US20090277608A1

US20090277608A1 - Thermal Control Via Adjustable Thermal Links

Info

Publication number: US20090277608A1
Application number: US12/242,438
Authority: US
Inventors: Theodore I. Kamins; Duncan Stewart; Alexandre Bratkovski
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2008-05-07
Filing date: 2008-09-30
Publication date: 2009-11-12

Abstract

An apparatus for thermal control includes a first component; a second component; an adjustable thermal link disposed between the first component and the second component; and a controller for selectively varying a thermal conductance of the adjustable thermal link. A method of controlling a temperature includes sensing a temperature of a first component; and adjusting a thermal conductance of an adjustable thermal link, the adjustable thermal link forming a thermal path between the first component and a second component; the thermal conductance of the adjustable thermal link being adjusted such that the temperature of the first component is controlled.

Description

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from provisional application Ser. No. 61/051,081, filed May 7, 2008, the contents of which are incorporated herein by reference in their entirety.

BACKGROUND

Controlling the temperature of components within integrated electronic systems can be challenging. Most electronic components generate heat as a result of normal operation. This excess heat can dissipate into surrounding components before leaving the system.
Integrated electronic systems can have a number of components that are temperature sensitive. For example, the properties of resistors, diodes, transistors, and resonant frequencies of oscillator crystals are all sensitive to temperature. The physical dimensions, material stability, electron behavior, chemical interactions, strength, and other properties are also affected by temperature. In integrated electronic systems that incorporate optical components, temperature variations can adversely affect the ability of the optical components to generate, modulate, and detect light.
As the integrated electronic system operates, it can produce varying levels of heat that are proportional to the duty cycle of various heat generating components. These changing heat loads can cause undesirable temperature fluctuations within the system. Consequently, temperature control and stabilization are major design issues for integrated electronic systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of the principles described herein and are a part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the claims.

FIG. 1 is a diagram showing one illustrative configuration for applying an electric field to piezoelectric materials, according to one embodiment of the principles described herein.

FIG. 2 is a diagram illustrating the response of piezoelectric materials to an applied electric field, according to one embodiment of the principles described herein.

FIG. 3 is a diagram of an illustrative ring resonator and tangential waveguide, according to one embodiment of the principles described herein.

FIGS. 4A and 4B are cross-sectional views of an illustrative ring resonator, a heat source/sink, and an adjustable thermal link, according to one embodiment of the principles described herein.

FIGS. 5A and 5B are cross-sectional diagrams showing an illustrative adjustable thermal link, according to one embodiment of the principles described herein.

FIGS. 6A and 6B are cross-sectional diagrams showing an illustrative adjustable thermal link, according to one embodiment of the principles described herein.

FIGS. 7A and 7B are cross-sectional diagrams showing an illustrative adjustable thermal link, according to one embodiment of the principles described herein.

FIGS. 8A and 8B are cross-sectional diagrams showing an illustrative adjustable thermal link, according to one embodiment of the principles described herein.

FIGS. 9A and 9B are cross-sectional diagrams of an apparatus using an adjustable thermal link for temperature control, according to one embodiment of the principles described herein.

FIGS. 10A and 10B are cross-sectional diagrams of an apparatus using an adjustable thermal link for temperature control, according to one embodiment of the principles described herein.

FIG. 11 is a cross-sectional diagram of an apparatus using an adjustable thermal link for temperature control, according to one embodiment of the principles described herein.

FIG. 12 is a cross-sectional diagram illustrating a capacitive actuator to alter the thermal conductivity of an adjustable thermal link, according to one embodiment of the principles described herein.

FIGS. 13A and 13B are cross-sectional diagrams illustrating a micro-electro mechanical mechanism to control the temperature of a thermally sensitive component, according to one embodiment of the principles described herein.

FIG. 14 is a flowchart showing one illustrative method of utilizing an adjustable thermal link to control the temperature of a component, according to one embodiment of the principles described herein.

Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.

DETAILED DESCRIPTION

Controlling the temperature of components within an integrated electronic system can be challenging. Integrated electronic systems can have a number of components that are temperature sensitive. For example, resonant frequencies of resonant system elements are very sensitive to temperature. The physical dimensions, material stability, electron behavior, chemical interactions, strength, and other properties of various electronic components are affected by temperature. In integrated electronic systems that incorporate optical components, temperature variations can adversely affect the ability of the system to generate, transmit, modulate and detect light.
As the integrated electronic system operates, it can produce varying levels of heat that are proportional to the duty cycle of various heat generating components. These changing heat loads can cause undesirable temperature fluctuations within the system. For example, in electro-optical systems that utilize ring resonators as light sources, detectors, or modulators, the temperature of the ring resonator directly affects its resonant optical frequency. Significant changes in the resonant frequency of the ring resonator can drastically alter the ring resonator's performance.
In compact designs, a variety of heat generating and heat sensitive components can be in close proximity to each other. The heat generated by one component can quickly and directly affect adjacent components.
Methods of stabilizing the temperature within a system include active methods of control, for example, using heaters to raise the temperature of heat sensitive components above the maximum temperature of surrounding components. Active methods typically require additional power and introduce additional heat into the system.
Passive methods of thermal control can also be used. For example, components can be spaced out to minimize thermal cross talk between components, a structure can be used to conduct heat away from the components, or a specialized radiating surface can be incorporated to dissipate heat. These methods can increase the size, mass, and cost of the system. In many instances, passive methods of thermal control lack the precision required for optimal performance of thermally sensitive components.
As used in the specification and appended claims, the term “thermal conductivity” refers to a material property that indicates a homogeneous material's ability to conduct heat. More generally, thermal conductivity can be used to describe the heat transferring ability of a material or component through any internal means including conduction, convection, radiation, or phase change. As used in the specification and appended claims, the term “thermal conductance” refers to the heat conducting ability of a structure composed of various materials.
The current specification describes the use of an electrical actuator to control the thermal conductance of an adjustable thermal link between a first component and a second component. In one embodiment, the first component may be a temperature-sensitive component and the second component may be a heat source or sink. The thermal conductance of the adjustable thermal link between the temperature-sensitive component and the heat source/sink is actively throttled to control the amount of heat flowing between the components. By controlling the amount of heat entering a temperature-sensitive component, its temperature can be controlled or stabilized.
In an alternative embodiment, the adjustable thermal link connects a heat source to a heat sink. The thermal conductance of the adjustable thermal link is varied to maintain a constant temperature at the heat source. For example, when the heat source generates a large amount of heat, the thermal conductance of the adjustable thermal link is increased so that the excess heat is channeled through the adjustable thermal link and into the heat sink. When the heat source is generating less heat, the thermal conductance of the adjustable thermal link is reduced, decreasing the amount of heat that is channeled into the heat sink. This stabilizes the temperature of the heat source and reduces its negative effects on surrounding components.
The concept of using an adjustable thermal link can be applied to a wide variety of temperature control problems. For purposes of explanation, optical ring resonators are used as one specific example of a temperature-sensitive component. A number of adjustable thermal link concepts are discussed, many of which include piezoelectric or piezoelectrically active materials as actuators which act on the thermal links. Piezoelectric materials are materials whose shape can be altered by application of an electric field across the material. Some examples of piezoelectric materials are quartz and silicon. The speed of shape change in response to a voltage across the piezoelectric material can be very fast. Additionally, there is very little current flow through the piezoelectric material; current flow would lead to power consumption and heat dissipation.
As used in the specification and appended claims, the term “controller” will be used generally to refer to the various components that perform the function of altering or controlling the thermal conductance of an adjustable thermal link. Thus, a “controller” may include a piezo electric device, a voltage source, an electric field generator, a micro-electrical mechanical system (MEMS) and other components, examples of which are provided below.
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present systems and methods. It will be apparent, however, to one skilled in the art that the present systems and methods may be practiced without these specific details. Reference in the specification to “an embodiment,” “an example” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least that one embodiment, but not necessarily in other embodiments. The various instances of the phrase “in one embodiment” or similar phrases in various places in the specification are not necessarily all referring to the same embodiment.
FIG. 1 is an illustration of a circuit (100) shown to demonstrate illustrative piezoelectric material qualities. A piezoelectric material (105), which may be made of any material that exhibits piezoelectric properties, is shown in series with a voltage source (110) and a switch (115) in an open position.
The piezoelectric material (105) is sandwiched between two plates (120, 121). The two plates (120, 121) are made of a conductive or semiconducting material and are used to create an electric field across the piezoelectric material (105). Because the switch (115) is open, there is no applied voltage differential across the plates (120, 121) and therefore no electric field across the piezoelectric material (105). The piezoelectric material's dimensions (106, 107, 108) are the dimensions of the piezoelectric material (105) in its natural or unaltered state. As will be apparent to one skilled in the art, the actual shape and geometry of the piezoelectric material can vary greatly as best suits a particular application.
FIG. 2 represents the same circuit (100) and piezoelectric material (105) except that the switch (115) is in its closed position. Because the switch (115) position is closed, the voltage source (110) is able to apply a voltage difference to the plates (120, 121) creating an electric field across the piezoelectric material (105). The piezoelectric properties of the piezoelectric material (105), responding to the applied voltage bias from the voltage source (110), result in a change in the physical geometry of the piezoelectric material (105). The physical change of the piezoelectric material is apparent in the new dimensions (206, 207, 208) of the piezoelectric material (105).
The changes between the dimensions (106, 107, 108) of the piezoelectric material (105) in FIG. 1 and the dimensions (206, 207, 208) of the piezoelectric material (105) in FIG. 2 are greatly exaggerated for illustrative purposes. The actual changes in the dimensions of a piezoelectric material in response to an applied voltage or electric field are usually much less dramatic and dependent on the type of piezoelectric material used, the unbiased or natural geometry of the piezoelectric material, and the magnitude and distribution of the electric field applied across the piezoelectric material.
FIG. 3 is a diagram of an illustrative optical modulator (300). The illustrative optical modulator (300) has a first optical waveguide or “ring resonator” (301) arranged in a loop. A second “tangential” optical waveguide (303) is tangentially arranged with respect to the first optical waveguide (301) and is optically coupled to the ring resonator (301). The ring resonator (301) and tangential waveguide (303) are in optical communication with each other near the point where the tangential waveguide (303) passes closest to the ring resonator (301).
A heat generating electrical component (311) is shown adjacent to the ring resonator (301). In other situations, the heat generating electrical component may be under the optical structure rather than coplanar with the optical structure.
In some cases, it can be desirable for the optical resonant frequency of the ring resonator (301) to substantially match the optical frequency of a portion of energy passing through the tangential waveguide (303). For example, in wavelength division multiplexing, the optical energy transmitted through the tangential waveguide can be made up of optical energy divided among a plurality of different wavelength bands or “lines.” Each line may be narrow, on the order of 1 MHz to 1 GHz in width. Each different line can serve as a carrier signal for a separate data signal.
Ring resonators (301) that are optically coupled to the tangential waveguide (303) can be tuned to specific lines for modulation, detection, and de-multiplexing operations. The ring resonator (301) has a resonant frequency which is at least partially determined by its geometry and operating temperature. The ring resonator (301) is configured to sustain optical energy having a wavelength corresponding to the resonant frequency or a range of wavelengths centered on the resonant frequency of the resonator (301). Other optical energy of any different wavelength is attenuated or suppressed within the ring resonator (301) by destructive interference.
Changes in the operating environment can cause the resonant frequency of a ring resonator to undesirably shift away from the line frequency that the ring resonator is paired with. For example, when the adjacent electrical component (311) is heavily utilized and generates a large amount of thermal energy, the thermal energy is transferred to the surrounding components. In this case, the thermal energy will be partially absorbed by the ring resonator (301) and tangential wave guide (303).
The internal temperature of the ring resonator (301) rises proportionally to the amount of thermal energy it absorbs. Increases in the internal temperature of the ring resonator (301) may cause dimensional and optical changes which alter the resonant optical frequency of the ring resonator (301). Thus, as the duty cycle of the adjacent electrical component (311) changes, the temperature of the ring resonator (301) may also change and adversely affects its performance.
Thermal control and stabilization of the ring resonator (301) can be desirable to limit the thermal variations within the system. Additionally, it can be desirable to shift the optical resonant frequency of a ring resonator (301) from one line to another. By actively controlling one or more factors that affect the ring's optical resonant frequency, the ring (301) can be “tuned” to keep its optical frequency aligned with a given line or switch from one line to another.
In one method of controlling or stabilizing the temperature of a ring resonator (301), a heating element is incorporated into the design that is controlled to obtain the desired temperature and therefore the desired resonant frequency of the ring resonator. The stabilized temperature is typically higher than the maximum operating temperature of the surrounding components. However, because of the inclusion of heating elements, a greater amount of power and heat dissipation occurs within the system. Further, in high density applications, thermal cross talk between different heated ring resonators can become a problem. In applications where lower heat dissipation and high component densities are desired, alternative methods of tuning the ring resonators can be used.
Optical energy is generated by an optical source (307), such as a laser or a light emitting diode (LED), and enters the modulator (300) through a first end of the tangential optical waveguide (303). The optical energy is then transmitted through the optical coupling between the tangential waveguide (303) and the ring resonator (301) into the ring resonator (301). As noted above, optical energy having the correct wavelength (i.e. at or near the resonant frequency of the ring resonator(301)) will be sustained by the resonator (301), while optical energy at other wavelengths will be suppressed by destructive interference.
A wavelength that is being used to transmit data between the optical source (307) and an optical receiver (309) may be referred to as the carrier wavelength. The carrier wavelength may or may not correspond to the resonant frequency of the ring (301) depending on how the modulator (300) is configured to perform.
For example, the ring resonator (301) may act as a modulator of the carrier wavelength by selectively varying the amount of coupling between the ring resonator (301) and the tangential waveguide (303) or the amount of absorption of the carrier wavelength within the ring resonator (301) by detuning the resonant frequency of the ring (301) away from the carrier wavelength. The modulated optical energy may then be transmitted through the remainder of the tangential optical waveguide (303) to the optical receiver (309) disposed at a second end of the tangential optical waveguide (303). The receiver (309) may be, for example, a photodetector or another waveguide.
FIGS. 4A and 4B are cross-sectional views of an illustrative ring resonator (402, 410, 412), a heat source/sink (404), and an adjustable thermal link (414). According to one exemplary embodiment, the ring resonator (402, 410, 412) and heat source/sink (404) are supported by a substrate (406). Above the substrate (406), an insulating layer (408) electrically isolates the components from the substrate (406). The ring resonator (402, 410, 412) is comprised of a core (402) and surrounding electrodes (410, 412). As discussed above, the core (402) is optically transparent over a range of optical energy which passes through it. The electrodes (410, 412) are configured to inject charge carriers into the core (402), thereby influencing the optical energy that passes through the core (402).
According to one embodiment, the heat source/sink (404) shares a common support with the ring resonator (402) and is placed on the insulating layer (408). In alternative embodiments, the heat source/sink (404) may be located under or above the ring resonator (402) rather than coplanar with the ring resonator (402). The heat source/sink (404) may represent a variety of electrical or other components. By way of example and not limitation, the heat source/sink (404) may be a resistor, capacitor, memory chip, processor, light emitting diode, detector, cooling fin, heat pipe, etc. When the heat source/sink (404) is a heat sink, it may represent a radiator, large surface area conductor, passive electrical component, or other element capable of dissipating heat absorbed from surrounding components.
An adjustable thermal link (414) is placed above the core (402) and the heat source/sink (404). According to one exemplary embodiment, the adjustable thermal link (414) is in thermal contact with the upper surface of the core (402) and the upper surface of the heat source/sink (404). Above the adjustable thermal link (414) is a piezo active layer (416) and a restraining layer (420). The piezo active layer (416) is sandwiched between the adjustable thermal link (414) and the restraining layer (420) such that the expansion of the piezo active layer (416) compresses the adjustable thermal link (414).
According to one exemplary embodiment, the adjustable thermal link (414) has a low thermal conductivity in an uncompressed state and a high thermal conductivity in a compressed state. In FIG. 4A, the piezoelectrically active layer (416) is in a relaxed state and the adjustable thermal link (414) is in a low thermal conductivity state. The adjustable thermal link (414) may be comprised of a variety of materials in a variety of configurations. According to one exemplary embodiment, the adjustable thermal link (414) is comprised of a compressible matrix (422) in which a number of thermally conductive particles (424) are embedded. In the embodiment illustrated in FIG. 4, the thermally conductive particles (424) are randomly oriented and distributed throughout the matrix (422).
FIG. 4B shows the adjustable thermal link (414) in a compressed state. To compress the adjustable thermal link (414), a voltage difference is applied across the electrodes (120, 121; FIG. 1), which causes the piezoelectrically active layer (416) to alter its dimensions, thereby compressing the adjustable thermal link (414). As the adjustable thermal link (414) is squeezed between the piezo active layer (416) and the upper surface of the core (402) and the upper surface heat source/sink (404), the matrix (422) is compressed and deformed. The compression and deformation of the matrix (422) allows the thermally conductive particles (424) to come into closer contact with each other and with the surface of the components (402, 404). The thermal contact conductance between the coupling layer (414) and the components (402, 404) is also increased because of higher contact pressure and increased contact surface area. This creates a more thermally conductive path through the thermal coupling layer (414) which allows heat to flow between the two components (402, 404) at a greater rate.
It will be recognized that a variety of factors may influence the rate at which heat flows between the two components (402, 404). By way of example and not limitation, these factors may include the relative temperature differential between the two components (402, 404), the thermal conductivity of the components (402, 404), the amount of compression exerted by the piezo active layer (416), the thermal conductance of the adjustable thermal link (414), the surface contact resistance between the adjustable thermal link (414) and the component, the geometry and thermal conductivity the particles (424), the thermal conductivity of the matrix (122), and other factors.
By varying the voltage differential across the piezoelectrically active layer (416) the compression of the adjustable thermal link (414) is altered, thereby changing the thermal conductance. The temperatures of the various components (402, 404) are dependent on the incoming heat flux, outgoing heat flux, and heat generated within the component itself. In the embodiment shown in FIGS. 4A and 4B, the temperature of the components (402, 404) is controlled/stabilized by controlling the amount of heat flux which flows between the two components. For example, when the heat source/sink (404) is a heat generating component and the ring resonator (402) is a thermally passive element, the heat source (404) may have a significantly higher temperature than the ring resonator (402). In many electronic devices that generate a significant amount of heat, the heat generated is directly dependent on the usage or duty cycle of the component (404). As the usage of the component (404) increases, the amount of heat generated within the component also changes. Without the adjustable thermal link (414) and active piezoelectric layer (416), the heat from the heat source (404) would pass through the substrate (406) and insulating layer (408) and into the ring resonator (402, 410, 412). As the heat generated from the component varied over time, the temperature of the ring resonator would also vary. As described above, variations in temperature of a ring resonator (402, 410, 412) can be detrimental to its performance.
With the addition of the adjustable thermal link (414) and a means for actuating the adjustable thermal link (414), the temperature of the ring resonator (402, 410, 412) can be stabilized by varying the amount of heat that passes through the adjustable thermal link (414). When the heat generating component (404) generates less heat, the piezoelectrically active layer (416) could compress the adjustable thermal link (414) and allow a greater portion of the heat generated by heat source (404) to pass into the ring resonator (402). When the heat source (404) generates more heat, the piezo active layer 416) is relaxed and the thermal conductance of the adjustable thermal link (414) is reduced, thereby allowing less thermal energy to pass through the adjustable thermal link (414) into the ring resonator (402). In this manner, the temperature of the ring resonator (402) can be stabilized over wide variety of operating conditions.
According to one exemplary embodiment, a temperature sensor in proximity to the ring resonator (402) and/or heat source (404) may sense the temperature and provide information, allowing the desired voltage differential across the piezoelectrically active layer (416) to be calculated. The temperature of the ring resonator (402) may also be indirectly determined in a variety of ways including sensing the optical resonant frequency of the ring resonator (402).
FIGS. 5A and 5B are cross-sectional diagrams showing an illustrative embodiment of an adjustable thermal link (500). According to this exemplary embodiment, changes in the contact area (506) between an upper subelement (504) and a lower subelement (502) change the amount of heat which flows between the two surfaces. In the example shown in FIGS. 5A and 5B, the subelements are shown as having regular saw tooth profile. In the uncompressed state shown in FIG. 5A, only small portions near the tips of the upper and lower sawtooth profiles are touching. The contact areas (506) serve as the only conduction path between the two surfaces. In the compressed state shown in FIG. 5B, the contact area is significantly increased, thereby increasing the overall thermal conductance through the adjustable thermal link (500).
According to one exemplary embodiment, other modes of heat transfer are minimized within the adjustable thermal link (500). By way of example and not limitation, upper and lower surface may be polished to reduce radiative heat transfer. The size of the teeth could be chosen to reduce the convection in the air pockets between the teeth. In one embodiment, compressible solid insulating material could be used to displace the air in the voids between the teeth.
FIGS. 6A and 6B are cross-sectional diagrams showing an illustrative embodiment of an adjustable thermal link (600). In some circumstances, the shape and orientation of particles (604) within the adjustable thermal link may be selected to increase the variation in thermal conductance when the compressible layer (610) is compressed.
FIG. 6A shows the adjustable thermal link (600) in an uncompressed state. Similar to the embodiment shown in FIGS. 4A and 4B, a number of thermally conducting particles (604) are embedded in a compressible matrix (602). In this illustrative embodiment, the particles (604) are substantially similar in geometry and are positioned within the matrix in a regular periodic orientation. In one exemplary embodiment, the particles (604) may exhibit a higher thermal conductivity while the matrix (602) has a lower thermal conductivity. The orientation and repeating pattern of the particles within the matrix is only one example of the particles' orientations and patterns which could be used to change the thermal conductance of the adjustable thermal link when a dimension is altered.
In an uncompressed state, the adjustable thermal link (600) has low thermal conductance through its thickness and along its length. The thermally conductive particles (604) have little contact with the upper component (608), the lower component (606) or with each other. Consequently, heat which enters a thermally conductive particle (604) must follow a contorted path to travel either horizontally or vertically through the compressible layer (610).
FIG. 6B shows the adjustable thermal link (600) in its compressed state. In its compressed state, the upper component (608) and a lower component (606) are brought closer together, thereby compressing the compressible layer 610). As the compressible layer (610) is compressed, the particles (604) are re-oriented and become interlocked with the adjoining rows of particles. Additionally, the surface contact area between the particles and the component surfaces is substantially increased by bringing the longitudinal sides into contact with the upper and lower surfaces. Heat can then pass relatively easily between and within the two components (608, 606) in both vertical and horizontal paths, respectively. Heat passing vertically from one component to another allows an equalization of temperatures between two components (608, 606). The ability of heat to pass horizontally through the adjustable thermal link (610) minimizes temperature variations within the individual components and allows for more uniform heat flux between the two components (608, 606).
FIGS. 7A and 7B are cross-sectional diagrams showing an adjustable thermal link (704) interposed between a heat-generating component (710) and a temperature-sensitive component (702). In this embodiment, the heat-generating component (710) is on the bottom of the assembly. The heat-generating component (710) may represent the upper surface of an integrated circuit that has internal electronic elements which generate heat. The temperature-sensitive component (702) is placed on top of an adjustable thermal link (704). In alternative embodiments, the adjustable thermal link (704) can be above the temperature-sensitive component (702) and connect it to a heat sink.
According to one exemplary embodiment, the adjustable thermal link (704) may be comprised of a liquid or semi liquid matrix (706) with a number of suspended particles (708). In one embodiment, the liquid or semi liquid matrix (706) has a low thermal conductivity, while the particles have a higher thermal conductivity (708). In FIG. 7A the suspended particles (708) are shown in an at-rest state, with the majority of the particles having their major axis perpendicular to the heat path from the heat-generating component (710) to the temperature-sensitive component (702). This represents a minimally conductive state. The thermal energy passing from the heat-generating component (710) must pass into the liquid (706) and across the width of a particle (708) and back into the liquid many times before reaching the temperature-sensitive component (702).
FIG. 7B represents the adjustable thermal link (704) in a more highly conductive state. According to one exemplary embodiment, the particles (708) are reoriented under the influence of charges (716, 714) which have been introduced onto the surfaces surrounding the thermal coupling layer (704) by an applied voltage source (712). The particles (708) have been reoriented so that their major axes are parallel to the heat path. Thermal energy passing from the heat-generating component (710) into the temperature-sensitive component (702) will only have to pass through a minimal amount of liquid (706) before entering a particle (708) and being conducted down its major axis. By reducing the number of particle/liquid interfaces through which the heat must pass and maximizing the portion of the heat path that extends through the major axis of the particles (708), the overall thermal conductance of the adjustable thermal link (704) is increased.
This embodiment has a number of advantages. For example, the voltage may be varied in an analog manner to selectively throttle the flow of heat into the temperature-sensitive component (702). This allows for more precise control of the temperature of the various components. When the adjustable thermal link is an electrical insulator, the charges (714, 716) are trapped in a situation that is analogous to a capacitor. This capacitive charging of the surfaces surrounding the adjustable thermal link (704) reduces the power consumption and heat generation of the adjustable thermal link (704).
FIGS. 8A and 8B are cross-sectional diagrams showing an illustrative adjustable thermal link (800). In this exemplary embodiment, the adjustable thermal link (800) is comprised of a number of spacers (604) and a number of pliable layers (802). As shown in FIG. 8A, the adjustable thermal link (800) in an uncompressed state exhibits a relatively low thermal conductance between an upper component (808) and a lower component (806). According to one exemplary embodiment, the spacers (804) have a high thermal conductivity while the pliable layers (802) have a lower thermal conductivity. The lowest row of spacers is in direct contact with the lower component (806). For heat to travel from the lower component (806) to the upper component (808) the heat must pass from a lower spacer into a first pliable layer, horizontally along the first pliable layer into a second spacer, from the second spacer into a second pliable layer, along the second pliable layer into a third spacer and so forth until the heat reaches the second component (808). This contorted path provides high thermal resistance and results in only a minimal amount of thermal energy passing between the upper component (808) and the lower component (806).
FIG. 8B shows the adjustable thermal link (800) in its compressed state. In its compressed state, the adjustable thermal link (800) creates direct thermal paths between the lower component (806) and the upper component (808). When compressed, the pliable layers (802) are deformed, allowing spacers (804) to be pressed vertically together. The compressed spacers form a column that provides a direct thermal path between the two components (808, 806). Additionally, the spacers (804) are pressed against the surfaces of the upper component (808) and lower component (806). This reduces the surface contact resistance and allows heat to flow more easily into the spacers (804). The thermal energy may then pass directly into a spacer, through the thicknesses of the pliable layers (802) that separate the first spacer from the second spacer and so forth and until the heat passes into the other component.
According to one exemplary embodiment, a piezoelectrically active layer (e.g. 416, FIG. 4) may provide the compressive force required to change the thermal conductance of the thermal coupling layer (800). In an alternative embodiment, the pliable layers (802) may be deformed by other means. By way of example and not limitation, the pliable layers (802) may be made out of a temperature-sensitive material, such as a memory alloy. When the temperature of the component or components rises above a transition temperature, the memory alloy layers (802) return to their original serpentine shape and bring the two components (808, 806) together. The use of memory alloy pliable layers (802) allows for passive control of the component temperatures and may reduce the complexity of the control system.
In an alternative embodiment, the electricity may be passed through the memory alloy pliable layers (802) causing their temperature to rise above the transition temperature and return to their original serpentine shape, thereby increasing the thermal conductance between the two components. In other embodiments, the spacers (804) may be thermal insulators and the layers (802) may have a high thermal conductivity. For example, the spacers (804) could have an open cell structure while the layers (802) are resilient metal foil.
The examples of adjustable thermal links given in FIGS. 4, 5, 6, 7, and 8 are examples of possible adjustable thermal links that exhibit a change in thermal conductance in response to a dimensional change or a reorientation of embedded elements. A variety of other embodiments could be used. By way of example and not limitation, magnetic particles could be reoriented within a matrix in response to an applied magnetic or electrical field. In an alternative embodiment, the adjustable thermal link could be comprised of a number of high thermal conductivity fibers embedded in a matrix. For example, carbon nanotubes exhibit high thermal conductivity along their long dimension and relatively low transverse thermal conductivity between adjacent carbon nanotubes. By pressing a bundle of carbon nanotubes between two surfaces, the heat could enter the nanotubes through the large transverse area and be carried longitudinally away from the component.
In an alternative embodiment, a doped semiconductor could be used as an adjustable thermal link. For example, some dopant ions move through the semiconductor lattice at room temperature in response to an applied electrical field. For example, lithium ions can move through a silicon lattice. The dopant atoms change the electrical and thermal conductivity of the bulk semiconductor material through which they move. Therefore, moving dopant atoms part way or all the way through the adjustable thermal link will increase its thermal conductance and the thermal communication between the components on either side of the adjustable thermal link. One advantage of this embodiment is that the bulk semiconductor material and dopant atoms can be deposited and formed through standard integrated-circuit techniques. Additionally, because the ions make up only a small portion of the lattice material, changes in the dimensions of the adjustable thermal link would be negligible. This allows the adjustable thermal link to provide a stable base upon which a temperature-sensitive component could be formed.
In many embodiments, a large number of air pockets are present in the uncompressed state of the adjustable thermal link. These small pockets of air provide a high level of thermal isolation between the two components. As the adjustable thermal link is compressed, the air pockets collapse and expel the air. This can lead to a much higher thermal conductance of the adjustable thermal link.
FIGS. 9, 10, 11, 12, and 13 illustrate various configurations and actuation methods for controlling the temperature of a component or components using an adjustable thermal link.
FIGS. 9A and 9B are cross-sectional diagrams of an apparatus (900) using an adjustable thermal link (906) to control temperature. The first component (912) and the second component (904) are supported by a common base (902). A piezoelectrically active layer (908) and an adjustable thermal link (906) are placed in the gap between the first component (912) and the second component (904). An upper restraining layer (910) prevents the upward motion of the piezoelectrically active layer (908).
FIG. 9A shows the adjustable thermal link (906) in an uncompressed state. In the uncompressed state, the voltage across the piezoelectrically active layer (908) is minimal and the piezoelectrically active layer (908) is not extended. As previously discussed, the adjustable thermal link (906) may take a variety of forms. In FIG. 9A, the adjustable thermal link (906) is comprised of a matrix with a number of randomly oriented particles.
FIG. 9B illustrates the compression of the adjustable thermal link (906) by the piezoelectrically active layer (908). As previously discussed, the piezoelectrically active layer (908) may be extended or compressed by applying a voltage across the electrodes (120, 121; FIG. 1). The voltage difference across the electrodes (120, 121; FIG. 1) may be selected to obtain the desired level of compression in the adjustable thermal link (906). Higher voltage differences across the electrodes results in greater extension of the piezoelectrically active layer (908) and correspondingly greater compression of the adjustable thermal link (906). As the adjustable thermal link (906) is compressed, the matrix collapses and brings the particles into closer contact with both each other and the surfaces of the joining components. In this exemplary embodiment, the extension of the piezoelectrically active layer (908) and resulting compression of the adjustable thermal link (906) have been exaggerated for illustration purposes. However, even small changes in displacement can greatly contribute to controlling the temperature of system components.
The thermal conductance between the first component (912) and the second component (904) is also increased by the reduction in surface contact resistance between the adjustable thermal link (906) and surrounding components (902, 904, 912) and the increase in surface contact area as the adjustable thermal link is deformed and pressed against greater portions of surrounding surfaces. As was previously mentioned, the conducting particles also come in greater contact with each other, thereby increasing the internal thermal conductance of the thermal coupling layer (906).
FIGS. 10A and 10B are cross-sectional diagrams of an apparatus (1000) using an adjustable thermal link (1006). A base (1002) provides support and structure for the apparatus (1000). According to one exemplary embodiment, a first component (1010) is rigidly affixed to the base (1002). A second component (1004) slidably interfaces with the base (1002). A piezoelectrically active layer (1008) is rigidly restrained at one end by the base (1002) and presses against the second component (1004) at the other end. Interposed between the first component (1010) and the second component (1004) is an adjustable thermal link (1006). In this exemplary embodiment, the adjustable thermal link (1006) has a configuration similar to the adjustable thermal link (800) illustrated in FIG. 8. However, any one of a number of adjustable thermal link configurations could be used.
In FIG. 10A, the adjustable thermal link (1006) is illustrated in an uncompressed state. The adjustable thermal link (1006) presents a low thermal conductance path between the first component (1010) and the second component (1004). Heat passing from one component to another must pass through a lengthy and contorted path to reach the other component.
FIG. 10B shows the thermal coupling layer (1006) in its compressed state. According to one exemplary embodiment, the adjustable thermal link (1006) is compressed by applying a voltage across electrodes integral to the piezoelectrically active layer (1008). The dimensions of the piezoelectrically active layer (1008) are then altered such that the second component (1004) slides over the base (1002) and narrows the gap between the first component (1010) and the second component (1004). As described above, the adjustable thermal link (1006) is interposed in the gap between the first component (1010) and the second component (1004). The adjustable thermal link (1006) is compressed as the spacing between the two components is reduced. As described in FIG. 8, the thermal conductance of the adjustable thermal link (1006) increases as more direct contact between the thermally conductive spacers is created.
FIG. 11 illustrates an alternative geometry (1100) in which an adjustable thermal link (1104) is used to control the temperature of an optical component (1102). In this illustrative embodiment, the heat source (1110) is separated from the adjustable thermal link (1104) by a heat diffuser layer (1108). By way of example and not limitation the heat source (1110) may be electronics embedded in an integrated circuit or on a printed circuit board. The heat diffuser layer (1108) can be used to smooth the spatial and temporal variations in temperature and heat flux. By way of example and not limitation, the heat diffuser layer (1108) may be comprised of a material that is a relatively good thermal conductor and has a high thermal capacity. The heat diffuser layer (1108) conducts heat (1111) away from heavily used electronic components to prevent them from overheating. This heat is spread over the area that the heat diffuser layer (1108) covers. Because of its high thermal capacity, the heat diffuser layer (1108) may also absorb a significant amount of heat with only small changes in overall temperature. This reduces the temporal variations in temperature and maintains a more stable operating environment for the device.
The heat diffuser layer (1108) is separated from the optical component (1102) by an adjustable thermal link (1104). The adjustable thermal link (1104) may take a variety of forms including, but not limited to the illustrative embodiments shown in FIGS. 4, 5, 6, 7, and 8.
A temperature-sensitive component is placed on top of the adjustable thermal link. In the embodiment illustrated in FIG. 11, the temperature-sensitive component is an optical component (1102). The optical component (1102) is cooled by radiation, convection, and possibly conduction heat transfer to the surroundings. The wavy arrows (1106) emanating from the upper surface of the optical component represent the heat loss of the optical component (1102) to the surroundings. When the heat passed through the adjustable thermal link (1104) is equal to the heat loss (1106) of the optical component (1102) the temperature of the optical component will remain constant. When changing environmental conditions change the amount of heat loss (1106) or the amount of heat generated (1111), the thermal conductance of the adjustable thermal link (1104) can be altered to compensate, thereby keeping the optical component (1102) at a substantially constant temperature.
FIG. 12 is a diagram showing one illustrative embodiment-of an alternative actuation method for adjusting the thermal conductance of an adjustable thermal link (1205). According to this exemplary embodiment, a heat source/sink (1215) forms a base upon which an optical component (1210) and adjustable thermal link (1205) are placed. The adjustable thermal link (1205) is sandwiched between the optical component (1210) and the heat source/sink (1215). The adjustable thermal link (1205) forms the primary heat path between the optical component (1210) and the heat source/sink (1215).
In this exemplary embodiment, the thermal conductance of the adjustable thermal link (1205) can be altered by adjusting its vertical thickness. Compressive or expansive pressure is exerted on the adjustable thermal link (1205) by applying a voltage (1220) across the adjustable thermal link (1205). This forces charges (1225, 1230) to the surfaces adjoining the adjustable thermal link (1205). In the illustrative embodiment shown in FIG. 12, the upper surface contains positive charges (1225) and the lower surface contains negative charges (1230). The positive charges (1225) and negative charges (1230) are mutually attracted to one another and exert a force that tends to pull the two surfaces together. The adjustable thermal link (1205) is sandwiched between the two surfaces and is compressed by the applied voltage.
In an alternative embodiment, the adjustable thermal link (1205) may have capacitive plates that form its upper and lower surfaces. These capacitive plates could be charged as a means of actuating the adjustable thermal link. The adjustable thermal link (1205) may take a variety of forms, including but not limited to the embodiments illustrated in FIGS. 4, 5, 6, 7, and 8.
FIGS. 13A and 13B show an illustrative micro-electro mechanical system (MEMS) for controlling the temperature of an optical component (1302). In the illustrative embodiment shown in FIG. 13A, MEMS arms (1304, 1310) extend to support a temperature-sensitive component (1302). The temperature-sensitive component (1302) is suspended over the surface of a heat source/sink (1306). An air gap (1308) separates the lower surface of the temperature-sensitive component (1302) from the upper surface of the heat source/sink (1306).
In FIG. 13B, the MEMS arms (1304, 1310) have been deflected to bring the lower surface of the temperature-sensitive component (1302) into contact with the heat source/sink (1306). By cycling between the first position shown in FIG. 13A and the second position shown in FIG. 13B and adjusting the fraction of time the temperature-sensitive component (1302) is in each position, the temperature of the component (1302) can be regulated. In alternative embodiments, the height of the air gap (1308) could be adjusted to increase or decrease the convective and radiative heat transfer between the temperature-sensitive component (1302) and the heat source/sink (1306). The small air gaps (1308) increase the heat transfer between the two bodies (1302, 1306), while greater air gaps (1308) decrease the heat transfer between the two bodies (1302, 1306).
In an alternative embodiment, an adjustable thermal link is interposed between the bottom surface of the temperature-sensitive component (1302) and the upper surface of the heat source/sink (1306). The MEMs arms (1304, 1310) then provide the compressive or tensile force to adjust the thermal conductance of the adjustable thermal link.
It will be appreciated by those of skill in the art that an adjustable thermal link can form a controllable thermal conduit that can act as a throttle for thermal transfer between various components in a variety of geometric configurations. By way of example and not limitation, the adjustable thermal link can be placed on the upper surface, sides, or underneath a thermally-sensitive component. Additionally, one or more adjustable thermal links may be used to control the temperature of a single component. By way of example and not limitation, a first adjustable thermal link may connect a temperature-sensitive component to a heat source and a second adjustable thermal link may connect the temperature-sensitive component to a separate heat sink. Additionally, the temperature-sensitive component may be connected to a heat source by a fixed thermal link and connected to a heat sink by an adjustable thermal link. Conversely, the temperature-sensitive component may be connected to a heat source by an adjustable thermal link and connected to a heat sink by a fixed thermal link.
The application of an adjustable thermal link for temperature control is not limited to component level tasks. For example an adjustable thermal link may provide thermal control for a plurality of components, a subsystem, or a complete system.
FIG. 14 is a flowchart showing one illustrative method of utilizing an adjustable thermal link to control the temperature of a component. In a first step, the temperatures of the components connected by the adjustable thermal link are determined (step 1400). This determination may be made in a variety of ways including direct temperature measurement, measurement of a temperature-sensitive component's performance, or other indirect means. For example, the temperature of a ring resonator may be estimated by comparing the magnitude of an optical signal contained within the ring resonator to the available magnitude of a same optical frequency within the tangential waveguide.
Next, the desired change in temperature of a target component is quantified (step 1410). In some circumstances, the change may be large, such as when the temperature of a ring resonator is used to shift the ring resonator frequency from one carrier band to another. Conversely, if it is desirable to correct the drift of a ring resonator frequency away from the centerline frequency of a given carrier band, the desired shift may be relatively small.
The voltage or current which is to be applied to the actuating mechanism is then calculated (step 1420). This calculation may involve accounting for individual differences between various components, including the application of various calibration equations or constants. For example, when the target component is a ring resonator, the applied voltage may vary depending on the desired shift in the optical resonant frequency, the geometry of the actuating mechanism, uniformity of the electrical field produced, and other factors.
The calculated voltage or current is then applied to the actuating mechanism which alters the thermal conductance of the adjustable thermal link (step 1430). Following the application of the calculated voltage or current, the temperatures of the components are again measured to see if the thermal transfer attained by the application of the voltage or current to the actuator (step 1440) resulted in the desired temperature change or stabilization. Further adjustments can be made as needed by repeating steps 1400 through 1440 (step 1450).
In sum, an adjustable thermal link that couples a heat source/sink to a second component can be created in a variety of ways including varying the contact area between two elements, varying the fraction of time (duty cycle) that the adjustable thermal link is in a high thermal conduction state, varying the orientation of anisotropic particles within the thermal coupling layer, and varying the thermal conduction of the thermal coupling layer by moving dopant atoms into and out of the thermal coupling layer.
Various actuation means can be used to manipulate the adjustable thermal link's thermal conductance. For example, a voltage can be applied across a piezo electric actuator to extend or compress the adjustable thermal link. Electrostatic forces can be exerted by applying a voltage across two plates to extend or compress the adjustable thermal link. MEMS devices can be used in a variety of configurations to provide motion which alters the thermal conductance of the adjustable thermal link. Additionally, electrical or magnetic fields align particles within the adjustable thermal link without deforming the adjustable thermal link.
The preceding description has been presented only to illustrate and describe embodiments and examples of the principles described. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching.

Claims

1. An apparatus for thermal control comprising:

a first component;

a second component;

an adjustable thermal link disposed between said first component and said second component; and

a controller for selectively varying a thermal conductance of said adjustable thermal link.

2. The apparatus of claim 1, wherein said adjustable thermal link comprises particles within a compliant matrix.

3. The apparatus of claim 2, wherein said adjustable thermal link comprises anisotropic particles suspended within a fluidic matrix.

4. The apparatus of claim 1, wherein said adjustable thermal link comprises alternating spacers and pliable layers, said spacers have a substantially higher thermal conductivity than said pliable layers, said spacers comprising a primary thermal path when said adjustable thermal link is compressed.

5. The apparatus of claim 1, wherein said controller comprises a piezoelectrically active element.

6. The apparatus of claim 1, wherein said controller comprises a micro-electrical mechanical mechanism.

7. The apparatus of claim 1, wherein said controller applies an electrical field or magnetic field across said adjustable thermal link.

8. The apparatus of claim 1, wherein said controller comprises two capacitive plates.

9. The apparatus of claim 1 wherein said controller comprises a shape memory alloy.

10. The apparatus of claim 1 wherein said controller selectively adjusts a dimension of said adjustable thermal link such that said thermal conductance of said thermal link is altered.

11. The apparatus of claim 10, wherein compressing said adjustable thermal link results in said thermal link having an increased thermal conductance in at least one dimension.

12. An apparatus for thermal control comprising:

a ring resonator;

a second component, said second component being a sub-chip level electronic element;

an adjustable thermal link between said ring resonator and said second component, said adjustable thermal link comprising a compressible matrix with embedded particles; and

a piezoelectrically active element; said piezoelectrically active element altering dimensions in proportion to a bias voltage applied across said piezoelectrically active element, said piezoelectrically active element adjusting a dimension of said adjustable thermal link such that thermal contact between said embedded particles increases and thermal contact between said embedded particles and surfaces of said ring resonator and said second component increases, thereby increasing a thermal conductance of said adjustable thermal link and controlling an amount of thermal energy passing between said ring resonator and said second component.

13. A method of controlling a temperature comprising:

sensing a temperature of a first component; and

adjusting a thermal conductance of an adjustable thermal link, said adjustable thermal link forming a thermal path between said first component and a second component; said thermal conductance of said adjustable thermal link being adjusted such that said temperature of said first component is controlled.

14. The method of claim 13, further comprising:

quantifying a desired shift in temperature of said first component;

calculating an electrical input;

applying said electrical input to an actuation means, said actuating means being configured to alter a dimension of said adjustable thermal link such that a thermal conductance of said adjustable thermal link is adjusted.

15. The method of claim 14, wherein:

said actuation means comprises a piezoelectrically active layer, said electrical input comprising a voltage applied across said piezoelectrically active layer such that said piezoelectrically active layer compresses said adjustable thermal link; and

said adjustable thermal link comprises a compressible matrix with embedded particles.