Abstract:
A heater plate is constructed with an embedded thermal diffusion layer of pyrolytic graphite to provide increased temperature uniformity in a critical heating surface. The heater has first and second metal plates with a heater element contained within the first plate and a core of the pyrolytic graphite diffusion layer sandwiched between the heater element and the second metal plate. The diffusion layer may be sputter metal coated to improve bonding of the layer to the plates.

Description:
CROSS-REFERENCE WITH RELATED APPLICATION 
     This application claims priority from U.S. provisional application Ser. No. 61/267,769,filed Dec. 8, 2009. 
    
    
     TECHNICAL FIELD 
     The present invention relates to heater plates and in particular to structural details of such heater plates specifically adapted to provide uniform heating. 
     BACKGROUND ART 
     Achieving the most uniform temperature on the surface of a heater can be limited due to the thermal conductivity of the materials of construction. Often, material options are limited by factors such as temperature rating, chemical compatibility, or thermal expansion. Geometry of the heater can have a significant impact on asymmetric losses and aggravate thermal non-uniformity. Typically, experience and thermal modeling are used for the heater design for the most effective power distribution. Heat homogenizing ceramic materials may be used for the outer plates. Metallic heat spreaders, e.g., a copper core, may be used. But, even with the most effective heater layout and construction, the thermal uniformity may need still further improvement, as a typical heating plate at 250° C. may have a maximum-minimum range of as much as 15-20° C. Examples of prior heaters are provided in U.S. Pat. No. 4,481,406 (Muka), U.S. Pat. No. 6,534,751 (Uchiyama et al.), U.S. Pat. No. 6,758,263 (Krassowski et al.) and U.S. Patent Application Publication No. 2009/0235866 (Kataigi et al.). 
     SUMMARY DISCLOSURE 
     Integrating a thermally annealed pyrolytic graphite (TPG) layer, between the heater and the critical surface of the plate dramatically improves the thermal uniformity. TPG is sometimes referred to as “hyper conductive” due to its having a thermal conductivity about four times that of copper. The high, in-plane thermal conductivity coefficient k allows for only shallow gradients. Thus, the provision of TPG material within a heater plate will help to distribute the heat from an isolated embedded heater element so that the operating surface of the plate has a more uniform temperature. 
     In summary, the invention provides a uniform heater having a core formed of a thermally-annealed pyrolytic graphite (TPG) diffuser sandwiched between a first metal plate containing a heater element and a second metal plate providing a critical surface. The plates and TPG diffuser may be vacuum thermal brazed together. The TPG diffuser may have a molybdenum coating and nickel braze alloy sheets may be present between the diffuser and the respective plates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of heater plate consistent with the present invention. 
         FIG. 2  is a side exploded view of an embodiment of a heater plate of the present invention. 
         FIG. 3  is a top view of a lower heater plate accommodating a heater coil. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a heater  11  has a critical heating surface on a thermally conductive upper plate  13 . Two electrodes  19  and  21  for in internal heater coil are seen to emerge from a side of the heater  11 , along with a ground electrode  20  for the plate  13 . 
     As seen in  FIG. 2 , the heater  11  includes upper and lower plates  13  and  15 , together with and a thermal pyrolytic graphite (TPG) diffusion layer  17  and an electrically isolated heating element  23  located between the two plates  13  and  15 . An interface material, not shown, fills voids between the various component parts  13 ,  15 ,  17  and  21 , and bonds the plates  13  and  15  together. 
     The upper and lower plates  13  and  15  may be made of metal. However, the plate material need not have especially high thermal conductivity in the plane of the plates because of the presence of the TPG diffusion layer  17  that serves to uniformly spread the heat from the heating element across the critical surface of the upper plate  13 . Thus, the plate material can be selected from a variety of metals, including stainless steels and nickel alloys, titanium, magnesium, molybdenum, tungsten, copper, aluminum, and combinations or alloys of the same. (The stainless steels and nickel alloys are sold under a number of trade names, including AISI 304 and 316 stainless steels, Incoloy®, Iconel®, Hastelloy®, and Nickel 600 (UNS N06600). These metals and others can be used.) 
     As seen in  FIG. 3 , the lower plate  15  may contain a spiral cavity to accept the heater element  23 . Alternatively, the cavity for the heater element  23  could be simply an open cavity with spaces between the coils of the heater element  23  filled with interface material. The upper plate may likewise contain a cavity to accept the TPG diffusion layer  17 . The TPG diffusion layer  17  may have a sputtered coating of molybdenum or other high-temperature sputter material that bonds to metal (where “high-temperature” refers to 500° C. or greater). Metals other than molybdenum that could be sputtered onto the TPG diffusion layer include nickel alloys, titanium, magnesium, tungsten, copper, aluminum, and combinations or alloys of the same. 
     Interface material is any material added to fill voids between the two plates  13  and  15  and heater element  23 , such as a potting compound, as well as material to bond the two plates  13  and  15 , such as a braze material or cement. In one embodiment, a braze material directly contacts the heater element  23  in the lower plate  15  to the coated TPG diffusion layer  17  in the upper plate  13 . A nickel braze clad, such as Nickel 4777 (82Ni-7Cr-4Si-3Fe-3B) foil, may be provided between the coated TPG diffusion layer  17  and each of the plates  13  and  15 , and the entire assembly then vacuum furnace brazed. 
     For the heater element&#39;s electrical isolation (using MgO insulation), electrical resistance between the internal heater wire and its insulating sheath has a tendency to break down significantly starting around 450° C. To overcome this problem, we have increased the sheath diameter from a 0.125″ (3.2 mm) diameter element to a 0.188″ (4.8 mm) diameter element in order to increase the dielectric distance and are able to achieve 600° C. without bad leakage current. Additionally, higher temperature dielectrics, namely boron nitride, could replace the MgO as the heater element&#39;s insulating sheath. The isolation material, while providing electrical resistance, should also have good thermal conductivity. Boron nitride has this combination of properties. 
     To determine the effect of the hyper-conductive diffusion layer  17  in heater plate  11 , we used an existing design for the lower plate  15  and heater element  23 , and made an upper plate  13  with the added diffusion layer  17  of TPG. Both heater plates  13  and  15  were made of stainless steel. The diffusion layer  17  was fused into a cavity between the heater element  23  and the upper plate  15 . The critical surfaces on the outside of the upper plate for both the embodiment of the present invention so made and a standard heater plate of the prior art without the TPG heater layer  17  were painted with a high temperature flat black paint to insure consistent emissivity for infrared evaluation. Both plates were placed in a chamber on small ceramic standoffs for side-by-side thermal imaging. Thermal images were taken in both atmosphere and vacuum. IR analysis settings were 21° C. ambient, 0.95 emissivity, lens factor  1 , 16″ focus, 6×4.5 cm field of view, high temperature range of 265.82° C., and low temperature range of 26.69° C. for a test at nominal heater temperature 250° C. The results for the heater  11  of the present invention with TPG diffusion layer  17  were a maximum temperature of 244.88° C., a minimum temperature of 230.02° C., an average temperature of 240.50° C., and a standard deviation of 4.19° C. The results for the standard heater plate without the TPG diffusion layer were a maximum temperature of 260.01° C., a minimum temperature of 225.97° C., an average temperature of 249.87° C., and a standard deviation of 9.69° C. The temperature uniformity across the plate improved from ±17° C. for the standard plate to ±7° C. by adding the diffusion layer, a 59% reduction in ΔT.