Abstract:
Thermal semi conductor is a mechanism for the control of conduction heat flux between two or more bodies with differing temperatures, promoting the unidirectional heat flux. It consists of composite slabs composed of materials with high and low thermal conductivity appropriately set, granting the thermal semi conduction. These slabs are able to displace allowing the contact between materials with high thermal conductivity respectively in order to promote higher heat flux or, in opposite, to displace facilitating the contact between materials with low thermal conductivity respectively to promote lower heat flux. The control of the heat transfer through the mechanism is defined by positioning the slabs appropriately. External devices can be used to promote the relative displacement of the slabs. On the other hand, an adequate design of the slabs can be used to promote or to avoid their thermal contact through thermal expansion or contraction. In this case, the control of the process can be reached automatically by the temperature gradient.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention, namely thermal semi conductor, relates to a device for heat flux control, which promotes unidirectional conduction heat transfer. The thermal semi conductor can be employed in any situation where a thermal insulator or a thermal conductor might otherwise be employed. However, the thermal semi conductor is superior to a thermal conductor/insulator in several aspects. The invention has particular utility in the promoting of the unidirectional heat transfer, improving the storage capacity of systems with time-dependent thermal energy sources or sinks, etc. 
         [0003]    2. State of the Art 
         [0004]    Although the present invention is quite similar from systems with thermal conductors or insulators, thermal conductors or insulators nevertheless provide a convenient reference point for purposes of comparison. As is well known, in principle thermal insulators consist of low thermal conductivity materials manufactured or combined in a way to achieve an even lower system thermal conductivity. Considering an unfavorable temperature gradient, this feature is useful to reduce the rate of heat transfer preserving the temperature of rooms, objects, process fluids, etc, avoiding thermal losses. However, if the temperature gradient is favorable, thermal insulators isolate the system preventing substantial thermal gains in addition. In that situation however, the use of thermal conductors are attractive for the reason that they increase the system rate of heat transfer, promoting thermal gains. Thus, methods and processes to control the rate of heat transfer to take advantage of the temperature gradient has been attempt. In this way, thermal semi conductors are able to perform this purpose. 
         [0005]    While thermal insulators and conductors have found wide application, they nevertheless have disadvantages in particular which are overcome by the present invention. One disadvantage of conventional thermal insulators and conductors systems is that they don&#39;t permit the transient control of the heat flux. Systems subjected to intermittent temperature gradient can take advantage of favorable temperature gradient to increase their operational performance. 
         [0006]    For instance, the objective of insulation traditionally used in buildings is to retard heat transfer. Thermal insulation is installed in buildings to provide thermal comfort and to reduce operating costs generated by heating, ventilation and air-conditioning. However, appropriate thermal semi conductors can be employed to maintain the building environment at a desired temperature range throughout the sun&#39;s daily and annual cycles. As a result it generally minimizes the use of active solar, renewable energy and especially fossil fuel technologies. 
         [0007]    Techniques for dissipation the heat generated by electronic devices and circuitry includes heat sinks and fans for air cooling. Particularly, an active region separated from the ventilated heat sink by a silicon substrate and a metal integrated heat spreader is the microprocessor heat transfer plate where heat exchange is achieved by conduction. Because of the localized heat source, the thermal spreading resistance of the interface region can be high. Appropriate thermal semi conductors configurations can alleviate on-chip hotspots much more effectively than any type of heat spreader and also contribute to a better chip temperature uniformity. 
       SUMMARY 
       [0008]    Accordingly, it is an object of the invention to provide techniques and processes for the unidirectional heat transfer. For that, composite slabs formed of materials with high and low thermal conductivity are appropriately set and adjusted permitting the thermal semi conduction. According to the proposed invention, the thermal semi conduction mechanism can be achieved by relative mechanical slabs positioning or automatically by temperature gradient. 
         [0009]    It is another object of the invention to provide such devices whose permit the transient control of the heat flux. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    While the novel features of the invention are set forth with particularity in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description, taken in conjunction with the drawings in which: 
           [0011]    FIG.  1 —Cross-sectional view of one form of thermal semi conductor device based on compound materials and thermal contact—minimal bulk thermal conductivity. 
           [0012]    FIG.  2 —Cross-sectional view of one form of thermal semi conductor device based on compound materials and thermal contact—intermediate bulk thermal conductivity. 
           [0013]    FIG.  3 —Cross-sectional view of one form of thermal semi conductor device based on compound materials and thermal contact—maximal bulk thermal conductivity. 
           [0014]    FIG.  4 —Cross-sectional view of one form of thermal semi conductor based on compound materials and thermal contact between extended surfaces—minimal bulk thermal conductivity. 
           [0015]    FIG.  5 —Cross-sectional view of one form of thermal semi conductor based on compound materials and thermal contact between extended surfaces—maximal bulk thermal conductivity. 
           [0016]    FIG.  6 —Cross-sectional view of one form of thermal semi conductor based on compound materials and thermal contact between extended surfaces—intermediate bulk thermal conductivity. 
           [0017]    FIG.  7 —Cross-sectional view of one form of thermal semi conductor based on compound materials, thermal contact between extended surfaces and thermal expansion/contraction—minimal bulk thermal conductivity. 
           [0018]    FIG.  8 —Cross-sectional view of one form of thermal semi conductor based on compound materials, thermal contact between extended surfaces and thermal expansion/contraction—maximal bulk thermal conductivity. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    In general, rate of heat transfer is proportional to the product of three factors: thermal conductivity, the area over which heat transfer occurs, and temperature gradient (temperature difference per unit distance). Applying this general relationship to the invention, the magnitude of the thermal conductivity for the system depends on its structure. Usually, the bulk thermal conductivity is subject to predictable constraints. In the present invention, such a bulk thermal conductivity can be systematically modified by combining the thermal contact effect with an adaptive structure consisting of materials with good and poor thermal conductivity. The area factor is also subject to predictable constraints. Considering that the area over which heat transfer takes place remains unchanged, it does not influence the heat transfer rate. The third factor, temperature gradient, is another matter, not subject to constraints which are obvious. 
         [0020]    A heat transfer device in accordance with the invention includes at least a pair of slabs adapted for positioning at respective locations with differing temperatures between which it is desired to control the direction and the heat transfer rate. A basic configuration consists in an arrangement of slideable composite slabs formed of good conductor material and good insulator material intercalated, namely, a thermal semi conductor device based on compound materials and thermal contact. The adequate displacement of these slabs permitting the contact between good thermal conductor materials respectively produces higher heat fluxes. On the contrary, the displacement of these slabs preventing the contact between good thermal conductor materials respectively produces lower heat fluxes. The thermal conductivity of the system and, consequently, the heat flux can be controlled through an intermediate positioning of the slabs. 
         [0021]    As another configuration is a system comprising two slideable parallel symmetric plates of a good conductor with internal extended surfaces, similar to fins, namely, the thermal semi conductor based on compound materials and thermal contact between extended surfaces. The remaining space is filled with a good insulator. Intentionally, a gap is left between fins with corresponding apparent surfaces, allowing the relative longitudinal displacement of the slabs. The longitudinal displacement of the plates works like an on-off switch promoting or preventing the thermal flux. By transversal dislocation, the thermal contact area can be reduced or increased reducing or increasing, respectively, the thermal conductivity of the system modifying the heat flux through the wall. 
         [0022]    The third configuration is similar to the previous one. This system consists of two parallel plates of a good conductor with internal fins. The fins of one wall are intentionally confined by fins of the other wall. The remaining space is filled with a good insulator and a gap is left between fins with corresponding apparent surfaces. In this proposed configuration, two right wall fins are confined by two left wall fins. By this way, higher temperatures at the left side of the wall promote its thermal expansion. On the other hand, lower temperatures at the right side of the wall promote its thermal contraction. This conjugated effect keeps the corresponding fin surfaces apart from each other enhancing, in this way, the thermal insulation between the left and the right regions. On the contrary, when higher temperatures occur at the left side and lower temperatures occur at the right side, thermal contraction and expansion takes place, respectively. Because of this conjugated effect, the contact between the corresponding fin surfaces takes place increasing through that the capability of the system to conduct thermal energy. In opposite of the previous system, this system is operated exclusively by defined temperature gradients and no external driven is necessary for its working. This system automatically switches between good or poor thermal conductivity by corresponding temperature gradients. 
         [0023]    Thus, with the three presented configurations, large quantities of heat are transported unidirectionally by providing the adequate design and the satisfactory slabs arrangement. 
         [0024]    In any given configuration, there is a most adequate configuration of the slabs thickness and segments width regarding the desired maximum and minimum heat flux. Particularly, the third configuration requires an optimum design concerning the temperature gradient between the reservoirs. If the gap is too large, then the thermal contact between the materials with good thermal conductivity does not occur and the system becomes a good thermal insulator. If the gap is too small, then the materials with good thermal conductivity remain contacted and the system becomes a good thermal conductor. 
         [0025]    It should be understood that the invention is not limited to the presented configurations, and that various changes (parts, dimensions, shapes, geometries, materials, configurations, arrangements, etc.) may be made by those skilled in the art without changing the essential characteristics and the basic concepts of the invention. 
         [0026]    As is known, the heat conduction between the slabs of a composite wall is strongly influenced by the contact resistance, especially if high-conductivity metals are involved. The contact resistance is dependent on the pressure which contact in maintained. Some representative data for contact resistances are presented by Mills (1995) and Hagen (1999). Good interfacial conductances can arrive at h i =2.5×10 4  W/m 2 ·K for a copper-copper interface and h i =4.0×10 4  W/m 2 ·K for an iron-aluminum interface at moderate pressure and usual finishes. As a first approach, the interfacial contact resistance between the slabs is neglected. 
         [0027]    Finally, serial and/or parallel and/or form of matrices arrangements of multiple slabs can be used to achieve the desired heat flux control. 
         [0028]    The following configurations are provided by way of illustration only and not by way of limitation. A variety of parameters can be changed of modified to yield essentially similar results and would be apparent to one skilled in the art. 
       Thermal Semi Conductor Based on Compound Materials and Thermal Contact 
       [0029]    Referring first to  FIG. 1 , an exemplary thermal semi conductor device  1  based on compound materials and thermal contact in accordance with the invention includes a pair of slabs  2  and  3  assembled with a intercalate sequence of segments with equivalent length of a relatively good and a poor thermal conductor materials adapted for positioning at respective locations of differing temperature between which it is desired to control the heat transfer. These slabs are maintained in contact but they are, at the same time, able to slide vertically. By way of example, the reservoir C is a relatively colder reservoir and is positioned so that to transfer heat from relatively hotter reservoir H. The slabs are positioned, so that the slab segments with materials with good thermal conductivities  4  and with poor thermal conductivities  5  are aligned. At this positioning, the system has the minimal bulk thermal conductivity and heat transfer achieves the lowest rates. 
         [0030]    By promoting the relative slabs longitudinal displacement as shown schematically in  FIG. 2 , segments with good heat conductivity  4  are set in contact and the bulk thermal conductivity of the system increases, increasing the heat transfer rate. The non-aligned slabs positioning allows a dynamic heat transfer control as desired. When the segments with higher thermal conductivity  4  are aligned, the bulk thermal conductivity of the system achieves its maximum value, as presented in  FIG. 3 . 
       Thermal Semi Conductor Based on Compound Materials and Thermal Contact Between Extended Surfaces 
       [0031]      FIG. 4  presents, an exemplary thermal semi conductor device  10  based on compound materials and thermal contact between extended surfaces in accordance with the invention. This design includes two internally finned slabs  8  of a material with good thermal conductivity filled with a material with poor thermal conductivity  7  adapted for positioning at respective locations with differing temperature between which it is desired to control the heat transfer. These slabs are maintained in contact but they are, at the same time, able to slide longitudinally and transversally. By way of example, the reservoir C is a relatively colder reservoir and is positioned so that to transfer heat from relatively hotter reservoir H. The gap  9  is filled with a compressible fluid with low thermal conductivity. By keeping the internal fins  6  apart from each other, the bulk thermal conductivity of the system is relatively lower. As shown in  FIG. 5 , when the slabs displace longitudinally and the internal fins of material with good thermal conductivity are set in contact, the bulk thermal conductivity increases, allowing relatively higher heat fluxes. At this position, the system presents its maximal bulk heat conductivity. When the slabs displace transversally apart from each other keeping the contact between the internal fins, according to  FIG. 6 , the bulk thermal conductivity decreases to a relatively intermediate value. Since the fluid inside the slabs is compressible, the slabs displacements in both transversal and longitudinally direction are guaranteed. This relative slabs displacement permits thus the control of the heat transfer between the reservoirs. 
       Thermal Semi Conductor Based on Compound Materials, Thermal Contact Between Extended Surfaces and Thermal Expansion/Contraction 
       [0032]      FIG. 7  introduces an exemplary thermal semi conductor device  16  based on compound materials, thermal contact between extended surfaces and thermal expansion/contraction in accordance with the invention. This system presents similarities with the system presented in  FIG. 4 . It consists of two parallel plates  12  e  13  of a good conductor with internal fins. The fins of the wall  13  are intentionally confined by fins of the wall  12 , as shown in  FIG. 7 . The remaining space  14  is filled with a good insulator and a gap  15  is left between fins with corresponding apparent surfaces, filled with a compressible gas. The appropriate system design permits the adequate thermal expansion or contraction of the plates  12  and  13  respectively, allowing or preventing the contact between the surfaces  11 . For instance, initially the reservoir H is a relatively hotter reservoir and is positioned so that to transfer heat to relatively colder reservoir C. In accordance with this arrangement, lower temperatures at the wall  13  promote its thermal contraction and higher temperatures at the wall  12  promote its expansion. This conjugated effect prevents the corresponding fin surfaces  11  contact keeping the good conductors away from each other, reducing the heat transfer rate of the system. On the other hand, by opposite temperature gradient, the inverse effect is expected, as shown in  FIG. 8 . Higher temperatures at the wall  13  promote its thermal expansion and lower temperatures at the wall  12  promote its contraction. This conjugated effect promotes the corresponding fin surfaces  11  contact allowing the contact between the good thermal conductors, increasing the heat transfer rate of the system. The great advantage of this system is that it can be drove exclusively by defined temperature gradients and no external driven is necessary for its working. This system automatically switches between good or poor heat transfer rates by corresponding temperature gradients. It is also suitable for places with vertically displacement constraints. 
         [0033]    While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous modifications, dimensions, proportions, configurations, arrangements, profiles, forms, outlines and changes for each part or for the whole invention will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention. 
         [0034]    Regardless of which configuration is used, the invention can be used to control the heat flux of a wide variety of substrates, including, but not limited to, roofs, ceilings, walls, containers, tanks, pipes, trucks, boats, barges and ships. 
         [0035]    It is important to recognize that heat transfer through any insulation/conduction system may include several modes: conduction through the solid materials; conduction or convection through the air in the void spaces; and radiation exchange between the surfaces of the solid matrix. 
         [0036]    References cited 
         [0037]    Mills, A. F. Basic Heat and Mass Transfer, Richard D. Irwin Inc., Concord, Mass., 1995. 
         [0038]    Hagen, K. D. Heat Transfer with Applications, Prentice-Hall, Upper Saddle River, N.J., 1999.