GB1583030A - Field emitters incorporating directionally solidified eutectics containing refractory metal carbides - Google Patents

Description

(54) FIELD EMITTERS INCORPORATING DIRECTIONALLY SOLIDIFIED EUTECTICS CONTAINING REFACTORY METAL CARBIDES (71) We, FULMER RESEARCH INSTITUTE LIMITED, a British Company, of Stoke Poges, Slough, Buckinghamshire, SL2 4QD, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to field emitters, that is to say cathodes which under the influence of an intense magnetic field are capable of emitting electrons without a need for auxiliary heating. Such an emitter usually comprises at least one sharp point and more usually an array of fibres of which the tips are sharp.The present invention provides a field emitter made from a direc tionally solidified eutectic of a refractory metallic carbide in a matrix which is metallic or semi-conductive.

There follows a more detailed description of the invention and the manner in which it may be performed.

Early attempts to extract electrons from a cold cathode included the fabrication of comb-like arrays of fine wires of which the ends were sharpened to a point. Electronic currents have been extracted also from single emitters such as carbon fibres and tungsten wires. Reference may be made to "Vacuum" Volume 25 Nos. 9 and 10, 1975 pages 425 and 426 for a description of the preparation and testing of field emitters constituted by carbon fibres.

It has also been variously proposed to produce a field emitter which comprises a regular array of fine conducting fibres, similar to wires, from a directionally solidified eutectic material. The material consists of an electrically conducting struc ture of fibres contained within a metal or semi-conductive matrix. Reference should be made to "Journal of Applied Physics, Volume 41 No. 1 pages 76 to 81" for a description of the manufacture of an array of filaments by the directional solidification of a nickel-tungsten eutectic and the subsequent production of a multi-needle field emission cathode by the selective etching and pointing of the tips of the filaments.

Further reference may be made to "Journal of Applied Physics, Volume 46 No. 4 April 1975 pages 1841 to 1843" for a description of the manufacture of multi-filament arrays of tungsten in uranium dioxide or zirconium dioxide and of molybdenum in gadolinium trioxide. Reference may also be made to page 153 of Phys. Stat. Sol. Volume 111972 for a description of the manufacture of a cathode plate with outgrowing spikes from a directionally solidified eutectic system consisting of filamentary metallic inclusions of NiSb or CrSb embedded within a semiconductive matrix such as InSb. However, the construction of efficient field emitters from directionally solified eutectics has not been successful owing either to mechanical weakness in the fibres or irregularity in the array and non-uniformity in the diameters of the fibres.

We have now found that emitters based upon the refractory metal carbide eutectics possess in general the attributes of a regular array of fibres of uniform diameter and high strength, of high melting point and a low sputter yield, that is to say a low rate of erosion when subjected to residual ionic bombardment.

Two examples of the manufacture of field emitters follow.

EXAMPLE I In this Example, the starting material comprises 10% by weight of vanadium and 2% by weight of carbon in a base of cobalt.

The starting material may be treated in the manner described in the aforementioned references, and particularly that described by Feeney, Chapman and Keener in Journal of Applied Physics, Volume 46, No. 4, April 1975 pages 1841 to 1843. The directional solidification of the eutectic produces an array of fibres, of diameter from 1 to 2 microns and composed of Vac08 occupying a volume fraction of 20% in a matrix of cobalt which contains a small fraction of vanadium in solid solution.

The accompanying Figure 1 illustrates the result of a test on a small emitter made according to this Example. For making the test, the emitter may be incorporated in a diode.

EXAMPLE 2 In this Example, the starting material is 16% by weight of tantalum carbide in a base of nickel containing 10% by weight of chromium. The fibres consist of tantalum carbide in a nickel-chromium matrix and occupy 10% by volume of the solidified ingot. The fibre diameter is again between 1 and 2 microns.

Another example of a suitable carbide is niobium carbide in a matrix of nickel chromium. Mixed carbides, such as niobium titanium carbide may also be used.

The accompanying Figure 2 shows a graph of current density against extraction voltage for a planar emitter made according to Example 1.

The field emitters may be produced in several forms.

A planar emitter may be made by taking a section through a solidified ingot of the refractory carbide eutectic at right angles to the direction of growth and subsequently etching the matrix, by chemical etching, electrolytic etching or ion beam etching, in order to expose the ends of the fibres. The fibres may be sharpened by additional etching. A planar emitter of an area as much as several square centimeters may be made in this manner.

A blade emitter may be made by cutting a thin laminar parallel to the fibres in the matrix, so that fibres lie within the plane of the laminar. A sharp razor edge may be ground in a direction across the fibres.

Etching of the matrix at the tip of the blade exposes row of fibres which may be sharpened as mentioned before.

An emitter in the form of a needle may be made by turning a cylinder of small diameter from the eutectic material. The axis of the cylinder may be parallel to the fibres. The end of the cylinder should be sharpened to a needle point. Subsequent etching of the matrix to expose and sharpen the points of the fibre produces the required emitter.

Finally, it is possible to make an emitter from a single fibre, which may be extracted from the matrix by chemical etching and may be subsequently sharpened at one end.

WHAT WE CLAIM IS: 1. A field emitter which is made from a directionally solidified eutectic of a refractory metallic carbide in a metallic or semiconductive matrix.

2. A field emitter according to claim 1, which comprises vanadium carbide in a matrix of cobalt.

3. A field emitter according to claim 1, which comprises tantalum carbide in a matrix of nickel and chromium.

4. A field emitter according to claim 1, which comprises niobium carbide in a matrix of nickel and chromium.

5. A field emitter according to claim 1 or claim 4 in which the carbide comprises the mixed carbide niobium titanium carbide.

6. A field emitter according to claim 1, substantially as hereinbefore described with reference to Example 1.

7. A field emitter according to claim 1, substantially as hereinbefore described with reference to Example 2.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. according to this Example. For making the test, the emitter may be incorporated in a diode. EXAMPLE 2 In this Example, the starting material is 16% by weight of tantalum carbide in a base of nickel containing 10% by weight of chromium. The fibres consist of tantalum carbide in a nickel-chromium matrix and occupy 10% by volume of the solidified ingot. The fibre diameter is again between 1 and 2 microns. Another example of a suitable carbide is niobium carbide in a matrix of nickel chromium. Mixed carbides, such as niobium titanium carbide may also be used. The accompanying Figure 2 shows a graph of current density against extraction voltage for a planar emitter made according to Example 1. The field emitters may be produced in several forms. A planar emitter may be made by taking a section through a solidified ingot of the refractory carbide eutectic at right angles to the direction of growth and subsequently etching the matrix, by chemical etching, electrolytic etching or ion beam etching, in order to expose the ends of the fibres. The fibres may be sharpened by additional etching. A planar emitter of an area as much as several square centimeters may be made in this manner. A blade emitter may be made by cutting a thin laminar parallel to the fibres in the matrix, so that fibres lie within the plane of the laminar. A sharp razor edge may be ground in a direction across the fibres. Etching of the matrix at the tip of the blade exposes row of fibres which may be sharpened as mentioned before. An emitter in the form of a needle may be made by turning a cylinder of small diameter from the eutectic material. The axis of the cylinder may be parallel to the fibres. The end of the cylinder should be sharpened to a needle point. Subsequent etching of the matrix to expose and sharpen the points of the fibre produces the required emitter. Finally, it is possible to make an emitter from a single fibre, which may be extracted from the matrix by chemical etching and may be subsequently sharpened at one end. WHAT WE CLAIM IS:

1. A field emitter which is made from a directionally solidified eutectic of a refractory metallic carbide in a metallic or semiconductive matrix.

2. A field emitter according to claim 1, which comprises vanadium carbide in a matrix of cobalt.

3. A field emitter according to claim 1, which comprises tantalum carbide in a matrix of nickel and chromium.

4. A field emitter according to claim 1, which comprises niobium carbide in a matrix of nickel and chromium.

5. A field emitter according to claim 1 or claim 4 in which the carbide comprises the mixed carbide niobium titanium carbide.

6. A field emitter according to claim 1, substantially as hereinbefore described with reference to Example 1.

7. A field emitter according to claim 1, substantially as hereinbefore described with reference to Example 2.