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Current Applications of MEMS Energy Harvesters

Thu, 16 Jun 2011 16:17:17 +0000

Current Applications

There are several commercialized MEMS-based energy harvesting products on the market. This report will focus on the Volture devices by Mide Technology Corporation, the Micro Generating System for a Watch by Kinetron, the Perpetuum Free-Standing Harvester, and attempts at commercializing shoe energy harvesters.

One piezoelectric energy harvester is the Volture System, which is able to convert mechanical vibrations from two separate frequency ranges, 60-140 Hz or 80-175 Hz, into electrical energy. The Volture System uses Mide’s patented QuickPack Piezoelectric transducer, which has a cantilevered inertial mass clamped at the free end.

The Volture System can be implemented in helicopters, where sensing is needed to allow the helicopter to remain at peak efficiency. Because powering sensors can be difficult, converting vibrational energy to electrical energy offers an economical and wireless solution. The Volture System can in fact be used in places where powering any kind of sensor is difficult, such as in factories and plants, where wireless sensors are useful in monitoring the status on machines and equipment. The Volture system can generate 500 µW at 113 Hz, and has a device size of 3.625 x 1.75 x 0.39 in³. Its price is $299 per module for an unturned device, or $399 per module if tuned, and can easily be mounted onto a vibration source with four through-holes. Electrical insulation allows for the device to function in harsh and humid environments. ^{Sources 1, 2}

The Micro Generating System for a Watch is made by the Dutch company Kinetron. Their device allows for the kinetic energy of the human wrist to be converted into electrical energy through a series of steps. In the first step, a mass winds up a mechanical spring through a rectifier. Next, the spring drives a micro generator shown below up to speeds of 5000 rpm.

The energy is then stored in a rechargeable battery. According to Kinetron, the average person will generate 400 mJ a day by causing the mass to turn 4000 complete revolutions. Wrist angles as small as 25 degrees are enough to keep the watch running. This micro energy generating system is compatible with the Swatch Autoquartz and the ETA 204 movement. ^{Source 3}

Another energy harvesting product is the Perpetuum Free Standing harvester, which can be mounted on industrial motors. This device converts vibrational energy into electrical energy using by using an oscillating mass connected to a coil that passes through a magnetic field created by a permanent magnet. A voltage is induced by Faraday’s law because of the changing magnetic flux. The Perpetuum device can provide power up to 20 mW, and has high reliability with no maintenance necessary for 20 years. Additionally, it has a high working temperature range and has a center-through hole for ease of installation. The device is shown below. ^{Source 4, 5, 6}

One more application of energy harvesting is in electricity generating shoes. A walking human who weighs 68 kg produces a theoretical 67 W of energy at the heel of the shoe. ^{Source from Jon’s MST review paper} Piezoelectric shoes have been demonstrated by the Electric Shoe Company in 2000, but nothing has been commercialized since then. Professor Kaajakari at Louisiana Tech has built prototypes of piezoelectric shoes that generate 2 mW per shoe in a normal walking pace. This is enough energy for GPS receivers and locator tags placed in shoes which require a mW power source. He claims that these cost less than a dollar and only weigh 6 grams. ^{Sources 7, 8,9}

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MEMS (Micro Electro Mechanical Systems) Notes

Thu, 16 Jun 2011 16:00:33 +0000

numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.

Phase shifting masks – selectively altering phase to take advantage of destructive interference

Soft shifters – molybdenum silicide layer

Hard shifters – etch quartz mask

Immersion Lithography – increasing numerical aperture with medium that has high index of refraction. Problems: bubbles, particles

Antireflective coating – prevents reflected light from being reflected back to structure.

Extreme UV –

Disadvantages: need vacuum and reflective optics

XRay –

Advantages: no vacuum, high throughput, high resolution and aspect ratio
Disadvantages: expensive, masks are hard to make

Imprint disadvantages: template wear, defects from air bubbles.

Image reversal allows you to create a negative PR profile using a positive photoresist. Perfect for lift-off, where a negative photoresist is required!

SU-8 – thicknesses from 1-500 microns with a single spin.

ITO – indium tin oxide. Transparent electrode. Can be deposited by sputtering.

Difficult to form plasmas at high pressure because collisions are not energetic enough (short mean free path).

Difficult to form plasmas at low pressure because not enough particles for avalanching.

Cathode – reduction (plating)

Anode – oxidation (etching)

Electroplating: brighteners reduce crystal grain size and make the surface smoother or brighter. Surfactants help wet hydrophobic surfaces for plating.

Extrinsic sources of thin-film stress – thermal expansion mismatch, plastic deformation

Intrinsic sources of thin-film stress – lattice mismatch, impurities (causes buckling or cracking)

Sputtering – compressive stress

PECVD – compressive

Fusion bonding – use high temperature to bond wafers. For bonding like wafers. Si to Si or SiO2 to SiO2.

Anodic bonding – Can bond Si to SiO2.

Wafer-wafer bonding for packaging – uses more area, has a high cost for batch production, lower temperature.

Thin film packing – lower cost, higher temperature, minimizes areal footprint.

Pirani gauge – measure pressure from the thermal conductance of gas. If the gas is at high pressure, gas molecules collide frequently with the filament and absorb energy from the filament which results in cooling of the filament. As the pressure of the gas molecules decreases the number of gas molecules inside the chamber also goes down resulting in fewer collisions with the filament. As a result the temperature of the filament increases because of decreased cooling. Electrical resistance of a wire varies with temperature.

Poly MUMPS – 3 layer poly micromachining. Can be used to make a micromotor.

Mass – volume L³

Heat transfer, diffusion, muscle strength, bone strength – area L²

Surface tension and surface energy – length – L

Adhesion – state of minimum energy attained when two solids are brought into intimate contact. Adhesion increases with increasing surface smoothness and material softness.

Stick-slip caused by roughness

Pyroelectricity – material develops voltage when heated or cooled.

Thermoelectric – seebeck effect – conductor with a temperature difference develops an electrical potential. Peltier effect – voltage difference creates temperature difference.

Shape memory alloys – metals that after being strained at a certain temperature revert back to their original shape.

Thermal bimorph – two layers stacked on top of each other with different coefficients of thermal expansion alpha.

Electrophoretic effect – Migration of ions in solution under the influence of an electric field.

Electroosmosis – DC voltage applied and positive charge ions migrate towards cathode.

Dielectrophoretic effect – force is exerted on a dielectric particle when it is subjected to a non uniform electric field.

Surface roughness in wetted state makes hydrophobic surface appear more hydrophobic, and hydrophilic surface appear more hydrophilic.

SEM relies on secondary or backscattered electrons.

Secondary electrons – an incoming electron excites an electron on the surface that can then be detected. Surface morphology sensitive.

Backscattered electrons- electrons collide with the nucleus of atoms and are scattered backwards, and can then be detected. Scattering is considered quasi-elastic. The higher the atomic number, the higher the probability of electrons being backscattered. Chemical composition sensitive

SEM samples are thick.

TEM – samples must be thin (less than 1 mm). The electrons do not interact with the specimen. Low atomic weight atoms transmit electrons and appear light in the TEM image, but heavy elements appear dark.

AFM – cantilever tip is bent and laser reflected.

STM – sharp tip brought close to sample, and when 10 angstroms away, electrons begin tunneling. Both sample and tip must be conductors or semiconductors.

CVD growth of nanowires – gold particles (20-100 nm) are deposited on a substrate that is heated to a high temperature. The vapor is absorbed into the gold catalyst particles. By increasing the temperature and vapor pressure, a second growth step can be accomplished.

Self-Assembled Monolayer – thiol group absorbs to gold surfaces, the alkyl chain is attached to the thiol group, and a functional group attaches to the alkyl chain.

Smaller quantum dots have a larger bandgap. So smaller quantum dots emit light at a higher energy (lower wavelength, higher frequency, more blue), whereas larger quantum dots emit at longer wavelengths, lower frequency, and more red. Blue is 400 nm, red is 700 nm.

Graphene is one atom thick planar sheet of sp2 bonded carbon atoms. Graphene is the strongest material ever tested, thinnest known 2D thin film, record high thermal conductivity.

CNTs are metallic if n-m is a multiple of 3. Otherwise they are semiconducting.

Fermi energy level – highest occupied energy level at absolute zero T.

Graphene transistor – Since graphene has no bandgap, you have to cut it into a narrow ribbon. Graphene continues to conduct a lot of electrons in its off state.

Dye-sensitized solar cell – titanium dioxide nanoparticles covered with a molecular dye that absorbs sunlight, like chlorophyll in green leaves. Like in a battery, an anode and a cathode are placed on either side of the electrolyte. Sunlight passes through the transparent electrode into the dye layer where it can excite electrons that then flow into the titanium dioxide. The electrons flow toward the transparent electrode where they are collected for powering a load. After flowing through the external circuit, they are re-introduced into the cell on a metal electrode on the back, flowing into the electrolyte. The electrolyte then transports the electrons back to the dye molecules. Efficiency can be increased further by adding quantum dots to the TiO2 nanoparticles.

Nanowires or tunable quantum dots can capture 90% of the solar spectrum leading to higher efficiencies.

Nanowires have high electron and hole transport properties. Reduced manufacturing costs because no vacuum or high temperature required. Cheap materials – TiO2 is very cheap. Challenges are low efficiencies.

Near field Scanning Optical Microscopy – uses optical imaging that has resolution beyond the diffraction limit. Has a sharp tip with optical fibers that scans the surface of the sample. The resolution is not limited by diffraction but by the size of the aperture of the optical fiber tip.

PCR – heat separates a single strand of DNA. You lower the temperature then add a synthetic DNA fragment (DNA primers), then heat up again to separate and repeat.

When a fluorophore or quantum dot is close to a quencher (<10 nm), the energy from the fluorophore or QD is transferred to the quencher, and no fluorescent light is emitted. Light is only emitted when the fluorophore and the quencher are separated.

Molecular Beacon – stem and loop. Loop contains the complementary probe sequence. The stem contains the fluorophore and quencher on each side of the stem. In absence of targets, probe is dark, since fluorophore is close to the quencher. When the probe encounters a target molecule, it forms a probe-target hybrid, and fluorophore and quencher are separated.

Endocytosis – cells absorb materials by engulfing it with a little piece of their cell membrane. After entering the cell, the physical forces that hold together the self-assembling nanoparticle no longer exist and the nanoparticle falls apart.

DNA array to detect normal and sickle-cell hemoglobin – use inkjet printer to print out a fluorophore and quencher at two ends of a stem, in different locations, one for sickle cell, and one for normal.

Folding springs allow for linear motion instead of a single spring that allows for arc motion.

Endocytosis

Wed, 23 Mar 2011 21:10:58 +0000

Endocytosis – process by which cells absorb material from outside the cell by engulfing it with their cell membrane. Nanoparticles can transport drugs into cells using this method. After entering the cells, the physical forces that hold together the self-assembled nanoparticle no longer exist, so the nanoparticle falls apart and the drugs entrapped within are …

Nano Electric Lithography

Wed, 23 Mar 2011 21:09:52 +0000

Nano Electric Lithography (NEL) – can pattern 10 nm nanoparticle assembly with high throughput.

A nanoscale lithographic method in which a reusable conductive mask, having a pattern of conductive surfaces and insulating surfaces, is positioned upon a substrate whose surface contains an electrically responsive resist layer over a buried conductive layer. When an electric field is applied between the conductive mask and buried conductive layer, the resist layer is altered in portions adjacent the conductive areas of the mask. Selective processing is performed on the surface of the substrate, after mask removal, to remove portions of the resist layer according to the pattern transferred from the mask.

Biomedical applications of nanotechnology

Wed, 23 Mar 2011 21:09:06 +0000

Fluoresent reportor – an incident photon of high enough energy can be absorbed raising an electron to the excited state. The electron can recombine with a hole in the valence band and emit a photon (fluorescent light).

Fluorescence Resonance Energy Transfer – When a fluorophore or QD is close to a quencer (<10 nm), the electron at the QD can transfer its energy to the quencher without emitting the fluorescent light. The light is only emitted when the fluorophore and quencher are separted.

Molecular Beacon – single stranded DNA probe that has a stem and loop structure. The loop contains the probe sequence that is complementary to the target sequence. In the absence of targets, the probe is dark since the stem puts the fluorophore close to the quencher, but when a target is encountered, the stem hybrid dissociates, and the fluorophore and quencher are separated giving off light.

How Polymerase Chain Reaction Works

Wed, 23 Mar 2011 21:02:17 +0000

Polymerase Chain Reaction (PCR) - method to generate millions of copies of a DNA sequence.

PCR uses thermal cycling, with repeated cycles of heating and cooling. Heat physically separates the two strands in the DNA double helix, and lower temperatures each strand is used as a template.

Primers (short DNA fragments) with sequences complementary to the target region along with DNA polymerase allow for repeated amplification. As PCR progresses, the DNA generated is used as a template for further replication allowing for exponential replication.

DNA

Wed, 23 Mar 2011 21:01:20 +0000

DNA:

Genes provide instructions for proteins to build new copies of a cell, or repair damage.

Each protein is a specialist that only does one particular job, so every time a cell wants to do something new, it has to produce a new protein.

Proteins are made from 20 different amino acids. One type of protein is an enzyme, which are tiny machines that alter other molecules.

A, T, G, and C are the 4 nucleotides. A pairs with T, and G pairs with C.

DNA transcription:

DNA sequence is read by RNA polymerase, which produces a complementary RNA strand. The RNA complement has uracil instead of thymine.

The steps of transcription are:

1. RNA polymerase unzips the DNA by breaking the hydrogen bonds between complimentary nucleotides

2. RNA nucleotides are paired with complementary DNA bases.

3. RNA sugar-phosphate backbone forms with help from RNA polymerase

4. Hydrogen bonds from the RNA and DNA helix break, freeing the newly made RNA strand

Can be used to make mRNA (if the gene transcribed encodes a protein), or tRNA

DNA replication:

1) DNA strands are separated

2) RNA primers are created on DNA template strands.

Nano Composites

Wed, 23 Mar 2011 20:59:05 +0000

Nano Composites – reinforcing material called fiber and a continuous matrix material.

The reinforcement material can be:

Particulates: 0D
Fibers: 1D, could be rods or whiskers
Platelets: 2D

Pressure Molding – a composite sheet material made by sandwiching chopped fiberglass between two layers of thick resin paste. To form the sheet, the resin paste transfers from a metering device onto a moving film carrier. Chopped glass fibers drop onto the paste, and a second film carrier places another layer of resin on top of the glass. Rollers compact the sheet to saturate the glass with resin and squeeze out entrapped air.

Resin Transfer Molding –resin is injected under pressure.

Problems in CNT composites

Adhesion between CNT and polymer matrix like epoxy
Homogeneous dispersion in the matrix
Aligned CNT along the loading direction

CNT-epoxy composites have an increased tensile strength and young’s modulus.

CNTs can be grown on a carbon fiber substrate using CVD

Near Field Scanning Optical Microscopy (NSOM)

Wed, 23 Mar 2011 20:58:02 +0000

Near Field Scanning Optical Microscopy (NSOM) – allows for optical resolution beyond the diffraction limit. The probe is an optical fiber with a sharp tip. The sidewall is coated with metal to improve reflection. Light can only enter at the sharp tip. The resolution is not limited by diffraction, but by the size of the …

How Plasmons Work

Wed, 23 Mar 2011 20:56:27 +0000

From http://www.nanocomposix.com/images/stories/Services/service-uv-vis/plasmon.jpg

Plasmons – a metal cube is placed in an external electric field pointing to the right. Electrons accumulate on the left side, and positive ions on the right side. When the electric field is switched off, the electrons and positive ions oscillate back and forth at the plasma frequency. Light of frequency higher than the plasma frequency is transmitted, whereas light of frequency below the plasma frequency is reflected.

Plasmons are electronic oscillations in metal under the influence of an external electric field.

When the metal nanoparticle size decreases, the plasmonic wavelength decreases resulting in a blue shift.

Lycurgus cup – green color is from reflection. Red and purple colors are from transmitted light.

Stained glass – different colors come from the size variation of embedded gold nanoparticles.

Single nanoparticles appear blue, two nanoparticles appear green, and multiple particles appear orange.

Plasmon resonance can be used to measure nm scale distances, since the scattering light wavelength of two nanoparticles shifts as a function of the distance them.

Plasmons also can enhance the electro-magnetic field at a metal-dielectric interface.

Raman Scattering – light is scattered with a different frequency from the original frequency of the incident photons.

Surface Enhanced Raman Scattering (SERS) – surface sensitive technique that results in the enhancement of Raman scattering from molecules adsorbed on metal surfaces. The local electric field can be enhanced by 10¹⁴-10¹⁵. The technique can be sensitive enough to detect single molecules.

Raman- AFM system – uses a sharp metal tip and illuminating tip with a laser beam.

Surface Plasmonic wave – electromagnetic wave that propagates at the interface between a metal and a dielectric. Metals have dielectric constants that are negative. The wavelength of the plasmonic wave can be reduced significantly.