Thermal actuator

Shape-Memory Alloys. Stoeckel defines a shape-memory alloy as the ability of some plastically deformed metals (and plastics) to resume their original shape upon heating. This effect has been observed in numerous metal alloys, notably the Ni—Ti and copper-based alloys, where commercial utilization of this effect lias been exploited. (An example is valve springs that respond automatically to change in transmission-fluid temperature.) Copper-based alloy systems also exhibit this effect. These have been Cu-Zn-Al and Cu-Al-Ni systems. In fact, the first thermal actuator to utilize this effect /a greenhouse window opener) uses a Cu—Zn-Al spring. 

Because the vibration is spontaneous (i.e., thermally actuated) and the readout is optical (Fig. 4.23), there are no electrical connections required for their operation even in conducting electrolyte solutions. This greatly simplifies the packaging and encapsulation for biosensing, for example. 

Computer-controlled automatic profile dies with electrical controlled sensors in closed-loop control systems have developed greater efficiency and accuracy to extrusion coating, cast film, and sheet lines. A scanner measures the web thickness and signals the computer, which then converts the readings to act on thermally actuated die bolts. The individual adjusting bolts expand or contract as ordered by the computer to control the profile. The more sophisticated systems measure adjusting bolt temperature and provide faster response time with less scrap and quicker startups. The scanner is typically an infrared, nuclear, or caliper-type gauge. 

Patents describing specific applications or formulations are available. Specific items which appeared include aryloxyphosphazenes for flame-retardant panels and fibers, MEEP-LiCFjSOj based thermally actuated secondary hydrogen... 

Diamond has a low heat capacity, a low thermal expansion coefficient, and a high mechanical and thermal stability. These properties are very useful for devices using high dynamic thermal stress such as ink-jet heads. Indeed, ink-jet heads were fabricated using diamond films [17, 422]. Figure 13.8 shows a thermal actuator... 

Figure 13.8. Diamond thermal actuator with two thermistors at each side to monitor the thermal transient [17, 422].

If an autotransformer is not used, the temperature of the heating fluid may be controlled by placing a thermally actuated switch (such as a Fenwal Thermoswitch) in series with the heating coil. The switch may be set for the desired temperature and will thereafter regulate the on and off periods to maintain this temperature. When such a switch is used, the bath may be connected directly to the current supply. 

Sandia s design thermally actuates an exothermic reaction of metal hydride with water to generate the hydrogen on board the microsystem. The pressure created by this reaction and the low viscosity of the hydrogen enable the high flow rate of the mobile phase, which, in turn, enables the desired 2 -second chromatographic separation. 

The Achilles heel of the AIMS system was the Redwood microvalves that were used for positive flow shut-off Failures generally occurred when the system was first powered up, which suggested that power transients were the cause of many of the problems. Specifically, these valves were thermally actuated, so opening one of the SOVs produced a large transient current draw of about one amp, but for less than % second. Adjustments were made in the LabVIEW control program to prevent the simultaneous actuation of multiple valves. In particular, the valves were not allowed to actuate when power was first applied to the chassis. 

With the possible exception of the thermal actuator, all of these structures tend to be much larger than the valve itself and most require fairly large areas. Besides these drawbacks, the long-term reliability of the valves will depend on striction and gradual seat degradation with extended use. 

Capillary Force Valves, Fig. 9 A volume of 25 pL is isolated by the creation of two new liquid-air interfaces, through the use of two thermal actuators. Two air bubbles isolate the separated volume from the rest of the liquid in the main channel [6]... 

Bullen D, Wang X, Zou J, Chung SW, Mirkin CA, Liu C (2004) Design, fabrication, and characterization of thermally actuated probe arrays for dip pen nanolithography. J Microelectromec Syst 13(4) 594-602... 

Electrothermal or Thermal Actuation Motion is generated by differential thermal expansion in materials such as sDicon or metals, while heat is typically injected into the actuator by means of Joule heat dissipation. Some actuator components expand more than others due to different cross-sections and therefore electrical resistance. Typical microactuator configurations include in-plane bimorph elements (Fig. 2a), out-of-plane bimorph plates, in-plane Chevron, or bent-beam elements (Fig. 2b) [3]. These types of actuators exhibit larger force capabiUties (in mN range), can achieve displacements of 100 pm or less but generally consume a lot of power (hundreds of mW), and have low bandwidth (Hz to KHz). Common methods of fabrication for electrothermal microactuators include surface micromachining... 

Butler JT, Bright VM, Cowan WD (1999) Average power control and positioning of polysilicon thermal actuators. Sens Actuators 72 88-97. doi 10.1016/ S0924-4247(98)00211-8... 

Thermal mechanical valves Thermally actuated mechanical valves Thermally driven mechanical valves... 

The actuators can be broadly classified based on their principle of operation as thermal actuators, electrohydrodynamic actuators, magnetohydro-dynamic actuators, momentum injection actuators, and moving surface actuators. 

Printing multiple layers could even produce three-dimensional structures. A thermal actuator has been... 

The most common macroscopic actuators are motors. Electromagnetic motors do not operate efficiently at the microscale, but alternative actuation mechanisms become favorable. In conventional MEMS, other types of actuators have been used to generate movement, such as electrostatic actuators, thermal actuators, piezoelectric crystals, SMAs, and magnetic actuators. 

Conventional actuators also suffer from relatively large footprint and this large size limits the degree of miniaturization and actuator densities that can be achieved. In contrast, arrays of polymer thermal actuators [108] have been fabricated that have a high actuator density and can be used to exploit parallel arrangements, such as for the handling of large objects. 

The brittleness of inorganic materials means that they readily break upon contact with macroscale objects. The polysiKcon legs used in the thermal actuator array above snapped when too much weight was appKed. Similarly, polysilicon microgrippers have been known to break if touched by a macro-object or if exposed to air velocities higher than 1 m/s [109]. 

There are a large number of polymer actuation mechanisms, each suitable for particular applications. The most ubiquitous for MEMS is thermal actuation, which is accomplished with conventional polymers and which makes use of differences between the CTE of the polymer and a second layer, which is often a metal. The actuator is heated by running current through the metal layer. To give some example metrics [108,111], stress can be up to 80 MPa and lifetimes greater than 10 cycles. Strains are up to 2%, but decrease above 1 Hz. They have intrinsically high power consumption because they turn the input energy into irrecoverable heat. 

FIGURE 14.9 Chip with V-groove polyimide thermally actuated legs (left). Cross-section schematic of one of the actuators (right). (From Ebefors, T., Polyimide V-groove joints for three-dimensional silicon transducers. Department of Signals, Sensors, and Systems. Royal Institute of Technology (KTH), Stockholm, Sweden, 2000.)... 

Copper pads for integration are electroplated. After this, seed layer is patterned to provide electrical continuity for the diamond bridge to be thermally actuated. 

A Cr/Au seed layer of 700A is deposited using an ion beam reactor following which a l- jm thick copper film is deposited by electroplating, which serves as the bi-metal for thermal actuation. 

Schmid P, et al. Diamond switch using new thermal actuation scheme. Diamond Relat Mater 2003. 

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