Manufacturing Bits: Feb. 10

Deadweight machines; silicene mystery; Love wave sensors.

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Deadweight machines
The National Institute of Standards and Technology (NIST) is in the process of cleaning, restoring and recalibrating its 4.45-million Newton deadweight machine.

NIST’s deadweight machine, the largest of its kind in the world, is equivalent to one million pounds-force. Built in 1965, the deadweight system consists of a stack of 20 stainless steel discs about three meters in diameter. The average mass of the discs is about 22,696 kg, or just over 50,000 pounds, each.

A deadweight machine is a mechanical structure that generates force by subjecting deadweights to the local gravitational field. Deadweight machines are used for the precise definition of the force scale. NIST’s machine is used by aerospace manufacturers, military laboratories and others. It is used to perform various calibrations.

NIST's deadweight machine. (Source: NIST)

NIST’s deadweight machine. (Source: NIST)

“Malfunctions have been appearing in the computers, interface components, and voltage-ratio measuring instrumentation that can no longer be corrected by NIST staff or repair facilities,” according to NIST, on the agency’s Web site. “Simple replacements with current generation instruments cannot be made on an individual unit basis, due to incompatibilities with former generation computers, interfaces, or software. A unified upgrade of these items is necessary to prevent degradation of the service to only those tasks that can be conducted on a manual basis.”

The first stage of the restoration is dismantling the top half of the three-story stack of weights. The individual pieces are being cleaned in preparation for recalibration. The machine is expected to be back online later this year.

Silicene mystery
The mystery continues to unfold for silicene. Like graphene and other 2D materials, silicene could enable futuristic FET devices.

Many have reported the epitaxial synthesis of silicene. But to date, there has been no report of silicene-based devices due to air stability issues. And recently, the U.S. Department of Energy’s Argonne National Laboratory has called into question the existence of silicene. The group basically debunked the material.

On the other hand, the University of Texas at Austin has devised the first transistors made of silicene, which is said to be the world’s thinnest silicon material. Researchers developed what they call a silicene field-effect transistor, which has a mobility of ∼100 cm2 V–1 s–1 at room temperature.

Honeycomb lattice structure of silicene. (Source: University of Texas at Austin)

Honeycomb lattice structure of silicene. (Source: University of Texas at Austin)

To address the challenges, researchers teamed up with the Institute for Microelectronics and Microsystems in Italy, to develop a new method for fabricating the material. First, a hot vapor of silicon atoms were condensed on a crystalline block of silver in a vacuum chamber. This, in turn, formed a silicene sheet on a thin layer of silver. Researchers added a layer of alumina on top.

Researchers transferred the sheet silver-side-up to a substrate. They scraped the silver. This enabled two metal electrodes, with a strip of silicene between them.

“Apart from introducing a new player in the playground of 2-D materials, silicene, with its close chemical affinity to silicon, suggests an opportunity in the road map of the semiconductor industry,” said Deji Akinwande, an assistant professor in the Cockrell School’s Department of Electrical and Computer Engineering, on the University of Texas’ Web site. “The major breakthrough here is the efficient low-temperature manufacturing and fabrication of silicene devices for the first time.”

Love wave sensors
Surface acoustic wave sensors are MEMS-based devices. These sensors rely on the modulation of surface acoustic waves to sense a physical phenomenon. They could be used in several applications, such as biology, weather forecast, medicine, among others.

Polymers are used for the acoustic waveguide layers, but the materials suffer from insertion losses. The Institute of Acoustics (IOA) of the Chinese Academy of Sciences (CAS) has found a new technology to get around the problem–Love waves. Named after mathematician Augustus Edward Hough Love, Love waves are basically horizontally polarized surface waves.

Researchers investigated humidity sensing by Love wave detectors. The detectors themselves were coated with different polymeric films. A Love wave humidity sensor has a waveguide layer above the substrate and the interdigital transducer (IDT). In theory, this technology can get higher sensitivities and lower insertion losses.

In the lab, researchers devised a Love wave device. It consisted of two IDTs with a period of 28μm. A polymer waveguide (SU-8) and a polyvinyl alcohol (PVA) were also used. The films, SU-8, SU-8+PVA and PVA, were spin-coated on the substrate surface.

Structure of a Love wave humidity sensor (Source: IOA)

Structure of a Love wave humidity sensor (Source: IOA)

A network analyzer was used to measure the operation frequency and insertion loss of the sensor at various humidity levels. Researchers found that the Love wave humidity sensor coated with PVA film had the best results.