripes running east-west. This oscillating pattern looks strange to the observer,” he notes. But, it is representative of how the conflicting magnetic fields beat against one another.

The result is that, when the polymer progressively solidifies, all of the particles assemble themselves into a pattern similar to a bee’s honeycomb. In keeping with that analogy, the polymer is the honey and the particles constitute the wax hexagonal patterns.

When the third magnetic field is brought down to the neighborhood of the first two fields—to 202.2 hertz—all three fields’ components beat against one another, and the magnetic oscillations take place in three dimensions, not just in two as with the checkerboard analogy. Martin relates that heterodyne magnetic fields knead the particles as if in bread. When the polymer solidifies, the triaxial magnetic fields have created a pattern that optimizes the technology to an even greater degree. The magnetic permeability of these materials is as high as it can possibly be, he states, and this also optimizes the electrical and thermal conductivities.

“In essence, we have found a way to use magnetic fields to make these particle composites as useful as they can be,” Martin declares. “They even are as good as the optimized structures that we can get from computer simulations.”

Now the laboratory is looking at varying the types of particles. Martin observes that the particles can range significantly in size, and this can produce vastly different effects.

For example, large particles of magnetic materials have different spins in what is known as their magnetic domain. Martin explains that if observers could peer into this large magnetic particle, they would see that different regions of the particle have spinning that is going in different directions.

However, the smallest particles have spins that are oriented identically. These small particles, known as superparamagnetic particles, are fully magnetized even without the presence of a magnetic field, and their magnetism is as strong as is possible. So, when these nanoparticles are placed in even a weak magnetic field, all of their magnetic spins align.

As the nanoparticles are embedded in a polymer resin, they form an interesting structure pattern. Martin relates that each particle in the chain ends up having another particle—above and below—with a huge magnetic moment. In this effect, which occurs only when the small particles are formed into chains, the nearby particles generate a “colossal” magnetic field that keeps all of the particles in the chain trying to magnetize in the same direction. The result is a material with 10 times the magnetic properties as the same compound with large particles.

This can increase the effects that earlier devices have demonstrated. A magneto-elastomer, such as the flexible worm, will contract even more in a very small magnetic field. In real terms, it may lead to even smaller actuators than currently envisioned.

This is the point at which laboratory research seeks more answers, Martin offers. “We’ve made the magnetic nanoparticles; we’ve demonstrated these huge permeability effects; but right now we are trying to put these small magnetic nanoparticles into polymers and test them for actuators,” he relates. “We hope to have actuators that are very strong but at very weak applied magnetic fields.”

Building actual materials with the nanoparticles is the scientists’ near-term goal. Concurrent with that effort is the development of commercially viable sensor systems. These would include pressure, chemical or temperature sensors with extreme response. The sensors would require very little electronic power for operation, which would be ideal for an individual warfighter on the battlefield.

Magnetic Research May Amend Laws of Physics

The Sandia National Laboratories scientists who are shepherding nanoparticles with magnetic fields have made some unusual findings. Technical staff member James E. Martin relates that researchers have noticed that the magnetic fields have a bizarre effect on large numbers of particles. Normally, random particles in motion tend to coalesce into bunches that resemble clusters of grapes. After a time, these grapelike clusters begin to gather into a form similar to latticework.

When particles are subjected to the triaxial magnetic fields, however, they form patterns that resemble molecules. “You scratch your head and say, ‘What on Earth is going on?’” Martin says. On a smaller scale, two particles normally would pair off, and then the third particle would join the pair to form a triangular structure. Instead, under the influence of the magnetic fields, the third particle completely alters the interaction of the two that have paired off, and the three particles tend to form a chain. Adding more particles seems to inspire them to form more complex—and strange—structures instead of conventional crystalline constructs.

“Most of physics is based on summing interactions between things, and these interactions are known as pairwise additivity,” Martin observes. “But, we have discovered a system where there is no pairwise additivity. It’s as complicated and bizarre as things can possibly be.”

Martin continues that scientists must start from scratch to solve the problem exactly for the three particles. “When you have a suspension of particles and you apply triaxial magnetic fields, the interactions between any two pair of particles depend on where all the other particles are,” he says. “It’s a little bit like trying to do quantum mechanics with magnetic fields.”


Web Resources
Sandia Basic Energy Sciences/Material Sciences:
Sandia Lab News report:
(PDF version)

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