GB2247348A - Peltier devices - Google Patents

Abstract

In a peltier device comprising a metallic heat absorption sink (15), a metallic heat dissipation sink (16) and a semiconductor material (17) disposed between the two sinks and electrically insulated therefrom, electrical connections to the semiconductor material (17) are via nickel pads (18) superimposed on copper pads (19) and these are insulated from their respective heat sinks by a microns-thin layer of a ceramic material such as alumina. The component parts are bonded together to form a consolidated structure. <IMAGE>

Description

PELTIER DEVICES THIS INVENTION concerns peltier devices and a method of constructing same.

Such a device comprises a metallic heat absorption sink, a metallic heat dissipation sink and a semiconductor material electrically driven and disposed between the two sinks but electrically insulated therefrom.

Peltier devices, which are intended to transfer heat energy by absorption and dissipation, can be operated efficiently only if it is possible to ensure efficient heat absorption at one side of the device, complete transfer of the absorbed heat across the device, and efficient heat dissipation from the output side.

Conventional peltier devices consist of an assembly of small semiconductor pieces mounted between two ceramic plates. This assembly is then clamped to and between a pair of metallic heat sinks one for absorption of heat and the other for heat dissipation.

The ceramic plates provide a mounting support for the semiconductor assembly and electrical insulation from the metallic sinks. In order to afford sufficient strength, the ceramic plates are usually 0.5mm or more in thickness. This strength is mainly in compression and the device is extremely vulnerable when subjected to sheer forces. The ceramic which in peltier devices is usually Alumina is an inefficient medium for heat transfer but is usually chosen for its electrical insulation properties. The thickness of the plates, which should be minimised for heat transfer, is however needed to achieve the mechanical requirements for mounting the device between the heat sinks.

Furthermore, heat transfer across the interface between two different materials is always much less effective than through a single material.

Peltier devices of this kind are usually assembled by clamping the parts tightly together using a thermally conductive substance to enhance the integrity of the surface contact of the ceramic plates with the metallic heat sinks. The manner of application of the clamping pressure is critical owing to the fragile nature of the ceramic plates when subjected to sheer forces. Any unevenness in pressure increases the risk of mechanical failure and prejudices the integrity of the surface contact at the interface between the ceramic and the metal and thus reduces the efficiency of heat transfer.

An object of the present invention is to provide an improved method of constructing a peltier device, wherein the aforementioned difficulties are substantially avoided and whereby a more effective peltier device is achieved.

According to the present invention there is provided a method of constructing a peltier device which comprises a metallic heat absorption sink, a metallic heat dissipation sink and a semiconductor material disposed between the two sinks and electrically insulated therefrom; the method including the steps of coating the heat sinks with a microns-thin layer of an electrically insulating material, placing between said coated surfaces a pair of electrically conductive layers, placing said semiconductor material between said electrically conductive layers and, with the application of heat, bonding the component parts together to form a consolidated structure.

Further according to the invention there is provided a peltier device comprising a metallic heat absorption sink, a metallic heat dissipation sink and a semiconductor material disposed between the two sinks and electrically insulated therefrom; characterised by a microns-thin layer of an electrically insulating material bonded to associated surface regions of said two sinks, an electrically conductive layer superimposed on each coating of electrically insulating material, and a layer of semiconductor material disposed between the two electrically conductive layers, the component parts of the entire device being bonded together as a consolidated structure.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: Fig. 1 is a cross-section through a peltier device made in accordance with conventional methods; and Fig. 2 is a cross-section through a peltier device made in accordance with the present invention.

Referring initially to Fig. 1, the conventional peltier device comprises a heat absorption sink 10 and a heat dissipation sink 11, the object being to transfer heat between the sinks 10 and 11.

Between the two sinks is an assembly comprising a series of semiconductor pieces 12 which typically will be approximately lmm across and mounted between a pair of ceramic plates 13 of some 0.5mm thickness. The device must be clamped together with the inclusion, between the plates 13 and their respective heat sinks, of a thermally conductive cream represented by double lines 14 in Fig. 1, and the entire device usually of rectangular form may, for example, occupy a space some 40mm, square.

Electrical connections are made to the semiconductor pieces 12 via electrically conductive pads 12a in the usual way.

As has been described above, this conventional construction suffers from the disadvantages of strength and heat transfer, in view of the delicate nature of the ceramic plates 13, and the non-intimate contact between the latter and the heat sinks.

Referring now to Fig. 2, the improved device in accordance with the invention is designed to overcome the difficulties of heat transfer efficiency whilst ensuring adequate electrical insulation and mechanical strength. Once again, the device comprises a metallic heat absorption sink 15 and a similar heat dissipation sink 16. The mass of sink 16 is preferably greater than that of sink 15.

Mounted centrally between the sinks 15 and 16 is a series of semiconductor pieces 17 electrically connected on each side to an electrically conductive layer consisting of an array of nickel pads 18 superimposed on copper pads. As can be seen from Fig.

2, there is a series electrical connection through the semiconductor pieces via the electrically conductive layers. Between the conductive layers and their heat sinks are microns-thin layers 20 of a ceramic material such as Alumina.

The device is constructed in accordance with the following method steps.

Onto a central region of each metallic heat sink 15 and 16 is fixed a microns-thin layer of the ceramic material which however is of sufficient thickness to provide electrical insulation without significant loss of thermal conductivity.

On to one of the ceramic surfaces are deposited the copper pads 19 superimposed by the nickel pads 18 which will serve for electrical connection to the semiconductor pieces 17 whilst also providing a means of adhesion therefor. The peltier pieces are positioned over the surfaces of the nickel pads and soldered thereto by application of heat at between 1500C and 2000C through the associated heat sink.

The other heat sink, pre-coated with its similar ceramic layer and conductive layer is then positioned precisely over the peltier pieces 17, and the assembly is completed by the soldering of the second conductive layer to the pieces 17, by the further application of heat via the associated heat sink.

If required, the entire device may be mechanically supported by the inclusion around the immediate vicinity of the semiconductor assembly of a strong thermally insulating material which will serve to maintain the two heat sinks in perfect parallelism thus preventing any damage to the assembled parts.

The method of construction as described has enabled the thickness of the electrical insulation material to be reduced from approximately 0.5mm to an order of microns, thus greatly improving thermal conductivity. By depositing the ceramic insulation material directly onto the aluminium substrate, the strength of the latter is assumed by the ceramic material and avoids the need for heat energy to transfer across between two non-intimate materials.

Furthermore, the need for critical pressure application and a thermally conductive medium has been avoided, and the strength of the entire device is increased when compared with the sheer weakness of thicker ceramic bodies.

The copper/nickel layer provides the necessary electrical connections to the semiconductor material, and the substances of this layer are such that it retains good thermal and electrical conductivity and is sufficiently malleable at operating temperatures to accommodate the differences in thermal expansion and contraction which effect the respective hot and cold sides of the device.

Whilst ceramic material, and particularly Alumina, has been stated as the electrically insulating material, any other low-voltage insulators may be used, such as anodised aluminium.

Claims (9)

1. A method of constructing a peltier device which comprises a metallic heat absorption sink, a metallic heat dissipation sink and a semiconductor material disposed between the two sinks and electrically insulated therefrom; the method including the steps of coating the heat sinks with a microns-thin layer of an electrically insulating material, placing between said coated surfaces a pair of electrically conductive layers, placing said semiconductor material between said electrically conductive layers and, with the application of heat, bonding the component parts together to form a consolidated structure.

2. A method according to Claim 1, including the steps of coating the surface of one of the heat sinks with said electrically insulating material, placing thereon one of said electrically conductive layers, positioning on said layer said semiconductor material, heating said one sink sufficiently to bond the semiconductor material, the electrically conductive layer, the electrically insulating material and the heat sink together thus to provide an electrical connection between the semiconductor material and the electrically conductive layer, and bonding the other sink, pre-coated with a similar microns-thin layer of an electrically insulating material and with the other electrically conductive layer, to the semiconductor material by the further application of heat.

3. A method according to Claim 1, wherein the semiconductor material and the electrically conductive layers associated therewith are assembled between said heat sinks pre-coated with said electrically insulating material, and the entire structure is bonded together in a single step.

4. A method according to any preceding claim, wherein said semiconductor material and said electrically conductive layers are bonded using solder at a temperature of between 1500C and 2000C.

5. A peltier device comprising a metallic heat absorption sink, a metallic heat dissipation sink and a semiconductor material disposed between the two sinks and electrically insulated therefrom; characterised by a microns-thin layer of an electrically insulating material bonded to associated surface regions of said two sinks, an electrically conductive layer superimposed on each coating of electrically insulating material, and a layer of semiconductor material disposed between the two electrically conductive layers, the component parts of the entire device being bonded together as a consolidated structure.

6. A peltier device according to Claim 5, wherein said electrically insulating material is alumina.

7. A peltier device according to Claim 5, wherein said heat sinks are of aluminium.

8. A peltier device according to Claim 5, wherein the mass of said heat dissipation sink is greater than that of said heat absorption sink.

9. A peltier device according to Claim 5, wherein said electrically conductive layer comprises an array of copper pads spaced apart on said electrically insulating material, each copper pad having a nickel pad super-imposed thereon remote from said electrically insulating material and in electrically conductive contact with said semiconductor material.