Q:What adhesives do you recommend to bond MFCs to a structure?

We recommend two component adhesives like 3M's DP 460 Epoxy or Loctite's E120 HP Epoxy. Best results are obtained if the adhesive is cured at 50°-60°C for 2 hours and the MFC is pressed against the structure with a fixture during curing.

Q: What is the best way to bond an MFC to a Host Structure?

Please watch our video tutorial How to Bond and Cure the MFC on our YouTube channel!
The tutorial walks you through the steps of how to prepare the MFC for bonding and explains three different methods to cure the MFC after attaching it to a glass fiber composite. The methods shown in this video tutorial can be also used for different substrate materials.

We would like to discourage simple clamping techniques to bond the MFC to a host structures. Clamping techniques, if not used properly, can damage the outer Kapton shell of the MFC which will lead to early failures of the MFC

In general vacuum bagging is a safe method to bond the MFC to structures and can be done using the following steps:

1. Apply Adhesive to the host material.
   Note: Always use a new mixing tip.
2. Apply MFC to Adhesive
3. Wrap with 1 layer of Perforated Release Film
4. Wrap with 1 layer of Breather Cloth (to absorb excess adhesive)
5. Place in Vacuum Bag Tube
6. Cure

• Perforated Release Film - https://www.acpsales.com/Perforated-Release-Film.html
• Breather Cloth - https://www.acpsales.com/Breather-Cloth.html
• Vacuum Bag Tube - https://www.acpsales.com/Nylon-Vacuum-Bag-Tubes.html

Q: What type of wire do you recommend to solder to the MFC?

We typically recommend 24-26 AWG stranded hook up wire, with an insulation rating of 3kV. For example CNC Tech partnumber 3239-24-1-0500, which also is available in different colors.

Q: I want to use the MFC as a strain sensor, but it seems I’m not getting any readings.

Make sure you have attached the MFC to a structure which actually is inducing a strain into the patch, i.e. stretching or compressing the fibers.

Q: What is the max force that an MFC can produce?

The MFC will expand at 1800 ppm over the length of the actuator (free strain). The blocking force is about 4kN/cm² for the active cross section of the MFC.

Q: Is the MFC porous or non-porous?

The MFC is non-porous due to its environmentally sealed packaging.

Q: What amount of force is a standard MFC generating, displacement?

The M8557P1 is generating about 900N blocking force and ~150µm displacement (free strain).

Q: Can the MFC be operated at frequencies higher than 10kHz?

Yes. The published 10kHz is in general the upper limit of operating the MFC as an actuator using a high electric field (i.e. voltages in excess of one third of the maximum operating voltage). Piezo ceramic can operate at much higher frequencies of up to 10-20 MHz.
As a sensor the MFC is currently used in applications to detect strain of up to several MHz. In low electric field operations it is also used as an actuator to generate ultrasound of up to 700kHz (i.e. SHM applications). Major criteria for operating the MFC as an actuator at higher frequencies is the heat built-up in the device due to dielectric and parasitic losses. Monitoring the device temperature is a good way to determine the upper frequency/voltage limit in the specific application. In general the temperature during operation should not exceed 80°C.

Q: What is the typical density of an MFC?

Typical areal density is 0.16g/cm² or volume density of 5.44 g/cm³.

Q: What is the mechanical efficiency of an MFC, meaning electrical energy transformed into mechanical energy?

This question requires a little more in-depth analysis:

1. In general a PZT 5A material used in the MFC has an effective coupling coefficient (k33) of about 0.69. That is it's first order electrical-to-mechanical energy conversion efficiency. k33 is a measure of efficiency, but not the actual efficiency.
2. k33² is the ratio of stored mechanical energy to input electrical energy (= 0.48), but this is not the same as output work energy efficiency, since one can not actually use all of the stored energy to do useful work.
3. Max. output work energy efficiency (under optimum loading condition) for the MFC will work out to about 0.16, so max 16% of input electrical energy can be converted into useful output work with an MFC.
4. Max. output-work energy efficiency is not the same as output-work to consumed electrical energy efficiency! Most (may be 97-99%, depending on dielectric loss of the package) of the electrical energy not converted to work is actually stored electrostatically, i.e., like in a capacitor. You can recover that energy, in principal, with a clever drive electronic design.

Q: How tight a radius of curvature can you bend the MFC before cracking? For example the standard size 3.4" x 2.2" MFC M8557P1.

For the 12-mil (0.3mm) thick, standard MFC package the minimum curvature diameter of the actuator is about 4.7 inches (120mm) curled in fiber direction and 4 inches (100mm) curled perpendicular to the fiber direction.

Please download the following publication for additional information about the tensile strain behavior of the MFC: Nonlinear Tensile and Shear Behavior of Macro Fiber Composite Actuators

Q: What type of electrodes are available for 1-3 Piezo Composite?

We offer Copper-Tin (CuSn) or Gold (Au) electrodes for our 1-3 composites. All electrodes are applied using a vacuum deposition process and range between 700nm (Au) and 2.5 µm (CuSn) in thickness.
Our standard electrode is the copper-tin electrode which combines cost effectivness with electrical properties similar to gold. Soldering wires to CuSn is much easier compared to gold. Our CuSn-electrode is about 1.5µm to 2.5µm thick (depending on resonance frequency) with a 500 nm tin-coating as passivation layer to prevent the copper from corroding.

Q: How do I connect wires to electroded 1-3 composites?

Most of our customers solder wires to the 1-3 composites using a low temperature solder paste (Sn42/Bi57.6/Ag0.4).
Soldering on a 1-3 composite using a solder iron requires experience due to the fact that a considerable area of the composite under the electrode is made of plastic. If the solder iron is too hot or applied too long, the plastic will become soft and the bonding with the sputtered electrode material will fail causing the electrode to come off.
While soldering, please pay attention to minimize the contact time between composite and solder bit. We use a low temp solder paste used for SMD soldering with temperatures around 138°C and a contact time less than 1-2 seconds. If you cannot attach the wire to the composite in the first attempt, please give the composite some time to cool down, otherwise this spot will become over-heated and the electrode will fail.

Q: What solders should I use? What is a good solder procedure?

Skin the wire from its insulation, then plate the stripped length with tin. Apply a drop of solder paste to a spot on the 1-3 composite where you want to attach the wire. Use low temp solder paste shipped in a syringe. Use a solder iron with a small tip for a short period of time. Do not apply any loads until the tin has cooled.

Materials:

Using solder paste: Low temp Sn42/Bi57.6/Ag0.4, i.e. Edsyn CR11, Chip Quik SMDLTLFPT5

Using solder iron: Solder Stannol HS10 TSC 0,5mm, heat solder bit to 270°C, do not keep solder iron on the 1-3 composite for more than 1-2s.

Wires: Stranded, 0,06...0,14 sq.mm or enameled copper wire

Q: Can I glue wires to an electroded 1-3 composites?

Yes, you can - by using a conductive glue for attaching wires to the composites. Conductive glues allow for a reliable way to connect wires to our 1-3 composites, with comparable and better adhesion as standard soldering. We recommend the following glues from Epoxy Technology (www.epotek.com): Epo-Tek 430, Epo-Tek H20E, Epo-Tek EE 129-4.

Q: What is the difference between “dice-and-fill” and “arrange-and-fill” manufacturing methods?

Dice-and-fill technique has been around for many years and is more expensive than the arrange-and-fill method used by Smart Material. Essentially, dice-and-fill begins with a bulk block of PZT, wherein many grooves are diced using a diamond saw, and the grooves are filled with polymer. The extensive cutting is the primary factor in the higher manufacturing costs. With arrange-and-fill, long strands of fibers are arranged randomly on end, and polymer is poured around them, forming a large block of piezo composite. This block can then be diced into different length pieces according to the frequency desired. This translates into a faster, more cost-effective process.

Q: What is a PEH?

PEH is the abbreviation for Piezo Electric Harvester. Sometimes a Piezo Electric Harvester is also referred to as PEG, Piezo Electric Generator.

Q: What is a Cantilever and its function in a PEH?

A Cantilever is a rigid structural element that extends horizontally and is supported or clamped at one end only. Cantilevers used as PEHs are typically made from glass fiber composites, like FR4, sheet metal or plastic. A piezo electric device, like a monolithic piezo ceramic plate or a MFC, is normally bonded close to the supported/clamped side of the cantilever. Bending the cantilever strains the attached piezo ceramic device, either compressing or stretching it, and causes the piezo electric device to generate an electric charge. The amount of generated electric charge, measured in Coulomb, is a function of the amount of strain imposed on the piezo electric device. The amount of strain is, in simple terms a function of how much the cantilever is bent and the distance of the piezo ceramic device from the middle (neutral) layer of the assembly.

Q: What is a Piezo Electric Harvesting Circuit?

A Piezo Harvesting Circuit or PHC is an electronic device which accepts the generated electric energy of a PEH/PEG as input and generates a stabilized DC (Direct Current) output. A Piezo Harvesting Circuit is often also referred to as Conditioner, PMC (Power Management Controller) or Electronic Piezo Harvesting Module.
A PEH transforms mechanical or kinetic energy into electric energy in form of an electric charge or, if its output is connected to any type of electric impedance, as electric current. A PEH is in first approximation a capacitor and an alternating electric current source. A PEHC retrieves the harvested energy from the PEH. The PEHC's basic functions are:

• Rectify the AC current input from the PEH.
• Store the harvested electric energy in a capacitor, which is typically larger than the capacitance of the PEH.
• Generate a stabilized DC voltage as output from the retrieved electric energy.
• Available DC output voltages are typically 1.8V, 3.3V or 5V DC.

Q: What is a unimorph, bimorph or trimorph PEH?

A unimorph or monomorph is a cantilever that consists of one active layer and one inactive layer.
A bimorph is a cantilever that consists of two active layers.
A trimorph is a cantilever that consists of an active layer, an inactive layer and a second active layer on the opposite side of the inactive layer of the first active layer.
In case of a PEH the active layer(s) is a piezo electric material.

Q: Are electrical energy and electrical power the same?

Electrical energy and electrical power are often used in a way suggesting that they are interchangeable. They are not! Energy and Power are closely related but they are not the same. This is an important fact to understand if designing energy harvesting applications.

Energy is defined as the capacity to do work. Work can be in the form of kinetic energy, potential energy and thermal energy. It is also correct to say, if you are doing work to an object, you give the object energy. Furthermore you can add energy to an object by transferring heat. This is the first law of thermodynamics: The total energy of a system can be increased by doing work on it or by adding heat.
Energy is measured in Joule, J (SI unit). One Joule is equivalent to 1 Nm (one Newton times one meter) and 1 Ws (one Watt times one second).

Power is the rate of doing work or the rate at which energy is used, produced, or transferred.
The unit of power is the unit for energy divided by the unit for time. In SI units this is Joule divided by seconds, which is given the name watt, W. Many people are familiar with the energy amount of kWh (Kilo Watt Hour).

It is important to understand that it is the same amount of energy if you are using 1kW for one hour or 1W for one thousand hours. Or, it is the same amount of potential energy, as when a crane is lifting a 1 ton stone block 20 m up in 1 min or 1 hour. But the required power for the motor of the crane is higher if you lift the block in 1min instead of 1 hour because the time interval is different.