METHOD FOR FORMING AN ELECTRICAL HEATING ELEMENT BY FLAME SPRAYING A METAL/METALLIC OXIDE MATRIX

Abstract

A method for forming an electrical heating element by flame spraying a metal/metallic oxide matrix, wherein a flame sprayed metal/metallic oxide matrix is deposited onto an insulating or conductive substrate such as to have a higher resistance than is required for a designed use, and an intermittently pulsed high voltage DC supply is applied across the matrix such as to produce continuous electrically conductive paths through the matrix which permanently increase the overall conduction and simultaneously reduce the overall resistance of the metal/metallic matrix to achieve a desired resistance value.

Description

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:-

FIG. 1 is a diagrammatic representation of one embodiment of a conditioning apparatus for use in performing the present invention.

FIG. 1 shows a typical sample 10 of an electrical heating element whose final operational resistance is to be established during its formation. The heating element in these cases comprises a substrate (not visible), which can be either conductive or non-conductive, carrying a layer of metal oxide 12 that has been deposited by flame spraying. As explained hereinbefore, it is found that such flame spraying produces areas of metal surrounded by areas of oxide in the resulting “oxide” layer 12. Metallic strips 14, 16 are formed/provided on opposite sides of the deposited oxide layer to enable electrical current to be passed through the latter layer.

An AC transformer 18 receives a variable AC input of 0-230 volts on its primary winding 19, the secondary winding 21 of this transformer presenting 0-5000 volts to a variable frequency pulsing switch 20 coupled to a control output 22 of a computer 24. The current in the secondary winding 21 of the transformer 18 is preferably limited to approximately 25 mA, but variable (0-25 mA) in 5 mA steps to result in a high voltage DC being presented across the sample 10 by the switch 20 via lines 23, 25.

Also connected across the sample 10 is a primary source of voltage 30 which can, for example, be 0-500 DC volts, with a current limit of 0-10 amps.

Finally, there is also connected across the sample 10 a resistance measuring means 26, using D.V.M., whose output is coupled at 28 to a monitoring input of the computer 24.

The computer is arranged to continuously monitor the resistance of the sample and to vary the applied DC pulsing voltage and the number of pulses.

In use, a metal/metallic oxide matrix is first applied to the insulating or conductive substrate by a flame spraying apparatus (not shown), which can itself be conventional, such that the matrix has initially a higher resistance than is required for a designed use of a heating element to be formed, the resistance measurement being made continuously by the resistance measuring means 26 and computer 24, preferably using OHM' s Law calculations based on the values of the continuously applied DC voltage and resulting current flow.

The supply 30 applies a first, continuous DC voltage to the metal/metallic oxide matrix in the direction in which the particular configuration of metal/metallic oxide matrix is intended to operate as an electrical resistance heating element.

A second DC voltage is applied by the pulsing switch 22 to the flame sprayed metal/metallic oxide matrix in the same direction as the continuously applied first DC voltage in a series of high frequency intermittent pulses to cause the overall conductivity of the metal/metallic oxide matrix to increase, with corresponding decrease in overall resistance.

The computer 24 monitors the increase in the current flowing through the metal/metallic oxide matrix by virtue of the continuously applied first DC voltage and detects when the overall resistance of the flame sprayed metal/metallic oxide matrix has been reduced to a value required for that particular design and configuration of flame sprayed deposited metal/metallic oxide matrix. The application of the pulsed, second DC voltage to the oxide matrix is then caused by the computer to be discontinued.

Claims

1. A method for forming an electrical heating element by flame spraying a metal/metallic oxide matrix, wherein a flame sprayed metal/metallic oxide matrix is deposited onto an insulating or conductive substrate such as to have a higher resistance than is required for a designed use, and an intermittently pulsed high voltage DC supply is applied across the matrix such as to produce continuous electrically conductive paths through the matrix which permanently increase the overall conduction and simultaneously reduce the overall resistance of the metal/metallic matrix to achieve a desired resistance value.

2. A method as claimed in claim 1, wherein the prevailing resistance of the metal/metallic oxide matrix is determined by applying a further continuous DC voltage to the matrix in the direction in which the particular configuration of oxide matrix is intended to operate as an electrical resistance heating element, and determining the resistance from OHM's Law calculations based on the values of continuously applied DC voltage and resulting current flow.

3. A method as claimed in claim 2, wherein said further DC voltage is applied at a level in the range from ten to one hundred percent more than the designed operating level of the resulting electrical resistance element.

4. A method as claimed in claim 1, comprising the steps of: (a) applying said further continuous DC voltage to the metal/metallic oxide matrix in the direction in which the particular configuration of metal/metallic oxide matrix is intended to operate as an electrical resistance heating element;(b) determining the resistance of the metal/metallic matrix fromOHM' s Law calculations based on the values of said further continuously applied DC voltage and resulting current flow;(c) applying said intermittently pulsed high voltage DC supply to the metal/metallic oxide matrix in the same direction as said further continuously applied DC voltage and in a series of high frequency intermittent pulses so as to cause the overall conductivity of the metal/metallic oxide matrix to increase, with corresponding decrease in overall resistance; and(d) continuously monitoring the increase in the current flowing through the metal/metallic oxide matrix by virtue of said further continuously applied DC voltage until a calculation using OHM's Law demonstrates that the overall resistance of the flame sprayed metal/metallic oxide matrix is at a value required for that particular design and configuration of flame sprayed deposited metal/metallic oxide matrix to operate as an electrically resistive heating element, and at this stage turning off both DC voltage supplies to the metal/metallic oxide matrix.

5. A method as claimed in claim 4, wherein said further continuous DC voltage is applied at a level ranging from ten to one hundred percent more than the designed operating level of the particular design or configuration of electrical resistance heating element.

6. A method as claimed in claim 5, wherein the intermittently pulsed DC voltage is applied such that the live and neutral contacts for both DC voltage sources are coincident.

7. A method as claimed in claim 6, wherein the intermittently pulsed DC voltage source is set successively at levels in a range lying between 500 and 5000 volts.

8. A method as claimed in claim 7, wherein the level of the intermittently applied DC voltage is initially set at a low level of the order of about 500 volts and progressively increased during steps (c) and (d) to a level of about 5000 volts or higher, as required by the different resistivities of the different metal/metallic oxide combinations produced by the flame spray deposited metal/metallic oxide matrices.

9. A method as claimed in any of claims 1 to 8 wherein the methodology for modifying the conductivity and resistance of the flame sprayed deposited metal/metallic oxide matrices intended for use as electrical resistance heating elements is applied as a rapid computer controlled process, independent of the flame spray element manufacturing process.

10. An apparatus for manufacturing an electrical heating element, comprising: (a) means for depositing a metal/metallic oxide matrix onto an insulating or conductive substrate by flame spraying, such that the matrix has initially a higher resistance than is required for a designed use of the heating element;(b) means for applying a first, continuous DC voltage to the metal/metallic oxide matrix in the direction in which the particular configuration of metal/metallic oxide matrix is intended to operate as an electrical resistance heating element;(c) means for determining the resistance of the metal/metallic matrix from OHM's Law calculations based on the values of the continuously applied DC voltage and resulting current flow;(d) means for applying a second DC voltage source to the flame sprayed metal/metallic oxide matrix in the same direction as the continuously applied first DC voltage, and in a series of high frequency intermittent pulses to cause the overall conductivity of the metal/metallic oxide matrix to increase, with corresponding decrease in overall resistance; and(e) means for monitoring the increase in the current flowing through the metal/metallic oxide matrix by virtue of the continuously applied first DC voltage until a calculation using OHM' s Law demonstrates that the overall resistance of the flame sprayed metal/metallic oxide matrix has been reduced to a value required for that particular design and configuration of flame sprayed deposited metal/metallic oxide matrix.