Patent Application
20130141207
Active thermal switches operate between states of thermal conductivity during which the switch transfers heat, and thermal insulation during which the switch conducts less or negligible heat. Miniaturized and/or arrayed active thermal switches could enable a range of new applications, including improving thermal management of integrated circuits and chip packages and new energy concepts. Current approaches have been unable to achieve distinct thermal contrast between the high heat conducting state and the low heat conducting state with small form-factors and fast actuation at temperatures suitable for many energy harvesting or cooling applications.
Issues may arise with thermal switches and their thermal conductivity contrast, switching speed, and the ease or difficulty of construction. Thermal conductivity contrast means the ratio of the thermal conductivity with the switch on to the thermal conductivity with the switch off. Many current approaches do not have good contrast. Similarly, many approaches have slow switching speeds between the thermal switch being on and off. Finally, many thermal switches have very complicated manufacturing processes, and use materials that can be difficult to handle or materials that are expensive. It becomes difficult to manufacture current heat switches efficiently and even more difficult to manufacture them in arrays.
An ‘active’ heat switch generally consists of a device with one or more thermal conductivities selectable by an input signal, such as a voltage. A two-port active heat switch has two contacts and accepts an input signal, shown in
A ‘single-pole, double-throw’ (SPDT) active heat switch selectively creates a high thermal conductance path between a common node and one of two target nodes in response to an input signal.
Note that the terms high and low as used here are relative to each other, where one path is higher or lower than the other. Additionally, k_on_A will typically approximately equal k_on_B, and k_off_A will equal k_off_B, but any pairs of relatively high and relatively low thermal conductance can be used.
An alternative to an SPDT switch includes a third state in which the thermal conductance between the common and both A and B has a low value. Yet another alternative allows a fourth state in which the thermal conductance between C and both A and B is relatively high. In addition, alternatives having more than one pole and/or more than two throws could also exist. Design of these and other similar variations is an obvious extension of this description and is not further discussed.
This discussion focuses on individual heat switches, with the understanding that these switches may reside in an array of individually addressed heat switches.
The contacts 20 and 22 have protrusions or other surfaces that allow the plug to connect the two contacts. In the OFF position, the plug 24 lies apart from the contacts 20 and 22, leaving a gap. The gap may have gas or liquid in it, or a low or high degree of vacuum. Higher vacuums result in lower thermal conductance in this state. Regardless if the gap has gas, including air, liquid, or vacuum, the path between the contacts has relatively low thermal conductance. The gas, liquid, or vacuum gap dominates the thermal resistance in the OFF position.
Upon receipt of a signal, the mechanical actuator 26 moves to bring the plug 24 into contact with the contacts 20 and 22. The thermal conductance between the two contacts is high, with the thermal resistance consisting of the sum of the resistance of the thermal connectors of the contacts, the interface material 28 and the plug 26. The contact may be made through a thermal connect, such as a volume of metal or silicon, or another relatively high thermal conductivity material. A thermal interface material 28, such as thermal grease, a carbon nanotube turf, an array of liquid metal droplets, a liquid metal film, etc., may reside at the contact interface. It may cover one or both sides of the interface.
In addition, depending upon the interface material, a higher pressure applied to the plug may generate a higher thermal conductance at the interface. The higher pressure may result from a higher impetus from the signal, an attractive force between the contacts and the plug, etc.
An array of switches such as the above may connect a single substrate to several contacts, with each switch connected to a same first contact but different second contacts. A high thermal conductivity path can form between the substrate and certain top contacts and not others. This is discussed in more detail in co-pending patent application, ______ (attorney docket number 20101117-US-NP-9841-0237).
The array of switches may be individually addressable at each actuator. The actuators may consist of one of many different mechanisms. For example, electrostatic actuation may be used. The plug attaches to the actuator, in this case a cantilever, positioned so that the OFF state requires no applied signal. One method of creating such a cantilever is with a stressed metal release process. When the switch is turned ON, such as by application of a voltage across the gap and another electrode, possibly on the substrate or on the thermal connectors on the contacts. The resulting charging of the interface attracts the plug, causing the actuator to move to close the gap. The displacement, spring constant of the actuator and applied force would be optimized for each design. Some general principles will apply to all designs, that the displacement be large enough that the thermal contact is poor in the OFF state, yet small enough that the force to overcome the spring force is able to be generated, and it must enable a large thermal conductivity at the interface in the ON state. Alternatively, the switch may be normally in the ON state and be switched to the OFF state upon application of the signal.
Another possibility involves piezoelectric cantilevers. The application of a voltage would cause a piezoelectric cantilever to bend, bringing the plug into contact. The actuator 36 may consist of a piezoelectric stack, such as one of lead zirconate titanate. This may generate up to 10 MPa of pressure or a 0.1% strain.
Further, the cantilever may consist of an electromagnetic cantilever. The cantilever may consist of a ferromagnetic material. Current applied to an electromagnet, such as a coil of conductive material on the substrate, would generate an attractive force.
The switch of
As mentioned above, a third signal may cause the actuator to move the plug to a neutral position, making no contact with either 30 or 32. Additionally, depending upon the shape of the plug and the thermal connectors of the contacts, it is possible that a greater movement would form contacts between the common terminal 38 and both devices or regions 30 and 32.
Other types of switch architectures are also possible.
The OFF position may occur because of a signal that causes the plug to release from the top contact, with the ‘passive’ state in which there is no signal having the plug in contact. Alternatively, the passive state may have the plug in the OFF state, with the application of a signal causing the plug to make contact. In the OFF state, the plug may submerge in the liquid 42, or remain above the surface. The high thermal resistance of the gap dominates the thermal conductivity of the switch.
In the ON position, the plug contacts the top contact 40, either by application or removal of the signal to the actuator 46. Again, a thermal interface material may exist on the contact between the plug and the contact. The thermal path of the switch includes the thermal conductance of the plug and the liquid, forming a high thermal conductance path to the bottom contact 48. This configuration may achieve a much higher fill-factor of an array of switches.
It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
This application is related to co-pending patent applications 13/______, (attorney docket number 20101117-US-NP-9841-0237), titled, “MECHANICAL HEAT SWITCH;” and 13/299,729, titled, “THERMAL SWITCH USING DROPLETS.”
