Q: What adhesives are you recommending 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.

A good alternative for using a liquid bonding agents are film adhesives, for example the film adhesive FM300-2M for strong permanent bonds or the 3M 465 film tape which can be used for re-using the MFCs. The 3M 465 is extremely thin. You have to apply it with a good 5mm overlap or more.

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

For this process we highly recommend using vacuum bagging techniques. 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.
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
The release film, cloth, and bagging supplies can be purchased from any composites manufacturing supply company. Examples of each are listed below.

Q: I want to use the MFC as a strain sensor but it seems I can not get any reading?

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.

Max. mechanical tensile strain for the MFC is approx. 4500 ppm, before fracture. This applies to a MFC without an electric field applied. The package might be still functional, although elastic properties will change. For the 12-mil (0.3mm) thick, standard MFC package, this works out to a minimum curvature diameter of the actuator of about 4.7 inches (120mm) curled in fiber direction and 4 inches (100mm) curled perpendicular to the fiber direction.